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  *

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 package java.util;

 import java.io.*;

 /**

  * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,

  * with predictable iteration order.  This implementation differs from

  * <tt>HashMap</tt> in that it maintains a doubly-linked list running through

  * all of its entries.  This linked list defines the iteration ordering,

  * which is normally the order in which keys were inserted into the map

  * (<i>insertion-order</i>).  Note that insertion order is not affected

  * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is

  * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when

  * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to

  * the invocation.)

  *

  * <p>This implementation spares its clients from the unspecified, generally

  * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),

  * without incurring the increased cost associated with {@link TreeMap}.  It

  * can be used to produce a copy of a map that has the same order as the

  * original, regardless of the original map's implementation:

  * <pre>

  *     void foo(Map m) {

  *         Map copy = new LinkedHashMap(m);

  *         ...

  *     }

  * </pre>

  * This technique is particularly useful if a module takes a map on input,

  * copies it, and later returns results whose order is determined by that of

  * the copy.  (Clients generally appreciate having things returned in the same

  * order they were presented.)

  *

  * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is

  * provided to create a linked hash map whose order of iteration is the order

  * in which its entries were last accessed, from least-recently accessed to

  * most-recently (<i>access-order</i>).  This kind of map is well-suited to

  * building LRU caches.  Invoking the <tt>put</tt> or <tt>get</tt> method

  * results in an access to the corresponding entry (assuming it exists after

  * the invocation completes).  The <tt>putAll</tt> method generates one entry

  * access for each mapping in the specified map, in the order that key-value

  * mappings are provided by the specified map's entry set iterator.  <i>No

  * other methods generate entry accesses.</i> In particular, operations on

  * collection-views do <i>not</i> affect the order of iteration of the backing

  * map.

  *

  * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to

  * impose a policy for removing stale mappings automatically when new mappings

  * are added to the map.

  *

  * <p>This class provides all of the optional <tt>Map</tt> operations, and

  * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time

  * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and

  * <tt>remove</tt>), assuming the hash function disperses elements

  * properly among the buckets.  Performance is likely to be just slightly

  * below that of <tt>HashMap</tt>, due to the added expense of maintaining the

  * linked list, with one exception: Iteration over the collection-views

  * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>

  * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>

  * is likely to be more expensive, requiring time proportional to its

  * <i>capacity</i>.

  *

  * <p>A linked hash map has two parameters that affect its performance:

  * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely

  * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an

  * excessively high value for initial capacity is less severe for this class

  * than for <tt>HashMap</tt>, as iteration times for this class are unaffected

  * by capacity.

  *

  * <p><strong>Note that this implementation is not synchronized.</strong>

  * If multiple threads access a linked hash map concurrently, and at least

  * one of the threads modifies the map structurally, it <em>must</em> be

  * synchronized externally.  This is typically accomplished by

  * synchronizing on some object that naturally encapsulates the map.

  *

  * If no such object exists, the map should be "wrapped" using the

  * {@link Collections#synchronizedMap Collections.synchronizedMap}

  * method.  This is best done at creation time, to prevent accidental

  * unsynchronized access to the map:<pre>

  *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>

  *

  * A structural modification is any operation that adds or deletes one or more

  * mappings or, in the case of access-ordered linked hash maps, affects

  * iteration order.  In insertion-ordered linked hash maps, merely changing

  * the value associated with a key that is already contained in the map is not

  * a structural modification.  <strong>In access-ordered linked hash maps,

  * merely querying the map with <tt>get</tt> is a structural

  * modification.</strong>)

  *

  * <p>The iterators returned by the <tt>iterator</tt> method of the collections

  * returned by all of this class's collection view methods are

  * <em>fail-fast</em>: if the map is structurally modified at any time after

  * the iterator is created, in any way except through the iterator's own

  * <tt>remove</tt> method, the iterator will throw a {@link

  * ConcurrentModificationException}.  Thus, in the face of concurrent

  * modification, the iterator fails quickly and cleanly, rather than risking

  * arbitrary, non-deterministic behavior at an undetermined time in the future.

  *

  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

  * as it is, generally speaking, impossible to make any hard guarantees in the

  * presence of unsynchronized concurrent modification.  Fail-fast iterators

  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.

  * Therefore, it would be wrong to write a program that depended on this

  * exception for its correctness:   <i>the fail-fast behavior of iterators

  * should be used only to detect bugs.</i>

  *

  * <p>This class is a member of the

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

  * Java Collections Framework</a>.

  *

  * @param <K> the type of keys maintained by this map

  * @param <V> the type of mapped values

  *

  * @author  Josh Bloch

  * @see     Object#hashCode()

  * @see     Collection

  * @see     Map

  * @see     HashMap

  * @see     TreeMap

  * @see     Hashtable

  * @since   1.4

  */

 public class LinkedHashMap<K,V>

     extends HashMap<K,V>

     implements Map<K,V>

 {

     private static final long serialVersionUID = 3801124242820219131L;

     /**

      * The head of the doubly linked list.

