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  *

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

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 package java.util;

 /**

  * The root interface in the <i>collection hierarchy</i>.  A collection

  * represents a group of objects, known as its <i>elements</i>.  Some

  * collections allow duplicate elements and others do not.  Some are ordered

  * and others unordered.  The JDK does not provide any <i>direct</i>

  * implementations of this interface: it provides implementations of more

  * specific subinterfaces like <tt>Set</tt> and <tt>List</tt>.  This interface

  * is typically used to pass collections around and manipulate them where

  * maximum generality is desired.

  *

  * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain

  * duplicate elements) should implement this interface directly.

  *

  * <p>All general-purpose <tt>Collection</tt> implementation classes (which

  * typically implement <tt>Collection</tt> indirectly through one of its

  * subinterfaces) should provide two "standard" constructors: a void (no

  * arguments) constructor, which creates an empty collection, and a

  * constructor with a single argument of type <tt>Collection</tt>, which

  * creates a new collection with the same elements as its argument.  In

  * effect, the latter constructor allows the user to copy any collection,

  * producing an equivalent collection of the desired implementation type.

  * There is no way to enforce this convention (as interfaces cannot contain

  * constructors) but all of the general-purpose <tt>Collection</tt>

  * implementations in the Java platform libraries comply.

  *

  * <p>The "destructive" methods contained in this interface, that is, the

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

  * throw <tt>UnsupportedOperationException</tt> if this collection does not

  * support the operation.  If this is the case, these methods may, but are not

  * required to, throw an <tt>UnsupportedOperationException</tt> if the

  * invocation would have no effect on the collection.  For example, invoking

  * the {@link #addAll(Collection)} method on an unmodifiable collection may,

  * but is not required to, throw the exception if the collection to be added

  * is empty.

  *

  * <p><a name="optional-restrictions"/>

  * Some collection implementations have restrictions on the elements that

  * they may contain.  For example, some implementations prohibit null elements,

  * and some have restrictions on the types of their elements.  Attempting to

  * add an ineligible element throws an unchecked exception, typically

  * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting

  * to query the presence of an ineligible element may throw an exception,

  * or it may simply return false; some implementations will exhibit the former

  * behavior and some will exhibit the latter.  More generally, attempting an

  * operation on an ineligible element whose completion would not result in

  * the insertion of an ineligible element into the collection may throw an

  * exception or it may succeed, at the option of the implementation.

  * Such exceptions are marked as "optional" in the specification for this

  * interface.

  *

  * <p>It is up to each collection to determine its own synchronization

  * policy.  In the absence of a stronger guarantee by the

  * implementation, undefined behavior may result from the invocation

  * of any method on a collection that is being mutated by another

  * thread; this includes direct invocations, passing the collection to

  * a method that might perform invocations, and using an existing

  * iterator to examine the collection.

  *

  * <p>Many methods in Collections Framework interfaces are defined in

  * terms of the {@link Object#equals(Object) equals} method.  For example,

  * the specification for the {@link #contains(Object) contains(Object o)}

  * method says: "returns <tt>true</tt> if and only if this collection

  * contains at least one element <tt>e</tt> such that

  * <tt>(o==null ? e==null : o.equals(e))</tt>."  This specification should

  * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt>

  * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be

  * invoked for any element <tt>e</tt>.  Implementations are free to implement

  * optimizations whereby the <tt>equals</tt> invocation is avoided, for

  * example, by first comparing the hash codes of the two elements.  (The

  * {@link Object#hashCode()} specification guarantees that two objects with

  * unequal hash codes cannot be equal.)  More generally, implementations of

  * the various Collections Framework interfaces are free to take advantage of

  * the specified behavior of underlying {@link Object} methods wherever the

  * implementor deems it appropriate.

  *

  * <p>This interface is a member of the

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

  * Java Collections Framework</a>.

