/*

  * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

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  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 package java.util;

 import java.io.Serializable;

 import java.io.IOException;

 import java.lang.reflect.Array;

 import org.apidesign.bck2brwsr.emul.lang.System;

 /**

  * This class consists exclusively of static methods that operate on or return

  * collections.  It contains polymorphic algorithms that operate on

  * collections, "wrappers", which return a new collection backed by a

  * specified collection, and a few other odds and ends.

  *

  * <p>The methods of this class all throw a <tt>NullPointerException</tt>

  * if the collections or class objects provided to them are null.

  *

  * <p>The documentation for the polymorphic algorithms contained in this class

  * generally includes a brief description of the <i>implementation</i>.  Such

  * descriptions should be regarded as <i>implementation notes</i>, rather than

  * parts of the <i>specification</i>.  Implementors should feel free to

  * substitute other algorithms, so long as the specification itself is adhered

  * to.  (For example, the algorithm used by <tt>sort</tt> does not have to be

  * a mergesort, but it does have to be <i>stable</i>.)

  *

  * <p>The "destructive" algorithms contained in this class, that is, the

  * algorithms that modify the collection on which they operate, are specified

  * to throw <tt>UnsupportedOperationException</tt> if the collection does not

  * support the appropriate mutation primitive(s), such as the <tt>set</tt>

  * method.  These algorithms may, but are not required to, throw this

  * exception if an invocation would have no effect on the collection.  For

  * example, invoking the <tt>sort</tt> method on an unmodifiable list that is

  * already sorted may or may not throw <tt>UnsupportedOperationException</tt>.

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @author  Josh Bloch

  * @author  Neal Gafter

  * @see     Collection

  * @see     Set

  * @see     List

  * @see     Map

  * @since   1.2

  */

 public class Collections {

     // Suppresses default constructor, ensuring non-instantiability.

     private Collections() {

     }

     // Algorithms

     /*

      * Tuning parameters for algorithms - Many of the List algorithms have

      * two implementations, one of which is appropriate for RandomAccess

      * lists, the other for "sequential."  Often, the random access variant

      * yields better performance on small sequential access lists.  The

      * tuning parameters below determine the cutoff point for what constitutes

      * a "small" sequential access list for each algorithm.  The values below

      * were empirically determined to work well for LinkedList. Hopefully

      * they should be reasonable for other sequential access List

      * implementations.  Those doing performance work on this code would

      * do well to validate the values of these parameters from time to time.

      * (The first word of each tuning parameter name is the algorithm to which

      * it applies.)

      */

     private static final int BINARYSEARCH_THRESHOLD   = 5000;

     private static final int REVERSE_THRESHOLD        =   18;

     private static final int SHUFFLE_THRESHOLD        =    5;

     private static final int FILL_THRESHOLD           =   25;

     private static final int ROTATE_THRESHOLD         =  100;

     private static final int COPY_THRESHOLD           =   10;

     private static final int REPLACEALL_THRESHOLD     =   11;

     private static final int INDEXOFSUBLIST_THRESHOLD =   35;

     /**

      * Sorts the specified list into ascending order, according to the

      * {@linkplain Comparable natural ordering} of its elements.

      * All elements in the list must implement the {@link Comparable}

      * interface.  Furthermore, all elements in the list must be

      * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}

      * must not throw a {@code ClassCastException} for any elements

      * {@code e1} and {@code e2} in the list).

      *

      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will

      * not be reordered as a result of the sort.

      *

      * <p>The specified list must be modifiable, but need not be resizable.

      *

      * <p>Implementation note: This implementation is a stable, adaptive,

      * iterative mergesort that requires far fewer than n lg(n) comparisons

      * when the input array is partially sorted, while offering the

      * performance of a traditional mergesort when the input array is

      * randomly ordered.  If the input array is nearly sorted, the

      * implementation requires approximately n comparisons.  Temporary

      * storage requirements vary from a small constant for nearly sorted

      * input arrays to n/2 object references for randomly ordered input

      * arrays.

      *

      * <p>The implementation takes equal advantage of ascending and

      * descending order in its input array, and can take advantage of

      * ascending and descending order in different parts of the same

      * input array.  It is well-suited to merging two or more sorted arrays:

      * simply concatenate the arrays and sort the resulting array.

      *

      * <p>The implementation was adapted from Tim Peters's list sort for Python

      * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">

      * TimSort</a>).  It uses techiques from Peter McIlroy's "Optimistic

      * Sorting and Information Theoretic Complexity", in Proceedings of the

      * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,

      * January 1993.

      *

      * <p>This implementation dumps the specified list into an array, sorts

      * the array, and iterates over the list resetting each element

      * from the corresponding position in the array.  This avoids the

      * n<sup>2</sup> log(n) performance that would result from attempting

      * to sort a linked list in place.

      *

      * @param  list the list to be sorted.

      * @throws ClassCastException if the list contains elements that are not

      *         <i>mutually comparable</i> (for example, strings and integers).

      * @throws UnsupportedOperationException if the specified list's

      *         list-iterator does not support the {@code set} operation.

      * @throws IllegalArgumentException (optional) if the implementation

      *         detects that the natural ordering of the list elements is

      *         found to violate the {@link Comparable} contract

      */

     public static <T extends Comparable<? super T>> void sort(List<T> list) {

         Object[] a = list.toArray();

         Arrays.sort(a);

         ListIterator<T> i = list.listIterator();

         for (int j=0; j<a.length; j++) {

             i.next();

             i.set((T)a[j]);

         }

     }

     /**

      * Sorts the specified list according to the order induced by the

      * specified comparator.  All elements in the list must be <i>mutually

      * comparable</i> using the specified comparator (that is,

      * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}

      * for any elements {@code e1} and {@code e2} in the list).

      *

      * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will

      * not be reordered as a result of the sort.

      *

      * <p>The specified list must be modifiable, but need not be resizable.

      *

      * <p>Implementation note: This implementation is a stable, adaptive,

      * iterative mergesort that requires far fewer than n lg(n) comparisons

      * when the input array is partially sorted, while offering the

      * performance of a traditional mergesort when the input array is

      * randomly ordered.  If the input array is nearly sorted, the

      * implementation requires approximately n comparisons.  Temporary

      * storage requirements vary from a small constant for nearly sorted

      * input arrays to n/2 object references for randomly ordered input

      * arrays.

      *

      * <p>The implementation takes equal advantage of ascending and

      * descending order in its input array, and can take advantage of

      * ascending and descending order in different parts of the same

      * input array.  It is well-suited to merging two or more sorted arrays:

      * simply concatenate the arrays and sort the resulting array.

      *

      * <p>The implementation was adapted from Tim Peters's list sort for Python

      * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">

      * TimSort</a>).  It uses techiques from Peter McIlroy's "Optimistic

      * Sorting and Information Theoretic Complexity", in Proceedings of the

      * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,

      * January 1993.

      *

      * <p>This implementation dumps the specified list into an array, sorts

      * the array, and iterates over the list resetting each element

      * from the corresponding position in the array.  This avoids the

      * n<sup>2</sup> log(n) performance that would result from attempting

      * to sort a linked list in place.

      *

      * @param  list the list to be sorted.

      * @param  c the comparator to determine the order of the list.  A

      *        {@code null} value indicates that the elements' <i>natural

      *        ordering</i> should be used.

      * @throws ClassCastException if the list contains elements that are not

      *         <i>mutually comparable</i> using the specified comparator.

      * @throws UnsupportedOperationException if the specified list's

      *         list-iterator does not support the {@code set} operation.

      * @throws IllegalArgumentException (optional) if the comparator is

      *         found to violate the {@link Comparator} contract

      */

     public static <T> void sort(List<T> list, Comparator<? super T> c) {

         Object[] a = list.toArray();

         Arrays.sort(a, (Comparator)c);

         ListIterator i = list.listIterator();

         for (int j=0; j<a.length; j++) {

             i.next();

             i.set(a[j]);

         }

     }

     /**

      * Searches the specified list for the specified object using the binary

      * search algorithm.  The list must be sorted into ascending order

      * according to the {@linkplain Comparable natural ordering} of its

      * elements (as by the {@link #sort(List)} method) prior to making this

      * call.  If it is not sorted, the results are undefined.  If the list

      * contains multiple elements equal to the specified object, there is no

      * guarantee which one will be found.

      *

      * <p>This method runs in log(n) time for a "random access" list (which

      * provides near-constant-time positional access).  If the specified list

      * does not implement the {@link RandomAccess} interface and is large,

      * this method will do an iterator-based binary search that performs

      * O(n) link traversals and O(log n) element comparisons.

      *

      * @param  list the list to be searched.

      * @param  key the key to be searched for.

      * @return the index of the search key, if it is contained in the list;

      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The

      *         <i>insertion point</i> is defined as the point at which the

      *         key would be inserted into the list: the index of the first

      *         element greater than the key, or <tt>list.size()</tt> if all

      *         elements in the list are less than the specified key.  Note

      *         that this guarantees that the return value will be >= 0 if

      *         and only if the key is found.

      * @throws ClassCastException if the list contains elements that are not

      *         <i>mutually comparable</i> (for example, strings and

      *         integers), or the search key is not mutually comparable

      *         with the elements of the list.

      */

     public static <T>

     int binarySearch(List<? extends Comparable<? super T>> list, T key) {

         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)

             return Collections.indexedBinarySearch(list, key);

         else

             return Collections.iteratorBinarySearch(list, key);

     }

     private static <T>

     int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key)

     {

         int low = 0;

         int high = list.size()-1;

         while (low <= high) {

             int mid = (low + high) >>> 1;

             Comparable<? super T> midVal = list.get(mid);

             int cmp = midVal.compareTo(key);

             if (cmp < 0)

                 low = mid + 1;

             else if (cmp > 0)

                 high = mid - 1;

             else

                 return mid; // key found

         }

         return -(low + 1);  // key not found

     }

     private static <T>

     int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)

     {

         int low = 0;

         int high = list.size()-1;

         ListIterator<? extends Comparable<? super T>> i = list.listIterator();

         while (low <= high) {

             int mid = (low + high) >>> 1;

             Comparable<? super T> midVal = get(i, mid);

             int cmp = midVal.compareTo(key);

             if (cmp < 0)

                 low = mid + 1;

             else if (cmp > 0)

                 high = mid - 1;

             else

                 return mid; // key found

         }

         return -(low + 1);  // key not found

     }

     /**

      * Gets the ith element from the given list by repositioning the specified

      * list listIterator.

