/* Set.java -- A collection that prohibits duplicates
   Copyright (C) 1998, 2001, 2004, 2005
   Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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02110-1301 USA.

Linking this library statically or dynamically with other modules is
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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package java.util;

/**
 * A collection that contains no duplicates. In other words, for two set
 * elements e1 and e2, <code>e1.equals(e2)</code> returns false. There
 * are additional stipulations on <code>add</code>, <code>equals</code>
 * and <code>hashCode</code>, as well as the requirements that constructors
 * do not permit duplicate elements. The Set interface is incompatible with
 * List; you cannot implement both simultaneously.
 * <p>
 *
 * Note: Be careful about using mutable objects in sets.  In particular,
 * if a mutable object changes to become equal to another set element, you
 * have violated the contract.  As a special case of this, a Set is not
 * allowed to be an element of itself, without risking undefined behavior.
 *
 * @author Original author unknown
 * @author Eric Blake (ebb9@email.byu.edu)
 * @see Collection
 * @see List
 * @see SortedSet
 * @see HashSet
 * @see TreeSet
 * @see LinkedHashSet
 * @see AbstractSet
 * @see Collections#singleton(Object)
 * @see Collections#EMPTY_SET
 * @since 1.2
 * @status updated to 1.4
 */
public interface Set<E> extends Collection<E>
{
  /**
   * Adds the specified element to the set if it is not already present
   * (optional operation). In particular, the comparison algorithm is
   * <code>o == null ? e == null : o.equals(e)</code>. Sets need not permit
   * all values, and may document what exceptions will be thrown if
   * a value is not permitted.
   *
   * @param o the object to add
   * @return true if the object was not previously in the set
   * @throws UnsupportedOperationException if this operation is not allowed
   * @throws ClassCastException if the class of o prevents it from being added
   * @throws IllegalArgumentException if some aspect of o prevents it from
   *         being added
   * @throws NullPointerException if null is not permitted in this set
   */
  boolean add(E o);

  /**
   * Adds all of the elements of the given collection to this set (optional
   * operation). If the argument is also a Set, this returns the mathematical
   * <i>union</i> of the two. The behavior is unspecified if the set is
   * modified while this is taking place.
   *
   * @param c the collection to add
   * @return true if the set changed as a result
   * @throws UnsupportedOperationException if this operation is not allowed
   * @throws ClassCastException if the class of an element prevents it from
   *         being added
   * @throws IllegalArgumentException if something about an element prevents
   *         it from being added
   * @throws NullPointerException if null is not permitted in this set, or
   *         if the argument c is null
   * @see #add(Object)
   */
  boolean addAll(Collection<? extends E> c);

  /**
   * Removes all elements from this set (optional operation). This set will
   * be empty afterwords, unless an exception occurs.
   *
   * @throws UnsupportedOperationException if this operation is not allowed
   */
  void clear();

  /**
   * Returns true if the set contains the specified element. In other words,
   * this looks for <code>o == null ? e == null : o.equals(e)</code>.
   *
   * @param o the object to look for
   * @return true if it is found in the set
   * @throws ClassCastException if the type of o is not a valid type
   *         for this set.
   * @throws NullPointerException if o is null and this set doesn't
   *         support null values.
   */
  boolean contains(Object o);

  /**
   * Returns true if this set contains all elements in the specified
   * collection. If the argument is also a set, this is the <i>subset</i>
   * relationship.
   *
   * @param c the collection to check membership in
   * @return true if all elements in this set are in c
   * @throws NullPointerException if c is null
   * @throws ClassCastException if the type of any element in c is not
   *         a valid type for this set.
   * @throws NullPointerException if some element of c is null and this
   *         set doesn't support null values.
   * @see #contains(Object)
   */
  boolean containsAll(Collection<?> c);

  /**
   * Compares the specified object to this for equality. For sets, the object
   * must be a set, the two must have the same size, and every element in
   * one must be in the other.
   *
   * @param o the object to compare to
   * @return true if it is an equal set
   */
  boolean equals(Object o);

  /**
   * Returns the hash code for this set. In order to satisfy the contract of
   * equals, this is the sum of the hashcode of all elements in the set.
   *
   * @return the sum of the hashcodes of all set elements
   * @see #equals(Object)
   */
  int hashCode();

  /**
   * Returns true if the set contains no elements.
   *
   * @return true if the set is empty
   */
  boolean isEmpty();

  /**
   * Returns an iterator over the set.  The iterator has no specific order,
   * unless further specified.
   *
   * @return a set iterator
   */
  Iterator<E> iterator();

  /**
   * Removes the specified element from this set (optional operation). If
   * an element e exists, <code>o == null ? e == null : o.equals(e)</code>,
   * it is removed from the set.
   *
   * @param o the object to remove
   * @return true if the set changed (an object was removed)
   * @throws UnsupportedOperationException if this operation is not allowed
   * @throws ClassCastException if the type of o is not a valid type
   *         for this set.
   * @throws NullPointerException if o is null and this set doesn't allow
   *         the removal of a null value.
   */
  boolean remove(Object o);

  /**
   * Removes from this set all elements contained in the specified collection
   * (optional operation). If the argument is a set, this returns the
   * <i>asymmetric set difference</i> of the two sets.
   *
   * @param c the collection to remove from this set
   * @return true if this set changed as a result
   * @throws UnsupportedOperationException if this operation is not allowed
   * @throws NullPointerException if c is null
   * @throws ClassCastException if the type of any element in c is not
   *         a valid type for this set.
   * @throws NullPointerException if some element of c is null and this
   *         set doesn't support removing null values.
   * @see #remove(Object)
   */
  boolean removeAll(Collection<?> c);

  /**
   * Retains only the elements in this set that are also in the specified
   * collection (optional operation). If the argument is also a set, this
   * performs the <i>intersection</i> of the two sets.
   *
   * @param c the collection to keep
   * @return true if this set was modified
   * @throws UnsupportedOperationException if this operation is not allowed
   * @throws NullPointerException if c is null
   * @throws ClassCastException if the type of any element in c is not
   *         a valid type for this set.
   * @throws NullPointerException if some element of c is null and this
   *         set doesn't support retaining null values.
   * @see #remove(Object)
   */
  boolean retainAll(Collection<?> c);

  /**
   * Returns the number of elements in the set. If there are more
   * than Integer.MAX_VALUE mappings, return Integer.MAX_VALUE. This is
   * the <i>cardinality</i> of the set.
   *
   * @return the number of elements
   */
  int size();

  /**
   * Returns an array containing the elements of this set. If the set
   * makes a guarantee about iteration order, the array has the same
   * order. The array is distinct from the set; modifying one does not
   * affect the other.
   *
   * @return an array of this set's elements
   * @see #toArray(Object[])
   */
  Object[] toArray();

  /**
   * Returns an array containing the elements of this set, of the same runtime
   * type of the argument. If the given set is large enough, it is reused,
   * and null is inserted in the first unused slot. Otherwise, reflection
   * is used to build a new array. If the set makes a guarantee about iteration
   * order, the array has the same order. The array is distinct from the set;
   * modifying one does not affect the other.
   *
   * @param a the array to determine the return type; if it is big enough
   *        it is used and returned
   * @return an array holding the elements of the set
   * @throws ArrayStoreException if the runtime type of a is not a supertype
   *         of all elements in the set
   * @throws NullPointerException if a is null
   * @see #toArray()
   */
  <T> T[] toArray(T[] a);
}