      */

     private transient Entry<K,V> header;

     /**

      * The iteration ordering method for this linked hash map: <tt>true</tt>

      * for access-order, <tt>false</tt> for insertion-order.

      *

      * @serial

      */

     private final boolean accessOrder;

     /**

      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

      * with the specified initial capacity and load factor.

      *

      * @param  initialCapacity the initial capacity

      * @param  loadFactor      the load factor

      * @throws IllegalArgumentException if the initial capacity is negative

      *         or the load factor is nonpositive

      */

     public LinkedHashMap(int initialCapacity, float loadFactor) {

         super(initialCapacity, loadFactor);

         accessOrder = false;

     }

     /**

      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

      * with the specified initial capacity and a default load factor (0.75).

      *

      * @param  initialCapacity the initial capacity

      * @throws IllegalArgumentException if the initial capacity is negative

      */

     public LinkedHashMap(int initialCapacity) {

         super(initialCapacity);

         accessOrder = false;

     }

     /**

      * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance

      * with the default initial capacity (16) and load factor (0.75).

      */

     public LinkedHashMap() {

         super();

         accessOrder = false;

     }

     /**

      * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with

      * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>

      * instance is created with a default load factor (0.75) and an initial

      * capacity sufficient to hold the mappings in the specified map.

      *

      * @param  m the map whose mappings are to be placed in this map

      * @throws NullPointerException if the specified map is null

      */

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

         super(m);

         accessOrder = false;

     }

     /**

      * Constructs an empty <tt>LinkedHashMap</tt> instance with the

      * specified initial capacity, load factor and ordering mode.

      *

      * @param  initialCapacity the initial capacity

      * @param  loadFactor      the load factor

      * @param  accessOrder     the ordering mode - <tt>true</tt> for

      *         access-order, <tt>false</tt> for insertion-order

      * @throws IllegalArgumentException if the initial capacity is negative

      *         or the load factor is nonpositive

      */

     public LinkedHashMap(int initialCapacity,

                          float loadFactor,

                          boolean accessOrder) {

         super(initialCapacity, loadFactor);

         this.accessOrder = accessOrder;

     }

     /**

      * Called by superclass constructors and pseudoconstructors (clone,

      * readObject) before any entries are inserted into the map.  Initializes

      * the chain.

      */

     void init() {

         header = new Entry<>(-1, null, null, null);

         header.before = header.after = header;

     }

     /**

      * Transfers all entries to new table array.  This method is called

      * by superclass resize.  It is overridden for performance, as it is

      * faster to iterate using our linked list.

      */

     void transfer(HashMap.Entry[] newTable) {

         int newCapacity = newTable.length;

         for (Entry<K,V> e = header.after; e != header; e = e.after) {

             int index = indexFor(e.hash, newCapacity);

             e.next = newTable[index];

             newTable[index] = e;

         }

     }

     /**

      * Returns <tt>true</tt> if this map maps one or more keys to the

      * specified value.

      *

      * @param value value whose presence in this map is to be tested

      * @return <tt>true</tt> if this map maps one or more keys to the

      *         specified value

      */

     public boolean containsValue(Object value) {

         // Overridden to take advantage of faster iterator

         if (value==null) {

             for (Entry e = header.after; e != header; e = e.after)

                 if (e.value==null)

                     return true;

         } else {

             for (Entry e = header.after; e != header; e = e.after)

                 if (value.equals(e.value))

                     return true;

         }

         return false;

     }

     /**

      * Returns the value to which the specified key is mapped,

      * or {@code null} if this map contains no mapping for the key.

      *

      * <p>More formally, if this map contains a mapping from a key

      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :

      * key.equals(k))}, then this method returns {@code v}; otherwise

      * it returns {@code null}.  (There can be at most one such mapping.)

      *

      * <p>A return value of {@code null} does not <i>necessarily</i>

      * indicate that the map contains no mapping for the key; it's also

      * possible that the map explicitly maps the key to {@code null}.

      * The {@link #containsKey containsKey} operation may be used to

      * distinguish these two cases.

      */

     public V get(Object key) {

         Entry<K,V> e = (Entry<K,V>)getEntry(key);

         if (e == null)

             return null;

         e.recordAccess(this);

         return e.value;

     }

     /**

      * Removes all of the mappings from this map.

      * The map will be empty after this call returns.

      */

     public void clear() {

         super.clear();

         header.before = header.after = header;

     }

     /**

      * LinkedHashMap entry.

      */

     private static class Entry<K,V> extends HashMap.Entry<K,V> {

         // These fields comprise the doubly linked list used for iteration.

         Entry<K,V> before, after;

         Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {

             super(hash, key, value, next);

         }

         /**

          * Removes this entry from the linked list.

          */

         private void remove() {

             before.after = after;

             after.before = before;

         }

         /**

          * Inserts this entry before the specified existing entry in the list.