      */

     private static <T> T get(ListIterator<? extends T> i, int index) {

         T obj = null;

         int pos = i.nextIndex();

         if (pos <= index) {

             do {

                 obj = i.next();

             } while (pos++ < index);

         } else {

             do {

                 obj = i.previous();

             } while (--pos > index);

         }

         return obj;

     }

     /**

      * Searches the specified list for the specified object using the binary

      * search algorithm.  The list must be sorted into ascending order

      * according to the specified comparator (as by the

      * {@link #sort(List, Comparator) sort(List, Comparator)}

      * method), prior to making this call.  If it is

      * not sorted, the results are undefined.  If the list contains multiple

      * elements equal to the specified object, there is no guarantee which one

      * will be found.

      *

      * <p>This method runs in log(n) time for a "random access" list (which

      * provides near-constant-time positional access).  If the specified list

      * does not implement the {@link RandomAccess} interface and is large,

      * this method will do an iterator-based binary search that performs

      * O(n) link traversals and O(log n) element comparisons.

      *

      * @param  list the list to be searched.

      * @param  key the key to be searched for.

      * @param  c the comparator by which the list is ordered.

      *         A <tt>null</tt> value indicates that the elements'

      *         {@linkplain Comparable natural ordering} should be used.

      * @return the index of the search key, if it is contained in the list;

      *         otherwise, <tt>(-(<i>insertion point</i>) - 1)</tt>.  The

      *         <i>insertion point</i> is defined as the point at which the

      *         key would be inserted into the list: the index of the first

      *         element greater than the key, or <tt>list.size()</tt> if all

      *         elements in the list are less than the specified key.  Note

      *         that this guarantees that the return value will be >= 0 if

      *         and only if the key is found.

      * @throws ClassCastException if the list contains elements that are not

      *         <i>mutually comparable</i> using the specified comparator,

      *         or the search key is not mutually comparable with the

      *         elements of the list using this comparator.

      */

     public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {

         if (c==null)

             return binarySearch((List) list, key);

         if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)

             return Collections.indexedBinarySearch(list, key, c);

         else

             return Collections.iteratorBinarySearch(list, key, c);

     }

     private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {

         int low = 0;

         int high = l.size()-1;

         while (low <= high) {

             int mid = (low + high) >>> 1;

             T midVal = l.get(mid);

             int cmp = c.compare(midVal, key);

             if (cmp < 0)

                 low = mid + 1;

             else if (cmp > 0)

                 high = mid - 1;

             else

                 return mid; // key found

         }

         return -(low + 1);  // key not found

     }

     private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {

         int low = 0;

         int high = l.size()-1;

         ListIterator<? extends T> i = l.listIterator();

         while (low <= high) {

             int mid = (low + high) >>> 1;

             T midVal = get(i, mid);

             int cmp = c.compare(midVal, key);

             if (cmp < 0)

                 low = mid + 1;

             else if (cmp > 0)

                 high = mid - 1;

             else

                 return mid; // key found

         }

         return -(low + 1);  // key not found

     }

     private interface SelfComparable extends Comparable<SelfComparable> {}

     /**

      * Reverses the order of the elements in the specified list.<p>

      *

      * This method runs in linear time.

      *

      * @param  list the list whose elements are to be reversed.

      * @throws UnsupportedOperationException if the specified list or

      *         its list-iterator does not support the <tt>set</tt> operation.

      */

     public static void reverse(List<?> list) {

         int size = list.size();

         if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {

             for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)

                 swap(list, i, j);

         } else {

             ListIterator fwd = list.listIterator();

             ListIterator rev = list.listIterator(size);

             for (int i=0, mid=list.size()>>1; i<mid; i++) {

                 Object tmp = fwd.next();

                 fwd.set(rev.previous());

                 rev.set(tmp);

             }

         }

     }

     /**

      * Randomly permutes the specified list using a default source of

      * randomness.  All permutations occur with approximately equal

      * likelihood.<p>

      *

      * The hedge "approximately" is used in the foregoing description because

      * default source of randomness is only approximately an unbiased source

      * of independently chosen bits. If it were a perfect source of randomly

      * chosen bits, then the algorithm would choose permutations with perfect

      * uniformity.<p>

      *

      * This implementation traverses the list backwards, from the last element

      * up to the second, repeatedly swapping a randomly selected element into

      * the "current position".  Elements are randomly selected from the

      * portion of the list that runs from the first element to the current

      * position, inclusive.<p>

      *

      * This method runs in linear time.  If the specified list does not

      * implement the {@link RandomAccess} interface and is large, this

      * implementation dumps the specified list into an array before shuffling

      * it, and dumps the shuffled array back into the list.  This avoids the

      * quadratic behavior that would result from shuffling a "sequential

      * access" list in place.

      *

      * @param  list the list to be shuffled.

      * @throws UnsupportedOperationException if the specified list or

      *         its list-iterator does not support the <tt>set</tt> operation.

      */

     public static void shuffle(List<?> list) {

         Random rnd = r;

         if (rnd == null)

             r = rnd = new Random();

         shuffle(list, rnd);

     }

     private static Random r;

     /**

      * Randomly permute the specified list using the specified source of

      * randomness.  All permutations occur with equal likelihood

      * assuming that the source of randomness is fair.<p>

      *

      * This implementation traverses the list backwards, from the last element

      * up to the second, repeatedly swapping a randomly selected element into

      * the "current position".  Elements are randomly selected from the

      * portion of the list that runs from the first element to the current

      * position, inclusive.<p>

      *

      * This method runs in linear time.  If the specified list does not

      * implement the {@link RandomAccess} interface and is large, this

      * implementation dumps the specified list into an array before shuffling

      * it, and dumps the shuffled array back into the list.  This avoids the

      * quadratic behavior that would result from shuffling a "sequential

      * access" list in place.

      *

      * @param  list the list to be shuffled.

      * @param  rnd the source of randomness to use to shuffle the list.

      * @throws UnsupportedOperationException if the specified list or its

      *         list-iterator does not support the <tt>set</tt> operation.

      */

     public static void shuffle(List<?> list, Random rnd) {

         int size = list.size();

         if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {

             for (int i=size; i>1; i--)

                 swap(list, i-1, rnd.nextInt(i));

         } else {

             Object arr[] = list.toArray();

             // Shuffle array

             for (int i=size; i>1; i--)

                 swap(arr, i-1, rnd.nextInt(i));

             // Dump array back into list

             ListIterator it = list.listIterator();

             for (int i=0; i<arr.length; i++) {

                 it.next();

                 it.set(arr[i]);

             }

         }

     }

     /**

      * Swaps the elements at the specified positions in the specified list.

      * (If the specified positions are equal, invoking this method leaves

      * the list unchanged.)

      *

      * @param list The list in which to swap elements.

      * @param i the index of one element to be swapped.

      * @param j the index of the other element to be swapped.

      * @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt>

      *         is out of range (i < 0 || i >= list.size()

      *         || j < 0 || j >= list.size()).

      * @since 1.4

      */

     public static void swap(List<?> list, int i, int j) {

         final List l = list;

         l.set(i, l.set(j, l.get(i)));

     }

     /**

      * Swaps the two specified elements in the specified array.

      */

     private static void swap(Object[] arr, int i, int j) {

         Object tmp = arr[i];

         arr[i] = arr[j];

         arr[j] = tmp;

     }

     /**

      * Replaces all of the elements of the specified list with the specified

      * element. <p>

      *

      * This method runs in linear time.

      *

      * @param  list the list to be filled with the specified element.

      * @param  obj The element with which to fill the specified list.

      * @throws UnsupportedOperationException if the specified list or its

      *         list-iterator does not support the <tt>set</tt> operation.

      */

     public static <T> void fill(List<? super T> list, T obj) {

         int size = list.size();

         if (size < FILL_THRESHOLD || list instanceof RandomAccess) {

             for (int i=0; i<size; i++)

                 list.set(i, obj);

         } else {

             ListIterator<? super T> itr = list.listIterator();

             for (int i=0; i<size; i++) {

                 itr.next();

                 itr.set(obj);

             }

         }

     }

     /**

      * Copies all of the elements from one list into another.  After the

      * operation, the index of each copied element in the destination list

      * will be identical to its index in the source list.  The destination

      * list must be at least as long as the source list.  If it is longer, the

      * remaining elements in the destination list are unaffected. <p>

      *

      * This method runs in linear time.

      *

      * @param  dest The destination list.

      * @param  src The source list.

      * @throws IndexOutOfBoundsException if the destination list is too small

      *         to contain the entire source List.

      * @throws UnsupportedOperationException if the destination list's

      *         list-iterator does not support the <tt>set</tt> operation.

      */

     public static <T> void copy(List<? super T> dest, List<? extends T> src) {

         int srcSize = src.size();

         if (srcSize > dest.size())

             throw new IndexOutOfBoundsException("Source does not fit in dest");

         if (srcSize < COPY_THRESHOLD ||

             (src instanceof RandomAccess && dest instanceof RandomAccess)) {

             for (int i=0; i<srcSize; i++)

                 dest.set(i, src.get(i));

         } else {

             ListIterator<? super T> di=dest.listIterator();

             ListIterator<? extends T> si=src.listIterator();

             for (int i=0; i<srcSize; i++) {

                 di.next();

                 di.set(si.next());

             }

         }

     }

     /**

      * Returns the minimum element of the given collection, according to the

      * <i>natural ordering</i> of its elements.  All elements in the

      * collection must implement the <tt>Comparable</tt> interface.

      * Furthermore, all elements in the collection must be <i>mutually

      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a

      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and

      * <tt>e2</tt> in the collection).<p>

      *

      * This method iterates over the entire collection, hence it requires

      * time proportional to the size of the collection.

      *

      * @param  coll the collection whose minimum element is to be determined.

      * @return the minimum element of the given collection, according

      *         to the <i>natural ordering</i> of its elements.

      * @throws ClassCastException if the collection contains elements that are

      *         not <i>mutually comparable</i> (for example, strings and

      *         integers).

      * @throws NoSuchElementException if the collection is empty.