  *

  * @param <E> the type of elements in this collection

  *

  * @author  Josh Bloch

  * @author  Neal Gafter

  * @see     Set

  * @see     List

  * @see     Map

  * @see     SortedSet

  * @see     SortedMap

  * @see     HashSet

  * @see     TreeSet

  * @see     ArrayList

  * @see     LinkedList

  * @see     Vector

  * @see     Collections

  * @see     Arrays

  * @see     AbstractCollection

  * @since 1.2

  */

 public interface Collection<E> extends Iterable<E> {

     // Query Operations

     /**

      * Returns the number of elements in this collection.  If this collection

      * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns

      * <tt>Integer.MAX_VALUE</tt>.

      *

      * @return the number of elements in this collection

      */

     int size();

     /**

      * Returns <tt>true</tt> if this collection contains no elements.

      *

      * @return <tt>true</tt> if this collection contains no elements

      */

     boolean isEmpty();

     /**

      * Returns <tt>true</tt> if this collection contains the specified element.

      * More formally, returns <tt>true</tt> if and only if this collection

      * contains at least one element <tt>e</tt> such that

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.

      *

      * @param o element whose presence in this collection is to be tested

      * @return <tt>true</tt> if this collection contains the specified

      *         element

      * @throws ClassCastException if the type of the specified element

      *         is incompatible with this collection

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws NullPointerException if the specified element is null and this

      *         collection does not permit null elements

      *         (<a href="#optional-restrictions">optional</a>)

      */

     boolean contains(Object o);

     /**

      * Returns an iterator over the elements in this collection.  There are no

      * guarantees concerning the order in which the elements are returned

      * (unless this collection is an instance of some class that provides a

      * guarantee).

      *

      * @return an <tt>Iterator</tt> over the elements in this collection

      */

     Iterator<E> iterator();

     /**

      * Returns an array containing all of the elements in this collection.

      * If this collection makes any guarantees as to what order its elements

      * are returned by its iterator, this method must return the elements in

      * the same order.

      *

      * <p>The returned array will be "safe" in that no references to it are

      * maintained by this collection.  (In other words, this method must

      * allocate a new array even if this collection is backed by an array).

      * The caller is thus free to modify the returned array.

      *

      * <p>This method acts as bridge between array-based and collection-based

      * APIs.

      *

      * @return an array containing all of the elements in this collection

      */

     Object[] toArray();

     /**

      * Returns an array containing all of the elements in this collection;

      * the runtime type of the returned array is that of the specified array.

      * If the collection fits in the specified array, it is returned therein.

      * Otherwise, a new array is allocated with the runtime type of the

      * specified array and the size of this collection.

      *

      * <p>If this collection fits in the specified array with room to spare

      * (i.e., the array has more elements than this collection), the element

      * in the array immediately following the end of the collection is set to

      * <tt>null</tt>.  (This is useful in determining the length of this

      * collection <i>only</i> if the caller knows that this collection does

      * not contain any <tt>null</tt> elements.)

      *

      * <p>If this collection makes any guarantees as to what order its elements

      * are returned by its iterator, this method must return the elements in

      * the same order.

      *

      * <p>Like the {@link #toArray()} method, this method acts as bridge between

      * array-based and collection-based APIs.  Further, this method allows

      * precise control over the runtime type of the output array, and may,

      * under certain circumstances, be used to save allocation costs.

      *

      * <p>Suppose <tt>x</tt> is a collection known to contain only strings.

      * The following code can be used to dump the collection into a newly

      * allocated array of <tt>String</tt>:

      *

      * <pre>

      *     String[] y = x.toArray(new String[0]);</pre>

      *

      * Note that <tt>toArray(new Object[0])</tt> is identical in function to

      * <tt>toArray()</tt>.

      *

      * @param a the array into which the elements of this collection are to be

      *        stored, if it is big enough; otherwise, a new array of the same

      *        runtime type is allocated for this purpose.

