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 package java.util;

 /**

  * A collection that contains no duplicate elements.  More formally, sets

  * contain no pair of elements <code>e1</code> and <code>e2</code> such that

  * <code>e1.equals(e2)</code>, and at most one null element.  As implied by

  * its name, this interface models the mathematical <i>set</i> abstraction.

  *

  * <p>The <tt>Set</tt> interface places additional stipulations, beyond those

  * inherited from the <tt>Collection</tt> interface, on the contracts of all

  * constructors and on the contracts of the <tt>add</tt>, <tt>equals</tt> and

  * <tt>hashCode</tt> methods.  Declarations for other inherited methods are

  * also included here for convenience.  (The specifications accompanying these

  * declarations have been tailored to the <tt>Set</tt> interface, but they do

  * not contain any additional stipulations.)

  *

  * <p>The additional stipulation on constructors is, not surprisingly,

  * that all constructors must create a set that contains no duplicate elements

  * (as defined above).

  *

  * <p>Note: Great care must be exercised if mutable objects are used as set

  * elements.  The behavior of a set is not specified if the value of an object

  * is changed in a manner that affects <tt>equals</tt> comparisons while the

  * object is an element in the set.  A special case of this prohibition is

  * that it is not permissible for a set to contain itself as an element.

  *

  * <p>Some set 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 set 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>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 maintained by this set

  *

  * @author  Josh Bloch

  * @author  Neal Gafter

  * @see Collection

  * @see List

  * @see SortedSet

  * @see HashSet

  * @see TreeSet

  * @see AbstractSet

  * @see Collections#singleton(java.lang.Object)

  * @see Collections#EMPTY_SET

  * @since 1.2

  */

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

     // Query Operations

     /**

      * Returns the number of elements in this set (its cardinality).  If this

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

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

      *

      * @return the number of elements in this set (its cardinality)

      */

     int size();

     /**

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

      *

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

      */

     boolean isEmpty();

     /**

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

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

      * contains an 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 set is to be tested

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

      * @throws ClassCastException if the type of the specified element

      *         is incompatible with this set

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

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

      *         set does not permit null elements

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

      */

     boolean contains(Object o);

     /**

      * Returns an iterator over the elements in this set.  The elements are

      * returned in no particular order (unless this set is an instance of some

      * class that provides a guarantee).

      *

      * @return an iterator over the elements in this set

      */

     Iterator<E> iterator();

     /**

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

      * If this set 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 set.  (In other words, this method must

      * allocate a new array even if this set 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 the elements in this set

      */

     Object[] toArray();

     /**

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

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

      * If the set 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 set.

      *

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

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

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

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

      * set <i>only</i> if the caller knows that this set does not contain

      * any null elements.)

      *

      * <p>If this set 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 set known to contain only strings.

      * The following code can be used to dump the set 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 set 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 the elements in this set

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

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

      *         set

      * @throws NullPointerException if the specified array is null

      */

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

     // Modification Operations

     /**

      * Adds the specified element to this set if it is not already present

      * (optional operation).  More formally, adds the specified element

      * <tt>e</tt> to this set if the set contains no element <tt>e2</tt>

      * such that

      * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.

      * If this set already contains the element, the call leaves the set

      * unchanged and returns <tt>false</tt>.  In combination with the

      * restriction on constructors, this ensures that sets never contain

      * duplicate elements.

      *

      * <p>The stipulation above does not imply that sets must accept all

      * elements; sets may refuse to add any particular element, including

      * <tt>null</tt>, and throw an exception, as described in the

      * specification for {@link Collection#add Collection.add}.

      * Individual set implementations should clearly document any

      * restrictions on the elements that they may contain.