      * @see Comparable

      */

     public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {

         Iterator<? extends T> i = coll.iterator();

         T candidate = i.next();

         while (i.hasNext()) {

             T next = i.next();

             if (next.compareTo(candidate) < 0)

                 candidate = next;

         }

         return candidate;

     }

     /**

      * Returns the minimum element of the given collection, according to the

      * order induced by the specified comparator.  All elements in the

      * collection must be <i>mutually comparable</i> by the specified

      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a

      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and

      * <tt>e2</tt> in the collection).<p>

      *

      * This method iterates over the entire collection, hence it requires

      * time proportional to the size of the collection.

      *

      * @param  coll the collection whose minimum element is to be determined.

      * @param  comp the comparator with which to determine the minimum element.

      *         A <tt>null</tt> value indicates that the elements' <i>natural

      *         ordering</i> should be used.

      * @return the minimum element of the given collection, according

      *         to the specified comparator.

      * @throws ClassCastException if the collection contains elements that are

      *         not <i>mutually comparable</i> using the specified comparator.

      * @throws NoSuchElementException if the collection is empty.

      * @see Comparable

      */

     public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {

         if (comp==null)

             return (T)min((Collection<SelfComparable>) (Collection) coll);

         Iterator<? extends T> i = coll.iterator();

         T candidate = i.next();

         while (i.hasNext()) {

             T next = i.next();

             if (comp.compare(next, candidate) < 0)

                 candidate = next;

         }

         return candidate;

     }

     /**

      * Returns the maximum element of the given collection, according to the

      * <i>natural ordering</i> of its elements.  All elements in the

      * collection must implement the <tt>Comparable</tt> interface.

      * Furthermore, all elements in the collection must be <i>mutually

      * comparable</i> (that is, <tt>e1.compareTo(e2)</tt> must not throw a

      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and

      * <tt>e2</tt> in the collection).<p>

      *

      * This method iterates over the entire collection, hence it requires

      * time proportional to the size of the collection.

      *

      * @param  coll the collection whose maximum element is to be determined.

      * @return the maximum element of the given collection, according

      *         to the <i>natural ordering</i> of its elements.

      * @throws ClassCastException if the collection contains elements that are

      *         not <i>mutually comparable</i> (for example, strings and

      *         integers).

      * @throws NoSuchElementException if the collection is empty.

      * @see Comparable

      */

     public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {

         Iterator<? extends T> i = coll.iterator();

         T candidate = i.next();

         while (i.hasNext()) {

             T next = i.next();

             if (next.compareTo(candidate) > 0)

                 candidate = next;

         }

         return candidate;

     }

     /**

      * Returns the maximum element of the given collection, according to the

      * order induced by the specified comparator.  All elements in the

      * collection must be <i>mutually comparable</i> by the specified

      * comparator (that is, <tt>comp.compare(e1, e2)</tt> must not throw a

      * <tt>ClassCastException</tt> for any elements <tt>e1</tt> and

      * <tt>e2</tt> in the collection).<p>

      *

      * This method iterates over the entire collection, hence it requires

      * time proportional to the size of the collection.

      *

      * @param  coll the collection whose maximum element is to be determined.

      * @param  comp the comparator with which to determine the maximum element.

      *         A <tt>null</tt> value indicates that the elements' <i>natural

      *        ordering</i> should be used.

      * @return the maximum element of the given collection, according

      *         to the specified comparator.

      * @throws ClassCastException if the collection contains elements that are

      *         not <i>mutually comparable</i> using the specified comparator.

      * @throws NoSuchElementException if the collection is empty.

      * @see Comparable

      */

     public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {

         if (comp==null)

             return (T)max((Collection<SelfComparable>) (Collection) coll);

         Iterator<? extends T> i = coll.iterator();

         T candidate = i.next();

         while (i.hasNext()) {

             T next = i.next();

             if (comp.compare(next, candidate) > 0)

                 candidate = next;

         }

         return candidate;

     }

     /**

      * Rotates the elements in the specified list by the specified distance.

      * After calling this method, the element at index <tt>i</tt> will be

      * the element previously at index <tt>(i - distance)</tt> mod

      * <tt>list.size()</tt>, for all values of <tt>i</tt> between <tt>0</tt>

      * and <tt>list.size()-1</tt>, inclusive.  (This method has no effect on

      * the size of the list.)

      *

      * <p>For example, suppose <tt>list</tt> comprises<tt> [t, a, n, k, s]</tt>.

      * After invoking <tt>Collections.rotate(list, 1)</tt> (or

      * <tt>Collections.rotate(list, -4)</tt>), <tt>list</tt> will comprise

      * <tt>[s, t, a, n, k]</tt>.

      *

      * <p>Note that this method can usefully be applied to sublists to

      * move one or more elements within a list while preserving the

      * order of the remaining elements.  For example, the following idiom

      * moves the element at index <tt>j</tt> forward to position

      * <tt>k</tt> (which must be greater than or equal to <tt>j</tt>):

      * <pre>

      *     Collections.rotate(list.subList(j, k+1), -1);

      * </pre>

      * To make this concrete, suppose <tt>list</tt> comprises

      * <tt>[a, b, c, d, e]</tt>.  To move the element at index <tt>1</tt>

      * (<tt>b</tt>) forward two positions, perform the following invocation:

      * <pre>

      *     Collections.rotate(l.subList(1, 4), -1);

      * </pre>

      * The resulting list is <tt>[a, c, d, b, e]</tt>.

      *

      * <p>To move more than one element forward, increase the absolute value

      * of the rotation distance.  To move elements backward, use a positive

      * shift distance.

      *

      * <p>If the specified list is small or implements the {@link

      * RandomAccess} interface, this implementation exchanges the first

      * element into the location it should go, and then repeatedly exchanges

      * the displaced element into the location it should go until a displaced

      * element is swapped into the first element.  If necessary, the process

      * is repeated on the second and successive elements, until the rotation

      * is complete.  If the specified list is large and doesn't implement the

      * <tt>RandomAccess</tt> interface, this implementation breaks the

      * list into two sublist views around index <tt>-distance mod size</tt>.

      * Then the {@link #reverse(List)} method is invoked on each sublist view,

      * and finally it is invoked on the entire list.  For a more complete

      * description of both algorithms, see Section 2.3 of Jon Bentley's

      * <i>Programming Pearls</i> (Addison-Wesley, 1986).

      *

      * @param list the list to be rotated.

      * @param distance the distance to rotate the list.  There are no

      *        constraints on this value; it may be zero, negative, or

      *        greater than <tt>list.size()</tt>.

      * @throws UnsupportedOperationException if the specified list or

      *         its list-iterator does not support the <tt>set</tt> operation.

      * @since 1.4

      */

     public static void rotate(List<?> list, int distance) {

         if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)

             rotate1(list, distance);

         else

             rotate2(list, distance);

     }

     private static <T> void rotate1(List<T> list, int distance) {

         int size = list.size();

         if (size == 0)

             return;

         distance = distance % size;

         if (distance < 0)

             distance += size;

         if (distance == 0)

             return;

         for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {

             T displaced = list.get(cycleStart);

             int i = cycleStart;

             do {

                 i += distance;

                 if (i >= size)

                     i -= size;

                 displaced = list.set(i, displaced);

                 nMoved ++;

             } while (i != cycleStart);

         }

     }

     private static void rotate2(List<?> list, int distance) {

         int size = list.size();

         if (size == 0)

             return;

         int mid =  -distance % size;

         if (mid < 0)

             mid += size;

         if (mid == 0)

             return;

         reverse(list.subList(0, mid));

         reverse(list.subList(mid, size));

         reverse(list);

     }

     /**

      * Replaces all occurrences of one specified value in a list with another.

      * More formally, replaces with <tt>newVal</tt> each element <tt>e</tt>

      * in <tt>list</tt> such that

      * <tt>(oldVal==null ? e==null : oldVal.equals(e))</tt>.

      * (This method has no effect on the size of the list.)

      *

      * @param list the list in which replacement is to occur.

      * @param oldVal the old value to be replaced.

      * @param newVal the new value with which <tt>oldVal</tt> is to be

      *        replaced.

      * @return <tt>true</tt> if <tt>list</tt> contained one or more elements

      *         <tt>e</tt> such that

      *         <tt>(oldVal==null ?  e==null : oldVal.equals(e))</tt>.

      * @throws UnsupportedOperationException if the specified list or

      *         its list-iterator does not support the <tt>set</tt> operation.

      * @since  1.4

      */

     public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {

         boolean result = false;

         int size = list.size();

         if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {

             if (oldVal==null) {

                 for (int i=0; i<size; i++) {

                     if (list.get(i)==null) {

                         list.set(i, newVal);

                         result = true;

                     }

                 }

             } else {

                 for (int i=0; i<size; i++) {

                     if (oldVal.equals(list.get(i))) {

                         list.set(i, newVal);

                         result = true;

                     }

                 }

             }

         } else {

             ListIterator<T> itr=list.listIterator();

             if (oldVal==null) {

                 for (int i=0; i<size; i++) {

                     if (itr.next()==null) {

                         itr.set(newVal);

                         result = true;

                     }

                 }

             } else {

                 for (int i=0; i<size; i++) {

                     if (oldVal.equals(itr.next())) {

                         itr.set(newVal);

                         result = true;

                     }

                 }

             }

         }

         return result;

     }

     /**

      * Returns the starting position of the first occurrence of the specified

      * target list within the specified source list, or -1 if there is no

      * such occurrence.  More formally, returns the lowest index <tt>i</tt>

      * such that <tt>source.subList(i, i+target.size()).equals(target)</tt>,

      * or -1 if there is no such index.  (Returns -1 if

      * <tt>target.size() > source.size()</tt>.)

      *

      * <p>This implementation uses the "brute force" technique of scanning

      * over the source list, looking for a match with the target at each

      * location in turn.

      *

      * @param source the list in which to search for the first occurrence

      *        of <tt>target</tt>.

      * @param target the list to search for as a subList of <tt>source</tt>.

      * @return the starting position of the first occurrence of the specified

      *         target list within the specified source list, or -1 if there

      *         is no such occurrence.