      * @return an array containing all of the elements in this collection

      * @throws ArrayStoreException if the runtime type of the specified array

      *         is not a supertype of the runtime type of every element in

      *         this collection

      * @throws NullPointerException if the specified array is null

      */

     <T> T[] toArray(T[] a);

     // Modification Operations

     /**

      * Ensures that this collection contains the specified element (optional

      * operation).  Returns <tt>true</tt> if this collection changed as a

      * result of the call.  (Returns <tt>false</tt> if this collection does

      * not permit duplicates and already contains the specified element.)<p>

      *

      * Collections that support this operation may place limitations on what

      * elements may be added to this collection.  In particular, some

      * collections will refuse to add <tt>null</tt> elements, and others will

      * impose restrictions on the type of elements that may be added.

      * Collection classes should clearly specify in their documentation any

      * restrictions on what elements may be added.<p>

      *

      * If a collection refuses to add a particular element for any reason

      * other than that it already contains the element, it <i>must</i> throw

      * an exception (rather than returning <tt>false</tt>).  This preserves

      * the invariant that a collection always contains the specified element

      * after this call returns.

      *

      * @param e element whose presence in this collection is to be ensured

      * @return <tt>true</tt> if this collection changed as a result of the

      *         call

      * @throws UnsupportedOperationException if the <tt>add</tt> operation

      *         is not supported by this collection

      * @throws ClassCastException if the class of the specified element

      *         prevents it from being added to this collection

      * @throws NullPointerException if the specified element is null and this

      *         collection does not permit null elements

      * @throws IllegalArgumentException if some property of the element

      *         prevents it from being added to this collection

      * @throws IllegalStateException if the element cannot be added at this

      *         time due to insertion restrictions

      */

     boolean add(E e);

     /**

      * Removes a single instance of the specified element from this

      * collection, if it is present (optional operation).  More formally,

      * removes an element <tt>e</tt> such that

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>, if

      * this collection contains one or more such elements.  Returns

      * <tt>true</tt> if this collection contained the specified element (or

      * equivalently, if this collection changed as a result of the call).

      *

      * @param o element to be removed from this collection, if present

      * @return <tt>true</tt> if an element was removed as a result of this call

      * @throws ClassCastException if the type of the specified element

      *         is incompatible with this collection

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws NullPointerException if the specified element is null and this

      *         collection does not permit null elements

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws UnsupportedOperationException if the <tt>remove</tt> operation

      *         is not supported by this collection

      */

     boolean remove(Object o);

     // Bulk Operations

     /**

      * Returns <tt>true</tt> if this collection contains all of the elements

      * in the specified collection.

      *

      * @param  c collection to be checked for containment in this collection

      * @return <tt>true</tt> if this collection contains all of the elements

      *         in the specified collection

      * @throws ClassCastException if the types of one or more elements

      *         in the specified collection are incompatible with this

      *         collection

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws NullPointerException if the specified collection contains one

      *         or more null elements and this collection does not permit null

      *         elements

      *         (<a href="#optional-restrictions">optional</a>),

      *         or if the specified collection is null.

      * @see    #contains(Object)

      */

     boolean containsAll(Collection<?> c);

     /**

      * Adds all of the elements in the specified collection to this collection

      * (optional operation).  The behavior of this operation is undefined if

      * the specified collection is modified while the operation is in progress.

      * (This implies that the behavior of this call is undefined if the

      * specified collection is this collection, and this collection is

      * nonempty.)

      *

      * @param c collection containing elements to be added to this collection

      * @return <tt>true</tt> if this collection changed as a result of the call

      * @throws UnsupportedOperationException if the <tt>addAll</tt> operation

      *         is not supported by this collection

      * @throws ClassCastException if the class of an element of the specified

      *         collection prevents it from being added to this collection

      * @throws NullPointerException if the specified collection contains a

      *         null element and this collection does not permit null elements,

      *         or if the specified collection is null

      * @throws IllegalArgumentException if some property of an element of the

      *         specified collection prevents it from being added to this

      *         collection

      * @throws IllegalStateException if not all the elements can be added at

      *         this time due to insertion restrictions

      * @see #add(Object)

      */

     boolean addAll(Collection<? extends E> c);

     /**

      * Removes all of this collection's elements that are also contained in the

      * specified collection (optional operation).  After this call returns,

      * this collection will contain no elements in common with the specified

      * collection.