      *

      * @param e element to be added to this set

      * @return <tt>true</tt> if this set did not already contain the specified

      *         element

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

      *         is not supported by this set

      * @throws ClassCastException if the class of the specified element

      *         prevents it from being added to this set

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

      *         set does not permit null elements

      * @throws IllegalArgumentException if some property of the specified element

      *         prevents it from being added to this set

      */

     boolean add(E e);

     /**

      * Removes the specified element from this set 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 set contains such an element.  Returns <tt>true</tt> if this set

      * contained the element (or equivalently, if this set changed as a

      * result of the call).  (This set will not contain the element once the

      * call returns.)

      *

      * @param o object to be removed from this set, if present

      * @return <tt>true</tt> if this set contained the specified element

      * @throws ClassCastException if the type of the specified element

      *         is incompatible with this set

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

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

      *         set does not permit null elements

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

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

      *         is not supported by this set

      */

     boolean remove(Object o);

     // Bulk Operations

     /**

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

      * specified collection.  If the specified collection is also a set, this

      * method returns <tt>true</tt> if it is a <i>subset</i> of this set.

      *

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

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

      *         specified collection

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

      *         in the specified collection are incompatible with this

      *         set

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

      * @throws NullPointerException if the specified collection contains one

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

      *         elements

      * (<a href="Collection.html#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 set if

      * they're not already present (optional operation).  If the specified

      * collection is also a set, the <tt>addAll</tt> operation effectively

      * modifies this set so that its value is the <i>union</i> of the two

      * sets.  The behavior of this operation is undefined if the specified

      * collection is modified while the operation is in progress.

      *

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

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

      *

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

      *         is not supported by this set

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

      *         specified collection prevents it from being added to this set

      * @throws NullPointerException if the specified collection contains one

      *         or more null elements and this set 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 set

      * @see #add(Object)

      */

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

     /**

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

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

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

      * specified collection.  If the specified collection is also a set, this

      * operation effectively modifies this set so that its value is the

      * <i>intersection</i> of the two sets.

      *

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

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

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

      *         is not supported by this set

      * @throws ClassCastException if the class of an element of this set

      *         is incompatible with the specified collection

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

      * @throws NullPointerException if this set contains a null element and the

      *         specified collection does not permit null elements

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

      *         or if the specified collection is null

      * @see #remove(Object)

      */

     boolean retainAll(Collection<?> c);

     /**

      * Removes from this set all of its elements that are contained in the

      * specified collection (optional operation).  If the specified

      * collection is also a set, this operation effectively modifies this

      * set so that its value is the <i>asymmetric set difference</i> of

      * the two sets.

      *

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

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

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

      *         is not supported by this set

      * @throws ClassCastException if the class of an element of this set

      *         is incompatible with the specified collection

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

      * @throws NullPointerException if this set contains a null element and the

      *         specified collection does not permit null elements

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

      *         or if the specified collection is null

      * @see #remove(Object)

      * @see #contains(Object)

      */

     boolean removeAll(Collection<?> c);

     /**

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

      * The set will be empty after this call returns.

      *

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

      *         is not supported by this set

      */

     void clear();

     // Comparison and hashing

     /**

      * Compares the specified object with this set for equality.  Returns

      * <tt>true</tt> if the specified object is also a set, the two sets

      * have the same size, and every member of the specified set is

      * contained in this set (or equivalently, every member of this set is

      * contained in the specified set).  This definition ensures that the

      * equals method works properly across different implementations of the

      * set interface.

      *

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

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

      */

     boolean equals(Object o);

     /**

      * Returns the hash code value for this set.  The hash code of a set is

      * defined to be the sum of the hash codes of the elements in the set,

      * where the hash code of a <tt>null</tt> element is defined to be zero.

      * This ensures that <tt>s1.equals(s2)</tt> implies that

      * <tt>s1.hashCode()==s2.hashCode()</tt> for any two sets <tt>s1</tt>

      * and <tt>s2</tt>, as required by the general contract of

      * {@link Object#hashCode}.

      *

      * @return the hash code value for this set

      * @see Object#equals(Object)

      * @see Set#equals(Object)

      */

     int hashCode();

 }