      * @since  1.4

      */

     public static int indexOfSubList(List<?> source, List<?> target) {

         int sourceSize = source.size();

         int targetSize = target.size();

         int maxCandidate = sourceSize - targetSize;

         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||

             (source instanceof RandomAccess&&target instanceof RandomAccess)) {

         nextCand:

             for (int candidate = 0; candidate <= maxCandidate; candidate++) {

                 for (int i=0, j=candidate; i<targetSize; i++, j++)

                     if (!eq(target.get(i), source.get(j)))

                         continue nextCand;  // Element mismatch, try next cand

                 return candidate;  // All elements of candidate matched target

             }

         } else {  // Iterator version of above algorithm

             ListIterator<?> si = source.listIterator();

         nextCand:

             for (int candidate = 0; candidate <= maxCandidate; candidate++) {

                 ListIterator<?> ti = target.listIterator();

                 for (int i=0; i<targetSize; i++) {

                     if (!eq(ti.next(), si.next())) {

                         // Back up source iterator to next candidate

                         for (int j=0; j<i; j++)

                             si.previous();

                         continue nextCand;

                     }

                 }

                 return candidate;

             }

         }

         return -1;  // No candidate matched the target

     }

     /**

      * Returns the starting position of the last occurrence of the specified

      * target list within the specified source list, or -1 if there is no such

      * occurrence.  More formally, returns the highest index <tt>i</tt>

      * such that <tt>source.subList(i, i+target.size()).equals(target)</tt>,

      * or -1 if there is no such index.  (Returns -1 if

      * <tt>target.size() > source.size()</tt>.)

      *

      * <p>This implementation uses the "brute force" technique of iterating

      * over the source list, looking for a match with the target at each

      * location in turn.

      *

      * @param source the list in which to search for the last occurrence

      *        of <tt>target</tt>.

      * @param target the list to search for as a subList of <tt>source</tt>.

      * @return the starting position of the last occurrence of the specified

      *         target list within the specified source list, or -1 if there

      *         is no such occurrence.

      * @since  1.4

      */

     public static int lastIndexOfSubList(List<?> source, List<?> target) {

         int sourceSize = source.size();

         int targetSize = target.size();

         int maxCandidate = sourceSize - targetSize;

         if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||

             source instanceof RandomAccess) {   // Index access version

         nextCand:

             for (int candidate = maxCandidate; candidate >= 0; candidate--) {

                 for (int i=0, j=candidate; i<targetSize; i++, j++)

                     if (!eq(target.get(i), source.get(j)))

                         continue nextCand;  // Element mismatch, try next cand

                 return candidate;  // All elements of candidate matched target

             }

         } else {  // Iterator version of above algorithm

             if (maxCandidate < 0)

                 return -1;

             ListIterator<?> si = source.listIterator(maxCandidate);

         nextCand:

             for (int candidate = maxCandidate; candidate >= 0; candidate--) {

                 ListIterator<?> ti = target.listIterator();

                 for (int i=0; i<targetSize; i++) {

                     if (!eq(ti.next(), si.next())) {

                         if (candidate != 0) {

                             // Back up source iterator to next candidate

                             for (int j=0; j<=i+1; j++)

                                 si.previous();

                         }

                         continue nextCand;

                     }

                 }

                 return candidate;

             }

         }

         return -1;  // No candidate matched the target

     }

     // Unmodifiable Wrappers

     /**

      * Returns an unmodifiable view of the specified collection.  This method

      * allows modules to provide users with "read-only" access to internal

      * collections.  Query operations on the returned collection "read through"

      * to the specified collection, and attempts to modify the returned

      * collection, whether direct or via its iterator, result in an

      * <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned collection does <i>not</i> pass the hashCode and equals

      * operations through to the backing collection, but relies on

      * <tt>Object</tt>'s <tt>equals</tt> and <tt>hashCode</tt> methods.  This

      * is necessary to preserve the contracts of these operations in the case

      * that the backing collection is a set or a list.<p>

      *

      * The returned collection will be serializable if the specified collection

      * is serializable.

      *

      * @param  c the collection for which an unmodifiable view is to be

      *         returned.

      * @return an unmodifiable view of the specified collection.

      */

     public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {

         return new UnmodifiableCollection<>(c);

     }

     /**

      * @serial include

      */

     static class UnmodifiableCollection<E> implements Collection<E>, Serializable {

         private static final long serialVersionUID = 1820017752578914078L;

         final Collection<? extends E> c;

         UnmodifiableCollection(Collection<? extends E> c) {

             if (c==null)

                 throw new NullPointerException();

             this.c = c;

         }

         public int size()                   {return c.size();}

         public boolean isEmpty()            {return c.isEmpty();}

         public boolean contains(Object o)   {return c.contains(o);}

         public Object[] toArray()           {return c.toArray();}

         public <T> T[] toArray(T[] a)       {return c.toArray(a);}

         public String toString()            {return c.toString();}

         public Iterator<E> iterator() {

             return new Iterator<E>() {

                 private final Iterator<? extends E> i = c.iterator();

                 public boolean hasNext() {return i.hasNext();}

                 public E next()          {return i.next();}

                 public void remove() {

                     throw new UnsupportedOperationException();

                 }

             };

         }

         public boolean add(E e) {

             throw new UnsupportedOperationException();

         }

         public boolean remove(Object o) {

             throw new UnsupportedOperationException();

         }

         public boolean containsAll(Collection<?> coll) {

             return c.containsAll(coll);

         }

         public boolean addAll(Collection<? extends E> coll) {

             throw new UnsupportedOperationException();

         }

         public boolean removeAll(Collection<?> coll) {

             throw new UnsupportedOperationException();

         }

         public boolean retainAll(Collection<?> coll) {

             throw new UnsupportedOperationException();

         }

         public void clear() {

             throw new UnsupportedOperationException();

         }

     }

     /**

      * Returns an unmodifiable view of the specified set.  This method allows

      * modules to provide users with "read-only" access to internal sets.

      * Query operations on the returned set "read through" to the specified

      * set, and attempts to modify the returned set, whether direct or via its

      * iterator, result in an <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned set will be serializable if the specified set

      * is serializable.

      *

      * @param  s the set for which an unmodifiable view is to be returned.

      * @return an unmodifiable view of the specified set.

      */

     public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {

         return new UnmodifiableSet<>(s);

     }

     /**

      * @serial include

      */

     static class UnmodifiableSet<E> extends UnmodifiableCollection<E>

                                  implements Set<E>, Serializable {

         private static final long serialVersionUID = -9215047833775013803L;

         UnmodifiableSet(Set<? extends E> s)     {super(s);}

         public boolean equals(Object o) {return o == this || c.equals(o);}

         public int hashCode()           {return c.hashCode();}

     }

     /**

      * Returns an unmodifiable view of the specified sorted set.  This method

      * allows modules to provide users with "read-only" access to internal

      * sorted sets.  Query operations on the returned sorted set "read

      * through" to the specified sorted set.  Attempts to modify the returned

      * sorted set, whether direct, via its iterator, or via its

      * <tt>subSet</tt>, <tt>headSet</tt>, or <tt>tailSet</tt> views, result in

      * an <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned sorted set will be serializable if the specified sorted set

      * is serializable.

      *

      * @param s the sorted set for which an unmodifiable view is to be

      *        returned.

      * @return an unmodifiable view of the specified sorted set.

      */

     public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {

         return new UnmodifiableSortedSet<>(s);

     }

     /**

      * @serial include

      */

     static class UnmodifiableSortedSet<E>

                              extends UnmodifiableSet<E>

                              implements SortedSet<E>, Serializable {

         private static final long serialVersionUID = -4929149591599911165L;

         private final SortedSet<E> ss;

         UnmodifiableSortedSet(SortedSet<E> s) {super(s); ss = s;}

         public Comparator<? super E> comparator() {return ss.comparator();}

         public SortedSet<E> subSet(E fromElement, E toElement) {

             return new UnmodifiableSortedSet<>(ss.subSet(fromElement,toElement));

         }

         public SortedSet<E> headSet(E toElement) {

             return new UnmodifiableSortedSet<>(ss.headSet(toElement));

         }

         public SortedSet<E> tailSet(E fromElement) {

             return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));

         }

         public E first()                   {return ss.first();}

         public E last()                    {return ss.last();}

     }

     /**

      * Returns an unmodifiable view of the specified list.  This method allows

      * modules to provide users with "read-only" access to internal

      * lists.  Query operations on the returned list "read through" to the

      * specified list, and attempts to modify the returned list, whether

      * direct or via its iterator, result in an

      * <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned list will be serializable if the specified list

      * is serializable. Similarly, the returned list will implement

      * {@link RandomAccess} if the specified list does.

      *

      * @param  list the list for which an unmodifiable view is to be returned.

      * @return an unmodifiable view of the specified list.

      */

     public static <T> List<T> unmodifiableList(List<? extends T> list) {

         return (list instanceof RandomAccess ?

                 new UnmodifiableRandomAccessList<>(list) :

                 new UnmodifiableList<>(list));

     }

     /**

      * @serial include

      */

     static class UnmodifiableList<E> extends UnmodifiableCollection<E>

                                   implements List<E> {

         private static final long serialVersionUID = -283967356065247728L;

         final List<? extends E> list;

         UnmodifiableList(List<? extends E> list) {

             super(list);

             this.list = list;

         }

         public boolean equals(Object o) {return o == this || list.equals(o);}

         public int hashCode()           {return list.hashCode();}

         public E get(int index) {return list.get(index);}

         public E set(int index, E element) {

             throw new UnsupportedOperationException();

         }

         public void add(int index, E element) {

             throw new UnsupportedOperationException();

         }

         public E remove(int index) {

             throw new UnsupportedOperationException();

         }

         public int indexOf(Object o)            {return list.indexOf(o);}

         public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}

         public boolean addAll(int index, Collection<? extends E> c) {

             throw new UnsupportedOperationException();

         }

         public ListIterator<E> listIterator()   {return listIterator(0);}

         public ListIterator<E> listIterator(final int index) {

             return new ListIterator<E>() {

                 private final ListIterator<? extends E> i

                     = list.listIterator(index);

                 public boolean hasNext()     {return i.hasNext();}

                 public E next()              {return i.next();}

                 public boolean hasPrevious() {return i.hasPrevious();}

                 public E previous()          {return i.previous();}

                 public int nextIndex()       {return i.nextIndex();}

                 public int previousIndex()   {return i.previousIndex();}

                 public void remove() {

                     throw new UnsupportedOperationException();

                 }

                 public void set(E e) {

                     throw new UnsupportedOperationException();

                 }

                 public void add(E e) {

                     throw new UnsupportedOperationException();

                 }

             };

         }

         public List<E> subList(int fromIndex, int toIndex) {

             return new UnmodifiableList<>(list.subList(fromIndex, toIndex));

         }

         /**

          * UnmodifiableRandomAccessList instances are serialized as

          * UnmodifiableList instances to allow them to be deserialized

          * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).