      *

      * @param c collection containing elements to be removed from this collection

      * @return <tt>true</tt> if this collection changed as a result of the

      *         call

      * @throws UnsupportedOperationException if the <tt>removeAll</tt> method

      *         is not supported by this collection

      * @throws ClassCastException if the types of one or more elements

      *         in this collection are incompatible with the specified

      *         collection

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws NullPointerException if this collection contains one or more

      *         null elements and the specified collection does not support

      *         null elements

      *         (<a href="#optional-restrictions">optional</a>),

      *         or if the specified collection is null

      * @see #remove(Object)

      * @see #contains(Object)

      */

     boolean removeAll(Collection<?> c);

     /**

      * Retains only the elements in this collection that are contained in the

      * specified collection (optional operation).  In other words, removes from

      * this collection all of its elements that are not contained in the

      * specified collection.

      *

      * @param c collection containing elements to be retained in this collection

      * @return <tt>true</tt> if this collection changed as a result of the call

      * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation

      *         is not supported by this collection

      * @throws ClassCastException if the types of one or more elements

      *         in this collection are incompatible with the specified

      *         collection

      *         (<a href="#optional-restrictions">optional</a>)

      * @throws NullPointerException if this collection contains one or more

      *         null elements and the specified collection does not permit null

      *         elements

      *         (<a href="#optional-restrictions">optional</a>),

      *         or if the specified collection is null

      * @see #remove(Object)

      * @see #contains(Object)

      */

     boolean retainAll(Collection<?> c);

     /**

      * Removes all of the elements from this collection (optional operation).

      * The collection will be empty after this method returns.

      *

      * @throws UnsupportedOperationException if the <tt>clear</tt> operation

      *         is not supported by this collection

      */

     void clear();

     // Comparison and hashing

     /**

      * Compares the specified object with this collection for equality. <p>

      *

      * While the <tt>Collection</tt> interface adds no stipulations to the

      * general contract for the <tt>Object.equals</tt>, programmers who

      * implement the <tt>Collection</tt> interface "directly" (in other words,

      * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>

      * or a <tt>List</tt>) must exercise care if they choose to override the

      * <tt>Object.equals</tt>.  It is not necessary to do so, and the simplest

      * course of action is to rely on <tt>Object</tt>'s implementation, but

      * the implementor may wish to implement a "value comparison" in place of

      * the default "reference comparison."  (The <tt>List</tt> and

      * <tt>Set</tt> interfaces mandate such value comparisons.)<p>

      *

      * The general contract for the <tt>Object.equals</tt> method states that

      * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and

      * only if <tt>b.equals(a)</tt>).  The contracts for <tt>List.equals</tt>

      * and <tt>Set.equals</tt> state that lists are only equal to other lists,

      * and sets to other sets.  Thus, a custom <tt>equals</tt> method for a

      * collection class that implements neither the <tt>List</tt> nor

      * <tt>Set</tt> interface must return <tt>false</tt> when this collection

      * is compared to any list or set.  (By the same logic, it is not possible

      * to write a class that correctly implements both the <tt>Set</tt> and

      * <tt>List</tt> interfaces.)

      *

      * @param o object to be compared for equality with this collection

      * @return <tt>true</tt> if the specified object is equal to this

      * collection

      *

      * @see Object#equals(Object)

      * @see Set#equals(Object)

      * @see List#equals(Object)

      */

     boolean equals(Object o);

     /**

      * Returns the hash code value for this collection.  While the

      * <tt>Collection</tt> interface adds no stipulations to the general

      * contract for the <tt>Object.hashCode</tt> method, programmers should

      * take note that any class that overrides the <tt>Object.equals</tt>

      * method must also override the <tt>Object.hashCode</tt> method in order

      * to satisfy the general contract for the <tt>Object.hashCode</tt> method.

      * In particular, <tt>c1.equals(c2)</tt> implies that

      * <tt>c1.hashCode()==c2.hashCode()</tt>.

      *

      * @return the hash code value for this collection

      *

      * @see Object#hashCode()

      * @see Object#equals(Object)

      */

     int hashCode();

 }