          */

         private void addBefore(Entry<K,V> existingEntry) {

             after  = existingEntry;

             before = existingEntry.before;

             before.after = this;

             after.before = this;

         }

         /**

          * This method is invoked by the superclass whenever the value

          * of a pre-existing entry is read by Map.get or modified by Map.set.

          * If the enclosing Map is access-ordered, it moves the entry

          * to the end of the list; otherwise, it does nothing.

          */

         void recordAccess(HashMap<K,V> m) {

             LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;

             if (lm.accessOrder) {

                 lm.modCount++;

                 remove();

                 addBefore(lm.header);

             }

         }

         void recordRemoval(HashMap<K,V> m) {

             remove();

         }

     }

     private abstract class LinkedHashIterator<T> implements Iterator<T> {

         Entry<K,V> nextEntry    = header.after;

         Entry<K,V> lastReturned = null;

         /**

          * The modCount value that the iterator believes that the backing

          * List should have.  If this expectation is violated, the iterator

          * has detected concurrent modification.

          */

         int expectedModCount = modCount;

         public boolean hasNext() {

             return nextEntry != header;

         }

         public void remove() {

             if (lastReturned == null)

                 throw new IllegalStateException();

             if (modCount != expectedModCount)

                 throw new ConcurrentModificationException();

             LinkedHashMap.this.remove(lastReturned.key);

             lastReturned = null;

             expectedModCount = modCount;

         }

         Entry<K,V> nextEntry() {

             if (modCount != expectedModCount)

                 throw new ConcurrentModificationException();

             if (nextEntry == header)

                 throw new NoSuchElementException();

             Entry<K,V> e = lastReturned = nextEntry;

             nextEntry = e.after;

             return e;

         }

     }

     private class KeyIterator extends LinkedHashIterator<K> {

         public K next() { return nextEntry().getKey(); }

     }

     private class ValueIterator extends LinkedHashIterator<V> {

         public V next() { return nextEntry().value; }

     }

     private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {

         public Map.Entry<K,V> next() { return nextEntry(); }

     }

     // These Overrides alter the behavior of superclass view iterator() methods

     Iterator<K> newKeyIterator()   { return new KeyIterator();   }

     Iterator<V> newValueIterator() { return new ValueIterator(); }

     Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }

     /**

      * This override alters behavior of superclass put method. It causes newly

      * allocated entry to get inserted at the end of the linked list and

      * removes the eldest entry if appropriate.

      */

     void addEntry(int hash, K key, V value, int bucketIndex) {

         createEntry(hash, key, value, bucketIndex);

         // Remove eldest entry if instructed, else grow capacity if appropriate

         Entry<K,V> eldest = header.after;

         if (removeEldestEntry(eldest)) {

             removeEntryForKey(eldest.key);

         } else {

             if (size >= threshold)

                 resize(2 * table.length);

         }

     }

     /**

      * This override differs from addEntry in that it doesn't resize the

      * table or remove the eldest entry.

      */

     void createEntry(int hash, K key, V value, int bucketIndex) {

         HashMap.Entry<K,V> old = table[bucketIndex];

         Entry<K,V> e = new Entry<>(hash, key, value, old);

         table[bucketIndex] = e;

         e.addBefore(header);

         size++;

     }

     /**

      * Returns <tt>true</tt> if this map should remove its eldest entry.

      * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after

      * inserting a new entry into the map.  It provides the implementor

      * with the opportunity to remove the eldest entry each time a new one

      * is added.  This is useful if the map represents a cache: it allows

      * the map to reduce memory consumption by deleting stale entries.

      *

      * <p>Sample use: this override will allow the map to grow up to 100

      * entries and then delete the eldest entry each time a new entry is

      * added, maintaining a steady state of 100 entries.

      * <pre>

      *     private static final int MAX_ENTRIES = 100;

      *

      *     protected boolean removeEldestEntry(Map.Entry eldest) {

      *        return size() > MAX_ENTRIES;

      *     }

      * </pre>

      *

      * <p>This method typically does not modify the map in any way,

      * instead allowing the map to modify itself as directed by its

      * return value.  It <i>is</i> permitted for this method to modify

      * the map directly, but if it does so, it <i>must</i> return

      * <tt>false</tt> (indicating that the map should not attempt any

      * further modification).  The effects of returning <tt>true</tt>

      * after modifying the map from within this method are unspecified.

      *

      * <p>This implementation merely returns <tt>false</tt> (so that this

      * map acts like a normal map - the eldest element is never removed).

      *

      * @param    eldest The least recently inserted entry in the map, or if

      *           this is an access-ordered map, the least recently accessed

      *           entry.  This is the entry that will be removed it this

      *           method returns <tt>true</tt>.  If the map was empty prior

      *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting

      *           in this invocation, this will be the entry that was just

      *           inserted; in other words, if the map contains a single

      *           entry, the eldest entry is also the newest.

      * @return   <tt>true</tt> if the eldest entry should be removed

      *           from the map; <tt>false</tt> if it should be retained.

      */

     protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {

         return false;

     }

 }