          * This method inverts the transformation.  As a beneficial

          * side-effect, it also grafts the RandomAccess marker onto

          * UnmodifiableList instances that were serialized in pre-1.4 JREs.

          *

          * Note: Unfortunately, UnmodifiableRandomAccessList instances

          * serialized in 1.4.1 and deserialized in 1.4 will become

          * UnmodifiableList instances, as this method was missing in 1.4.

          */

         private Object readResolve() {

             return (list instanceof RandomAccess

                     ? new UnmodifiableRandomAccessList<>(list)

                     : this);

         }

     }

     /**

      * @serial include

      */

     static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>

                                               implements RandomAccess

     {

         UnmodifiableRandomAccessList(List<? extends E> list) {

             super(list);

         }

         public List<E> subList(int fromIndex, int toIndex) {

             return new UnmodifiableRandomAccessList<>(

                 list.subList(fromIndex, toIndex));

         }

         private static final long serialVersionUID = -2542308836966382001L;

         /**

          * Allows instances to be deserialized in pre-1.4 JREs (which do

          * not have UnmodifiableRandomAccessList).  UnmodifiableList has

          * a readResolve method that inverts this transformation upon

          * deserialization.

          */

         private Object writeReplace() {

             return new UnmodifiableList<>(list);

         }

     }

     /**

      * Returns an unmodifiable view of the specified map.  This method

      * allows modules to provide users with "read-only" access to internal

      * maps.  Query operations on the returned map "read through"

      * to the specified map, and attempts to modify the returned

      * map, whether direct or via its collection views, result in an

      * <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned map will be serializable if the specified map

      * is serializable.

      *

      * @param  m the map for which an unmodifiable view is to be returned.

      * @return an unmodifiable view of the specified map.

      */

     public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {

         return new UnmodifiableMap<>(m);

     }

     /**

      * @serial include

      */

     private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {

         private static final long serialVersionUID = -1034234728574286014L;

         private final Map<? extends K, ? extends V> m;

         UnmodifiableMap(Map<? extends K, ? extends V> m) {

             if (m==null)

                 throw new NullPointerException();

             this.m = m;

         }

         public int size()                        {return m.size();}

         public boolean isEmpty()                 {return m.isEmpty();}

         public boolean containsKey(Object key)   {return m.containsKey(key);}

         public boolean containsValue(Object val) {return m.containsValue(val);}

         public V get(Object key)                 {return m.get(key);}

         public V put(K key, V value) {

             throw new UnsupportedOperationException();

         }

         public V remove(Object key) {

             throw new UnsupportedOperationException();

         }

         public void putAll(Map<? extends K, ? extends V> m) {

             throw new UnsupportedOperationException();

         }

         public void clear() {

             throw new UnsupportedOperationException();

         }

         private transient Set<K> keySet = null;

         private transient Set<Map.Entry<K,V>> entrySet = null;

         private transient Collection<V> values = null;

         public Set<K> keySet() {

             if (keySet==null)

                 keySet = unmodifiableSet(m.keySet());

             return keySet;

         }

         public Set<Map.Entry<K,V>> entrySet() {

             if (entrySet==null)

                 entrySet = new UnmodifiableEntrySet<>(m.entrySet());

             return entrySet;

         }

         public Collection<V> values() {

             if (values==null)

                 values = unmodifiableCollection(m.values());

             return values;

         }

         public boolean equals(Object o) {return o == this || m.equals(o);}

         public int hashCode()           {return m.hashCode();}

         public String toString()        {return m.toString();}

         /**

          * We need this class in addition to UnmodifiableSet as

          * Map.Entries themselves permit modification of the backing Map

          * via their setValue operation.  This class is subtle: there are

          * many possible attacks that must be thwarted.

          *

          * @serial include

          */

         static class UnmodifiableEntrySet<K,V>

             extends UnmodifiableSet<Map.Entry<K,V>> {

             private static final long serialVersionUID = 7854390611657943733L;

             UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {

                 super((Set)s);

             }

             public Iterator<Map.Entry<K,V>> iterator() {

                 return new Iterator<Map.Entry<K,V>>() {

                     private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();

                     public boolean hasNext() {

                         return i.hasNext();

                     }

                     public Map.Entry<K,V> next() {

                         return new UnmodifiableEntry<>(i.next());

                     }

                     public void remove() {

                         throw new UnsupportedOperationException();

                     }

                 };

             }

             public Object[] toArray() {

                 Object[] a = c.toArray();

                 for (int i=0; i<a.length; i++)

                     a[i] = new UnmodifiableEntry<>((Map.Entry<K,V>)a[i]);

                 return a;

             }

             public <T> T[] toArray(T[] a) {

                 // We don't pass a to c.toArray, to avoid window of

                 // vulnerability wherein an unscrupulous multithreaded client

                 // could get his hands on raw (unwrapped) Entries from c.

                 Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));

                 for (int i=0; i<arr.length; i++)

                     arr[i] = new UnmodifiableEntry<>((Map.Entry<K,V>)arr[i]);

                 if (arr.length > a.length)

                     return (T[])arr;

                 System.arraycopy(arr, 0, a, 0, arr.length);

                 if (a.length > arr.length)

                     a[arr.length] = null;

                 return a;

             }

             /**

              * This method is overridden to protect the backing set against

              * an object with a nefarious equals function that senses

              * that the equality-candidate is Map.Entry and calls its

              * setValue method.

              */

             public boolean contains(Object o) {

                 if (!(o instanceof Map.Entry))

                     return false;

                 return c.contains(

                     new UnmodifiableEntry<>((Map.Entry<?,?>) o));

             }

             /**

              * The next two methods are overridden to protect against

              * an unscrupulous List whose contains(Object o) method senses

              * when o is a Map.Entry, and calls o.setValue.

              */

             public boolean containsAll(Collection<?> coll) {

                 for (Object e : coll) {

                     if (!contains(e)) // Invokes safe contains() above

                         return false;

                 }

                 return true;

             }

             public boolean equals(Object o) {

                 if (o == this)

                     return true;

                 if (!(o instanceof Set))

                     return false;

                 Set s = (Set) o;

                 if (s.size() != c.size())

                     return false;

                 return containsAll(s); // Invokes safe containsAll() above

             }

             /**

              * This "wrapper class" serves two purposes: it prevents

              * the client from modifying the backing Map, by short-circuiting

              * the setValue method, and it protects the backing Map against

              * an ill-behaved Map.Entry that attempts to modify another

              * Map Entry when asked to perform an equality check.

              */

             private static class UnmodifiableEntry<K,V> implements Map.Entry<K,V> {

                 private Map.Entry<? extends K, ? extends V> e;

                 UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e) {this.e = e;}

                 public K getKey()        {return e.getKey();}

                 public V getValue()      {return e.getValue();}

                 public V setValue(V value) {

                     throw new UnsupportedOperationException();

                 }

                 public int hashCode()    {return e.hashCode();}

                 public boolean equals(Object o) {

                     if (!(o instanceof Map.Entry))

                         return false;

                     Map.Entry t = (Map.Entry)o;

                     return eq(e.getKey(),   t.getKey()) &&

                            eq(e.getValue(), t.getValue());

                 }

                 public String toString() {return e.toString();}

             }

         }

     }

     /**

      * Returns an unmodifiable view of the specified sorted map.  This method

      * allows modules to provide users with "read-only" access to internal

      * sorted maps.  Query operations on the returned sorted map "read through"

      * to the specified sorted map.  Attempts to modify the returned

      * sorted map, whether direct, via its collection views, or via its

      * <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in

      * an <tt>UnsupportedOperationException</tt>.<p>

      *

      * The returned sorted map will be serializable if the specified sorted map

      * is serializable.

      *

      * @param m the sorted map for which an unmodifiable view is to be

      *        returned.

      * @return an unmodifiable view of the specified sorted map.

      */

     public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {

         return new UnmodifiableSortedMap<>(m);

     }

     /**

      * @serial include

      */

     static class UnmodifiableSortedMap<K,V>

           extends UnmodifiableMap<K,V>

           implements SortedMap<K,V>, Serializable {

         private static final long serialVersionUID = -8806743815996713206L;

         private final SortedMap<K, ? extends V> sm;

         UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m;}

         public Comparator<? super K> comparator() {return sm.comparator();}

         public SortedMap<K,V> subMap(K fromKey, K toKey) {

             return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey));

         }

         public SortedMap<K,V> headMap(K toKey) {

             return new UnmodifiableSortedMap<>(sm.headMap(toKey));

         }

         public SortedMap<K,V> tailMap(K fromKey) {

             return new UnmodifiableSortedMap<>(sm.tailMap(fromKey));

         }

         public K firstKey()           {return sm.firstKey();}

         public K lastKey()            {return sm.lastKey();}

     }

     // Synch Wrappers

     /**

      * Returns a synchronized (thread-safe) collection backed by the specified

      * collection.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing collection is accomplished

      * through the returned collection.<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * collection when iterating over it:

      * <pre>

      *  Collection c = Collections.synchronizedCollection(myCollection);

      *     ...

      *  synchronized (c) {

      *      Iterator i = c.iterator(); // Must be in the synchronized block

      *      while (i.hasNext())

      *         foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned collection does <i>not</i> pass the <tt>hashCode</tt>

      * and <tt>equals</tt> operations through to the backing collection, but

      * relies on <tt>Object</tt>'s equals and hashCode methods.  This is

      * necessary to preserve the contracts of these operations in the case

      * that the backing collection is a set or a list.<p>

      *

      * The returned collection will be serializable if the specified collection

      * is serializable.

      *

      * @param  c the collection to be "wrapped" in a synchronized collection.

      * @return a synchronized view of the specified collection.

