Set<E>

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

The Set interface places additional stipulations, beyond those inherited from the Collection interface, on the contracts of all constructors and on the contracts of the add, equals and hashCode methods. Declarations for other inherited methods are also included here for convenience. (The specifications accompanying these declarations have been tailored to the Set interface, but they do not contain any additional stipulations.)

The additional stipulation on constructors is, not surprisingly, that all constructors must create a set that contains no duplicate elements (as defined above).

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 equals 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.

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 NullPointerException or ClassCastException. 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.

This interface is a member of the Java Collections Framework.

Since: 1.2

Version: 1.33, 06/22/03

Author: Josh Bloch Neal Gafter

Method Summary
boolean	add(E o) Adds the specified element to this set if it is not already present (optional operation).
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
void	clear() Removes all of the elements from this set (optional operation).
boolean	contains(Object o) Returns `true` if this set contains the specified element.
boolean	containsAll(Collection<?> c) Returns `true` if this set contains all of the elements of the specified collection.
boolean	equals(Object o) Compares the specified object with this set for equality.
int	hashCode() Returns the hash code value for this set.
boolean	isEmpty() Returns `true` if this set contains no elements.
Iterator<E>	iterator() Returns an iterator over the elements in this set.
boolean	remove(Object o) Removes the specified element from this set if it is present (optional operation).
boolean	removeAll(Collection<?> c) Removes from this set all of its elements that are contained in the specified collection (optional operation).
boolean	retainAll(Collection<?> c) Retains only the elements in this set that are contained in the specified collection (optional operation).
int	size() Returns the number of elements in this set (its cardinality).
Object[]	toArray() Returns an array containing all of the elements in this set.
<T> T[]	toArray(T[] a) Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

Method Detail

add

Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element, o, to this set if this set contains no element e such that

(o==null ? e==null :
 o.equals(e))

. If this set already contains the specified element, the call leaves this set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.

The stipulation above does not imply that sets must accept all elements; sets may refuse to add any particular element, including null, and throwing an exception, as described in the specification for Collection.add. Individual set implementations should clearly document any restrictions on the the elements that they may contain.

Parameters: o element to be added to this set.

Returns: true if this set did not already contain the specified element.

Throws: UnsupportedOperationException if the add method is not supported by this set. ClassCastException if the class of the specified element prevents it from being added to this set. NullPointerException if the specified element is null and this set does not support null elements. IllegalArgumentException if some aspect of the specified element prevents it from being added to this set.

addAll

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 addAll operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is unspecified if the specified collection is modified while the operation is in progress.

Parameters: c collection whose elements are to be added to this set.

Returns: true if this set changed as a result of the call.

Throws: UnsupportedOperationException if the addAll method is not supported by this set. ClassCastException if the class of some element of the specified collection prevents it from being added to this set. NullPointerException if the specified collection contains one or more null elements and this set does not support null elements, or if the specified collection is null. IllegalArgumentException if some aspect of some element of the specified collection prevents it from being added to this set.

Interface Set<E>