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

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 * accompanied this code).

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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();

}