      */

     public static <T> Collection<T> synchronizedCollection(Collection<T> c) {

         return new SynchronizedCollection<>(c);

     }

     static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {

         return new SynchronizedCollection<>(c, mutex);

     }

     /**

      * @serial include

      */

     static class SynchronizedCollection<E> implements Collection<E>, Serializable {

         private static final long serialVersionUID = 3053995032091335093L;

         final Collection<E> c;  // Backing Collection

         final Object mutex;     // Object on which to synchronize

         SynchronizedCollection(Collection<E> c) {

             if (c==null)

                 throw new NullPointerException();

             this.c = c;

             mutex = this;

         }

         SynchronizedCollection(Collection<E> c, Object mutex) {

             this.c = c;

             this.mutex = mutex;

         }

         public int size() {

             synchronized (mutex) {return c.size();}

         }

         public boolean isEmpty() {

             synchronized (mutex) {return c.isEmpty();}

         }

         public boolean contains(Object o) {

             synchronized (mutex) {return c.contains(o);}

         }

         public Object[] toArray() {

             synchronized (mutex) {return c.toArray();}

         }

         public <T> T[] toArray(T[] a) {

             synchronized (mutex) {return c.toArray(a);}

         }

         public Iterator<E> iterator() {

             return c.iterator(); // Must be manually synched by user!

         }

         public boolean add(E e) {

             synchronized (mutex) {return c.add(e);}

         }

         public boolean remove(Object o) {

             synchronized (mutex) {return c.remove(o);}

         }

         public boolean containsAll(Collection<?> coll) {

             synchronized (mutex) {return c.containsAll(coll);}

         }

         public boolean addAll(Collection<? extends E> coll) {

             synchronized (mutex) {return c.addAll(coll);}

         }

         public boolean removeAll(Collection<?> coll) {

             synchronized (mutex) {return c.removeAll(coll);}

         }

         public boolean retainAll(Collection<?> coll) {

             synchronized (mutex) {return c.retainAll(coll);}

         }

         public void clear() {

             synchronized (mutex) {c.clear();}

         }

         public String toString() {

             synchronized (mutex) {return c.toString();}

         }

     }

     /**

      * Returns a synchronized (thread-safe) set backed by the specified

      * set.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing set is accomplished

      * through the returned set.<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * set when iterating over it:

      * <pre>

      *  Set s = Collections.synchronizedSet(new HashSet());

      *      ...

      *  synchronized (s) {

      *      Iterator i = s.iterator(); // Must be in the synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned set will be serializable if the specified set is

      * serializable.

      *

      * @param  s the set to be "wrapped" in a synchronized set.

      * @return a synchronized view of the specified set.

      */

     public static <T> Set<T> synchronizedSet(Set<T> s) {

         return new SynchronizedSet<>(s);

     }

     static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {

         return new SynchronizedSet<>(s, mutex);

     }

     /**

      * @serial include

      */

     static class SynchronizedSet<E>

           extends SynchronizedCollection<E>

           implements Set<E> {

         private static final long serialVersionUID = 487447009682186044L;

         SynchronizedSet(Set<E> s) {

             super(s);

         }

         SynchronizedSet(Set<E> s, Object mutex) {

             super(s, mutex);

         }

         public boolean equals(Object o) {

             synchronized (mutex) {return c.equals(o);}

         }

         public int hashCode() {

             synchronized (mutex) {return c.hashCode();}

         }

     }

     /**

      * Returns a synchronized (thread-safe) sorted set backed by the specified

      * sorted set.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing sorted set is accomplished

      * through the returned sorted set (or its views).<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * sorted set when iterating over it or any of its <tt>subSet</tt>,

      * <tt>headSet</tt>, or <tt>tailSet</tt> views.

      * <pre>

      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());

      *      ...

      *  synchronized (s) {

      *      Iterator i = s.iterator(); // Must be in the synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * or:

      * <pre>

      *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());

      *  SortedSet s2 = s.headSet(foo);

      *      ...

      *  synchronized (s) {  // Note: s, not s2!!!

      *      Iterator i = s2.iterator(); // Must be in the synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned sorted set will be serializable if the specified

      * sorted set is serializable.

      *

      * @param  s the sorted set to be "wrapped" in a synchronized sorted set.

      * @return a synchronized view of the specified sorted set.

      */

     public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {

         return new SynchronizedSortedSet<>(s);

     }

     /**

      * @serial include

      */

     static class SynchronizedSortedSet<E>

         extends SynchronizedSet<E>

         implements SortedSet<E>

     {

         private static final long serialVersionUID = 8695801310862127406L;

         private final SortedSet<E> ss;

         SynchronizedSortedSet(SortedSet<E> s) {

             super(s);

             ss = s;

         }

         SynchronizedSortedSet(SortedSet<E> s, Object mutex) {

             super(s, mutex);

             ss = s;

         }

         public Comparator<? super E> comparator() {

             synchronized (mutex) {return ss.comparator();}

         }

         public SortedSet<E> subSet(E fromElement, E toElement) {

             synchronized (mutex) {

                 return new SynchronizedSortedSet<>(

                     ss.subSet(fromElement, toElement), mutex);

             }

         }

         public SortedSet<E> headSet(E toElement) {

             synchronized (mutex) {

                 return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);

             }

         }

         public SortedSet<E> tailSet(E fromElement) {

             synchronized (mutex) {

                return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);

             }

         }

         public E first() {

             synchronized (mutex) {return ss.first();}

         }

         public E last() {

             synchronized (mutex) {return ss.last();}

         }

     }

     /**

      * Returns a synchronized (thread-safe) list backed by the specified

      * list.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing list is accomplished

      * through the returned list.<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * list when iterating over it:

      * <pre>

      *  List list = Collections.synchronizedList(new ArrayList());

      *      ...

      *  synchronized (list) {

      *      Iterator i = list.iterator(); // Must be in synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned list will be serializable if the specified list is

      * serializable.

      *

      * @param  list the list to be "wrapped" in a synchronized list.

      * @return a synchronized view of the specified list.

      */

     public static <T> List<T> synchronizedList(List<T> list) {

         return (list instanceof RandomAccess ?

                 new SynchronizedRandomAccessList<>(list) :

                 new SynchronizedList<>(list));

     }

     static <T> List<T> synchronizedList(List<T> list, Object mutex) {

         return (list instanceof RandomAccess ?

                 new SynchronizedRandomAccessList<>(list, mutex) :

                 new SynchronizedList<>(list, mutex));

     }

     /**

      * @serial include

      */

     static class SynchronizedList<E>

         extends SynchronizedCollection<E>

         implements List<E> {

         private static final long serialVersionUID = -7754090372962971524L;

         final List<E> list;

         SynchronizedList(List<E> list) {

             super(list);

             this.list = list;

         }

         SynchronizedList(List<E> list, Object mutex) {

             super(list, mutex);

             this.list = list;

         }

         public boolean equals(Object o) {

             synchronized (mutex) {return list.equals(o);}

         }

         public int hashCode() {

             synchronized (mutex) {return list.hashCode();}

         }

         public E get(int index) {

             synchronized (mutex) {return list.get(index);}

         }

         public E set(int index, E element) {

             synchronized (mutex) {return list.set(index, element);}

         }

         public void add(int index, E element) {

             synchronized (mutex) {list.add(index, element);}

         }

         public E remove(int index) {

             synchronized (mutex) {return list.remove(index);}

         }

         public int indexOf(Object o) {

             synchronized (mutex) {return list.indexOf(o);}

         }

         public int lastIndexOf(Object o) {

             synchronized (mutex) {return list.lastIndexOf(o);}

         }

         public boolean addAll(int index, Collection<? extends E> c) {

             synchronized (mutex) {return list.addAll(index, c);}

         }

         public ListIterator<E> listIterator() {

             return list.listIterator(); // Must be manually synched by user

         }

         public ListIterator<E> listIterator(int index) {

             return list.listIterator(index); // Must be manually synched by user

         }

         public List<E> subList(int fromIndex, int toIndex) {

             synchronized (mutex) {

                 return new SynchronizedList<>(list.subList(fromIndex, toIndex),

                                             mutex);

             }

         }

         /**

          * SynchronizedRandomAccessList instances are serialized as

          * SynchronizedList instances to allow them to be deserialized

          * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).

          * This method inverts the transformation.  As a beneficial

          * side-effect, it also grafts the RandomAccess marker onto

          * SynchronizedList instances that were serialized in pre-1.4 JREs.

          *

          * Note: Unfortunately, SynchronizedRandomAccessList instances

          * serialized in 1.4.1 and deserialized in 1.4 will become

          * SynchronizedList instances, as this method was missing in 1.4.

          */

         private Object readResolve() {

             return (list instanceof RandomAccess

                     ? new SynchronizedRandomAccessList<>(list)

                     : this);

         }

     }

     /**

      * @serial include

      */

     static class SynchronizedRandomAccessList<E>

         extends SynchronizedList<E>

         implements RandomAccess {

         SynchronizedRandomAccessList(List<E> list) {

             super(list);

         }

         SynchronizedRandomAccessList(List<E> list, Object mutex) {

             super(list, mutex);

         }

         public List<E> subList(int fromIndex, int toIndex) {

             synchronized (mutex) {

                 return new SynchronizedRandomAccessList<>(

                     list.subList(fromIndex, toIndex), mutex);

             }

         }

         private static final long serialVersionUID = 1530674583602358482L;

         /**

          * Allows instances to be deserialized in pre-1.4 JREs (which do

          * not have SynchronizedRandomAccessList).  SynchronizedList has

          * a readResolve method that inverts this transformation upon

          * deserialization.

          */

         private Object writeReplace() {

             return new SynchronizedList<>(list);

         }

     }

     /**

      * Returns a synchronized (thread-safe) map backed by the specified

      * map.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing map is accomplished

      * through the returned map.<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * map when iterating over any of its collection views:

      * <pre>

      *  Map m = Collections.synchronizedMap(new HashMap());

      *      ...

      *  Set s = m.keySet();  // Needn't be in synchronized block

      *      ...

      *  synchronized (m) {  // Synchronizing on m, not s!

      *      Iterator i = s.iterator(); // Must be in synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned map will be serializable if the specified map is

      * serializable.

      *

      * @param  m the map to be "wrapped" in a synchronized map.

      * @return a synchronized view of the specified map.

      */

     public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {

         return new SynchronizedMap<>(m);

     }

     /**

      * @serial include

      */

     private static class SynchronizedMap<K,V>

         implements Map<K,V>, Serializable {

         private static final long serialVersionUID = 1978198479659022715L;

         private final Map<K,V> m;     // Backing Map

         final Object      mutex;        // Object on which to synchronize

         SynchronizedMap(Map<K,V> m) {

             if (m==null)

                 throw new NullPointerException();

             this.m = m;

             mutex = this;

         }

         SynchronizedMap(Map<K,V> m, Object mutex) {

             this.m = m;

             this.mutex = mutex;

         }

         public int size() {

             synchronized (mutex) {return m.size();}

         }

         public boolean isEmpty() {

             synchronized (mutex) {return m.isEmpty();}

         }

         public boolean containsKey(Object key) {

             synchronized (mutex) {return m.containsKey(key);}

         }

         public boolean containsValue(Object value) {

             synchronized (mutex) {return m.containsValue(value);}

         }

         public V get(Object key) {

             synchronized (mutex) {return m.get(key);}

         }

         public V put(K key, V value) {

             synchronized (mutex) {return m.put(key, value);}

         }

         public V remove(Object key) {

             synchronized (mutex) {return m.remove(key);}

         }

         public void putAll(Map<? extends K, ? extends V> map) {

             synchronized (mutex) {m.putAll(map);}

         }

         public void clear() {

             synchronized (mutex) {m.clear();}

         }

         private transient Set<K> keySet = null;

         private transient Set<Map.Entry<K,V>> entrySet = null;

         private transient Collection<V> values = null;

         public Set<K> keySet() {

             synchronized (mutex) {

                 if (keySet==null)

                     keySet = new SynchronizedSet<>(m.keySet(), mutex);

                 return keySet;

             }

         }

         public Set<Map.Entry<K,V>> entrySet() {

             synchronized (mutex) {

                 if (entrySet==null)

                     entrySet = new SynchronizedSet<>(m.entrySet(), mutex);

                 return entrySet;

             }

         }

         public Collection<V> values() {

             synchronized (mutex) {

                 if (values==null)

                     values = new SynchronizedCollection<>(m.values(), mutex);

                 return values;

             }

         }

         public boolean equals(Object o) {

             synchronized (mutex) {return m.equals(o);}

         }

         public int hashCode() {

             synchronized (mutex) {return m.hashCode();}

         }

         public String toString() {

             synchronized (mutex) {return m.toString();}

         }

     }

     /**

      * Returns a synchronized (thread-safe) sorted map backed by the specified

      * sorted map.  In order to guarantee serial access, it is critical that

      * <strong>all</strong> access to the backing sorted map is accomplished

      * through the returned sorted map (or its views).<p>

      *

      * It is imperative that the user manually synchronize on the returned

      * sorted map when iterating over any of its collection views, or the

      * collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or

      * <tt>tailMap</tt> views.

      * <pre>

      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());

      *      ...

      *  Set s = m.keySet();  // Needn't be in synchronized block

      *      ...

      *  synchronized (m) {  // Synchronizing on m, not s!

      *      Iterator i = s.iterator(); // Must be in synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * or:

      * <pre>

      *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());

      *  SortedMap m2 = m.subMap(foo, bar);

      *      ...

      *  Set s2 = m2.keySet();  // Needn't be in synchronized block

      *      ...

      *  synchronized (m) {  // Synchronizing on m, not m2 or s2!

      *      Iterator i = s.iterator(); // Must be in synchronized block

      *      while (i.hasNext())

      *          foo(i.next());

      *  }

      * </pre>

      * Failure to follow this advice may result in non-deterministic behavior.

      *

      * <p>The returned sorted map will be serializable if the specified

      * sorted map is serializable.

      *

      * @param  m the sorted map to be "wrapped" in a synchronized sorted map.

      * @return a synchronized view of the specified sorted map.

      */

     public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {

         return new SynchronizedSortedMap<>(m);

     }

     /**

      * @serial include

      */

     static class SynchronizedSortedMap<K,V>

         extends SynchronizedMap<K,V>

         implements SortedMap<K,V>

     {

         private static final long serialVersionUID = -8798146769416483793L;

         private final SortedMap<K,V> sm;

         SynchronizedSortedMap(SortedMap<K,V> m) {

             super(m);

             sm = m;

         }

         SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {

             super(m, mutex);

             sm = m;

         }

         public Comparator<? super K> comparator() {

             synchronized (mutex) {return sm.comparator();}

         }

         public SortedMap<K,V> subMap(K fromKey, K toKey) {

             synchronized (mutex) {

                 return new SynchronizedSortedMap<>(

                     sm.subMap(fromKey, toKey), mutex);

             }

         }

         public SortedMap<K,V> headMap(K toKey) {

             synchronized (mutex) {

                 return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);

             }

         }

         public SortedMap<K,V> tailMap(K fromKey) {

             synchronized (mutex) {

                return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);

             }

         }

         public K firstKey() {

             synchronized (mutex) {return sm.firstKey();}

         }

         public K lastKey() {

             synchronized (mutex) {return sm.lastKey();}

         }

     }

     // Dynamically typesafe collection wrappers

     /**

      * Returns a dynamically typesafe view of the specified collection.

      * Any attempt to insert an element of the wrong type will result in an

      * immediate {@link ClassCastException}.  Assuming a collection

      * contains no incorrectly typed elements prior to the time a

      * dynamically typesafe view is generated, and that all subsequent

      * access to the collection takes place through the view, it is

      * <i>guaranteed</i> that the collection cannot contain an incorrectly

      * typed element.

      *

      * <p>The generics mechanism in the language provides compile-time

      * (static) type checking, but it is possible to defeat this mechanism

      * with unchecked casts.  Usually this is not a problem, as the compiler

      * issues warnings on all such unchecked operations.  There are, however,

      * times when static type checking alone is not sufficient.  For example,

      * suppose a collection is passed to a third-party library and it is

      * imperative that the library code not corrupt the collection by

      * inserting an element of the wrong type.

      *

      * <p>Another use of dynamically typesafe views is debugging.  Suppose a

      * program fails with a {@code ClassCastException}, indicating that an

      * incorrectly typed element was put into a parameterized collection.

      * Unfortunately, the exception can occur at any time after the erroneous

      * element is inserted, so it typically provides little or no information

      * as to the real source of the problem.  If the problem is reproducible,

      * one can quickly determine its source by temporarily modifying the

      * program to wrap the collection with a dynamically typesafe view.

      * For example, this declaration:

      *  <pre> {@code

      *     Collection<String> c = new HashSet<String>();

      * }</pre>

      * may be replaced temporarily by this one:

      *  <pre> {@code

      *     Collection<String> c = Collections.checkedCollection(

      *         new HashSet<String>(), String.class);

      * }</pre>

      * Running the program again will cause it to fail at the point where

      * an incorrectly typed element is inserted into the collection, clearly

      * identifying the source of the problem.  Once the problem is fixed, the

      * modified declaration may be reverted back to the original.

      *

      * <p>The returned collection does <i>not</i> pass the hashCode and equals

      * operations through to the backing collection, but relies on

      * {@code Object}'s {@code equals} and {@code hashCode} methods.  This

      * is necessary to preserve the contracts of these operations in the case

      * that the backing collection is a set or a list.

      *

      * <p>The returned collection will be serializable if the specified

      * collection is serializable.

      *

      * <p>Since {@code null} is considered to be a value of any reference

      * type, the returned collection permits insertion of null elements

      * whenever the backing collection does.

      *

      * @param c the collection for which a dynamically typesafe view is to be

      *          returned

      * @param type the type of element that {@code c} is permitted to hold

      * @return a dynamically typesafe view of the specified collection

      * @since 1.5

      */

     public static <E> Collection<E> checkedCollection(Collection<E> c,

                                                       Class<E> type) {

         return new CheckedCollection<>(c, type);

     }

     @SuppressWarnings("unchecked")

     static <T> T[] zeroLengthArray(Class<T> type) {

         return (T[]) Array.newInstance(type, 0);

     }

     /**

      * @serial include

      */

     static class CheckedCollection<E> implements Collection<E>, Serializable {

         private static final long serialVersionUID = 1578914078182001775L;

         final Collection<E> c;

         final Class<E> type;

         void typeCheck(Object o) {

             if (o != null && !type.isInstance(o))

                 throw new ClassCastException(badElementMsg(o));

         }

         private String badElementMsg(Object o) {

             return "Attempt to insert " + o.getClass() +

                 " element into collection with element type " + type;

         }

         CheckedCollection(Collection<E> c, Class<E> type) {

             if (c==null || type == null)

                 throw new NullPointerException();

             this.c = c;

             this.type = type;

         }

         public int size()                 { return c.size(); }

         public boolean isEmpty()          { return c.isEmpty(); }

         public boolean contains(Object o) { return c.contains(o); }

         public Object[] toArray()         { return c.toArray(); }

         public <T> T[] toArray(T[] a)     { return c.toArray(a); }

         public String toString()          { return c.toString(); }

         public boolean remove(Object o)   { return c.remove(o); }

         public void clear()               {        c.clear(); }

         public boolean containsAll(Collection<?> coll) {

             return c.containsAll(coll);

         }

         public boolean removeAll(Collection<?> coll) {

             return c.removeAll(coll);

         }

         public boolean retainAll(Collection<?> coll) {

             return c.retainAll(coll);

         }

         public Iterator<E> iterator() {

             final Iterator<E> it = c.iterator();

             return new Iterator<E>() {

                 public boolean hasNext() { return it.hasNext(); }

                 public E next()          { return it.next(); }

                 public void remove()     {        it.remove(); }};

         }

         public boolean add(E e) {

             typeCheck(e);

             return c.add(e);

         }

         private E[] zeroLengthElementArray = null; // Lazily initialized

         private E[] zeroLengthElementArray() {

             return zeroLengthElementArray != null ? zeroLengthElementArray :

                 (zeroLengthElementArray = zeroLengthArray(type));

         }

         @SuppressWarnings("unchecked")

         Collection<E> checkedCopyOf(Collection<? extends E> coll) {

             Object[] a = null;

             try {

                 E[] z = zeroLengthElementArray();

                 a = coll.toArray(z);

                 // Defend against coll violating the toArray contract

                 if (a.getClass() != z.getClass())

                     a = Arrays.copyOf(a, a.length, z.getClass());

             } catch (ArrayStoreException ignore) {

                 // To get better and consistent diagnostics,

                 // we call typeCheck explicitly on each element.

                 // We call clone() to defend against coll retaining a

                 // reference to the returned array and storing a bad

                 // element into it after it has been type checked.

                 a = coll.toArray().clone();

                 for (Object o : a)

                     typeCheck(o);

             }

             // A slight abuse of the type system, but safe here.

             return (Collection<E>) Arrays.asList(a);

         }

         public boolean addAll(Collection<? extends E> coll) {

             // Doing things this way insulates us from concurrent changes

             // in the contents of coll and provides all-or-nothing

             // semantics (which we wouldn't get if we type-checked each

             // element as we added it)

             return c.addAll(checkedCopyOf(coll));

         }

     }

     /**

      * Returns a dynamically typesafe view of the specified set.

      * Any attempt to insert an element of the wrong type will result in

      * an immediate {@link ClassCastException}.  Assuming a set contains

      * no incorrectly typed elements prior to the time a dynamically typesafe

      * view is generated, and that all subsequent access to the set

      * takes place through the view, it is <i>guaranteed</i> that the

      * set cannot contain an incorrectly typed element.

      *

      * <p>A discussion of the use of dynamically typesafe views may be

      * found in the documentation for the {@link #checkedCollection

      * checkedCollection} method.

      *

      * <p>The returned set will be serializable if the specified set is

      * serializable.

      *

      * <p>Since {@code null} is considered to be a value of any reference

      * type, the returned set permits insertion of null elements whenever

      * the backing set does.

      *

      * @param s the set for which a dynamically typesafe view is to be

      *          returned

      * @param type the type of element that {@code s} is permitted to hold

      * @return a dynamically typesafe view of the specified set

      * @since 1.5

      */

     public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {

         return new CheckedSet<>(s, type);

     }

     /**

      * @serial include

      */

     static class CheckedSet<E> extends CheckedCollection<E>

                                  implements Set<E>, Serializable

     {

         private static final long serialVersionUID = 4694047833775013803L;

         CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }

         public boolean equals(Object o) { return o == this || c.equals(o); }

         public int hashCode()           { return c.hashCode(); }

     }

     /**

      * Returns a dynamically typesafe view of the specified sorted set.

      * Any attempt to insert an element of the wrong type will result in an

      * immediate {@link ClassCastException}.  Assuming a sorted set

      * contains no incorrectly typed elements prior to the time a

      * dynamically typesafe view is generated, and that all subsequent

      * access to the sorted set takes place through the view, it is

      * <i>guaranteed</i> that the sorted set cannot contain an incorrectly

      * typed element.

      *

      * <p>A discussion of the use of dynamically typesafe views may be

      * found in the documentation for the {@link #checkedCollection

      * checkedCollection} method.

      *

      * <p>The returned sorted set will be serializable if the specified sorted

      * set is serializable.

      *

      * <p>Since {@code null} is considered to be a value of any reference

      * type, the returned sorted set permits insertion of null elements

      * whenever the backing sorted set does.

      *

      * @param s the sorted set for which a dynamically typesafe view is to be

      *          returned

      * @param type the type of element that {@code s} is permitted to hold

      * @return a dynamically typesafe view of the specified sorted set

      * @since 1.5

      */

     public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,

                                                     Class<E> type) {

         return new CheckedSortedSet<>(s, type);

     }

     /**

      * @serial include

      */

     static class CheckedSortedSet<E> extends CheckedSet<E>

         implements SortedSet<E>, Serializable

     {

         private static final long serialVersionUID = 1599911165492914959L;

         private final SortedSet<E> ss;

         CheckedSortedSet(SortedSet<E> s, Class<E> type) {

             super(s, type);

             ss = s;

         }

         public Comparator<? super E> comparator() { return ss.comparator(); }

         public E first()                   { return ss.first(); }

         public E last()                    { return ss.last(); }

         public SortedSet<E> subSet(E fromElement, E toElement) {

             return checkedSortedSet(ss.subSet(fromElement,toElement), type);

         }

         public SortedSet<E> headSet(E toElement) {

             return checkedSortedSet(ss.headSet(toElement), type);

         }

         public SortedSet<E> tailSet(E fromElement) {

             return checkedSortedSet(ss.tailSet(fromElement), type);

         }

     }

     /**

      * Returns a dynamically typesafe view of the specified list.

      * Any attempt to insert an element of the wrong type will result in

      * an immediate {@link ClassCastException}.  Assuming a list contains

      * no incorrectly typed elements prior to the time a dynamically typesafe

      * view is generated, and that all subsequent access to the list

      * takes place through the view, it is <i>guaranteed</i> that the

      * list cannot contain an incorrectly typed element.

      *

      * <p>A discussion of the use of dynamically typesafe views may be

      * found in the documentation for the {@link #checkedCollection

      * checkedCollection} method.

      *

      * <p>The returned list will be serializable if the specified list

      * is serializable.

      *

      * <p>Since {@code null} is considered to be a value of any reference

      * type, the returned list permits insertion of null elements whenever

      * the backing list does.

      *

      * @param list the list for which a dynamically typesafe view is to be

      *             returned

      * @param type the type of element that {@code list} is permitted to hold

      * @return a dynamically typesafe view of the specified list

      * @since 1.5

      */

     public static <E> List<E> checkedList(List<E> list, Class<E> type) {

         return (list instanceof RandomAccess ?

                 new CheckedRandomAccessList<>(list, type) :

                 new CheckedList<>(list, type));

     }

     /**

      * @serial include

      */

     static class CheckedList<E>

         extends CheckedCollection<E>

         implements List<E>

     {

         private static final long serialVersionUID = 65247728283967356L;

         final List<E> list;

         CheckedList(List<E> list, Class<E> type) {

             super(list, type);

             this.list = list;

         }

         public boolean equals(Object o)  { return o == this || list.equals(o); }

         public int hashCode()            { return list.hashCode(); }

         public E get(int index)          { return list.get(index); }

         public E remove(int index)       { return list.remove(index); }

         public int indexOf(Object o)     { return list.indexOf(o); }

         public int lastIndexOf(Object o) { return list.lastIndexOf(o); }

         public E set(int index, E element) {

             typeCheck(element);

             return list.set(index, element);

         }

         public void add(int index, E element) {

             typeCheck(element);

             list.add(index, element);

         }

         public boolean addAll(int index, Collection<? extends E> c) {

             return list.addAll(index, checkedCopyOf(c));

         }

         public ListIterator<E> listIterator()   { return listIterator(0); }

         public ListIterator<E> listIterator(final int index) {

             final ListIterator<E> i = list.listIterator(index);

             return new ListIterator<E>() {

                 public boolean hasNext()     { return i.hasNext(); }

                 public E next()              { return i.next(); }

                 public boolean hasPrevious() { return i.hasPrevious(); }

                 public E previous()          { return i.previous(); }

                 public int nextIndex()       { return i.nextIndex(); }

                 public int previousIndex()   { return i.previousIndex(); }

                 public void remove()         {        i.remove(); }

                 public void set(E e) {

                     typeCheck(e);

                     i.set(e);

                 }

                 public void add(E e) {

                     typeCheck(e);

                     i.add(e);

                 }

             };

         }

         public List<E> subList(int fromIndex, int toIndex) {

             return new CheckedList<>(list.subList(fromIndex, toIndex), type);

         }

     }

     /**

      * @serial include

      */

     static class CheckedRandomAccessList<E> extends CheckedList<E>

                                             implements RandomAccess

     {

         private static final long serialVersionUID = 1638200125423088369L;

         CheckedRandomAccessList(List<E> list, Class<E> type) {

             super(list, type);

         }

         public List<E> subList(int fromIndex, int toIndex) {

             return new CheckedRandomAccessList<>(

                 list.subList(fromIndex, toIndex), type);

         }

     }

     /**

      * Returns a dynamically typesafe view of the specified map.

      * Any attempt to insert a mapping whose key or value have the wrong

      * type will result in an immediate {@link ClassCastException}.

      * Similarly, any attempt to modify the value currently associated with

      * a key will result in an immediate {@link ClassCastException},

      * whether the modification is attempted directly through the map

      * itself, or through a {@link Map.Entry} instance obtained from the

      * map's {@link Map#entrySet() entry set} view.

      *

      * <p>Assuming a map contains no incorrectly typed keys or values

      * prior to the time a dynamically typesafe view is generated, and

      * that all subsequent access to the map takes place through the view

      * (or one of its collection views), it is <i>guaranteed</i> that the

      * map cannot contain an incorrectly typed key or value.

      *

      * <p>A discussion of the use of dynamically typesafe views may be

      * found in the documentation for the {@link #checkedCollection

      * checkedCollection} method.

      *

      * <p>The returned map will be serializable if the specified map is

      * serializable.

      *

      * <p>Since {@code null} is considered to be a value of any reference

      * type, the returned map permits insertion of null keys or values

      * whenever the backing map does.

      *

      * @param m the map for which a dynamically typesafe view is to be

      *          returned

      * @param keyType the type of key that {@code m} is permitted to hold

      * @param valueType the type of value that {@code m} is permitted to hold

      * @return a dynamically typesafe view of the specified map

      * @since 1.5

      */

     public static <K, V> Map<K, V> checkedMap(Map<K, V> m,

                                               Class<K> keyType,

                                               Class<V> valueType) {

         return new CheckedMap<>(m, keyType, valueType);

     }

     /**

      * @serial include

      */

     private static class CheckedMap<K,V>

         implements Map<K,V>, Serializable

     {

         private static final long serialVersionUID = 5742860141034234728L;

         private final Map<K, V> m;

         final Class<K> keyType;

         final Class<V> valueType;

         private void typeCheck(Object key, Object value) {

             if (key != null && !keyType.isInstance(key))

                 throw new ClassCastException(badKeyMsg(key));

             if (value != null && !valueType.isInstance(value))

                 throw new ClassCastException(badValueMsg(value));

         }

         private String badKeyMsg(Object key) {

             return "Attempt to insert " + key.getClass() +

                 " key into map with key type " + keyType;

         }

         private String badValueMsg(Object value) {

             return "Attempt to insert " + value.getClass() +

                 " value into map with value type " + valueType;

         }

         CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {

             if (m == null || keyType == null || valueType == null)

                 throw new NullPointerException();

             this.m = m;

             this.keyType = keyType;

             this.valueType = valueType;

         }

         public int size()                      { return m.size(); }

         public boolean isEmpty()               { return m.isEmpty(); }

         public boolean containsKey(Object key) { return m.containsKey(key); }

         public boolean containsValue(Object v) { return m.containsValue(v); }