java.util

Interface Set<E>

public interface Set<E> extends Collection<E>

A collection that contains no duplicates. In other words, for two set elements e1 and e2, e1.equals(e2) returns false. There are additional stipulations on add, equals and hashCode, as well as the requirements that constructors do not permit duplicate elements. The Set interface is incompatible with List; you cannot implement both simultaneously.

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.

Since: 1.2

UNKNOWN: updated to 1.4

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

Method Detail

add

public boolean add(E o)

Adds the specified element to the set if it is not already present (optional operation). In particular, the comparison algorithm is o == null ? e == null : o.equals(e). Sets need not permit all values, and may document what exceptions will be thrown if a value is not permitted.

Parameters: o the object to add

Returns: true if the object was not previously in the set

Throws: UnsupportedOperationException if this operation is not allowed ClassCastException if the class of o prevents it from being added IllegalArgumentException if some aspect of o prevents it from being added NullPointerException if null is not permitted in this set

addAll

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

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 union of the two. The behavior is unspecified if the set is modified while this is taking place.

Parameters: c the collection to add

Returns: true if the set changed as a result

Throws: UnsupportedOperationException if this operation is not allowed ClassCastException if the class of an element prevents it from being added IllegalArgumentException if something about an element prevents it from being added NullPointerException if null is not permitted in this set, or if the argument c is null

clear

public void clear()

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

contains

public boolean contains(Object o)

Returns true if the set contains the specified element. In other words, this looks for o == null ? e == null : o.equals(e).

Parameters: o the object to look for

Returns: true if it is found in the set

Throws: ClassCastException if the type of o is not a valid type for this set. NullPointerException if o is null and this set doesn't support null values.

containsAll

public boolean containsAll(Collection<?> c)

Returns true if this set contains all elements in the specified collection. If the argument is also a set, this is the subset relationship.

Parameters: c the collection to check membership in

Returns: true if all elements in this set are in c

Throws: NullPointerException if c is null ClassCastException if the type of any element in c is not a valid type for this set. NullPointerException if some element of c is null and this set doesn't support null values.

equals

public boolean equals(Object o)

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.

Parameters: o the object to compare to

Returns: true if it is an equal set

hashCode

public int hashCode()

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.

Returns: the sum of the hashcodes of all set elements

isEmpty

public boolean isEmpty()

Returns true if the set contains no elements.

Returns: true if the set is empty

iterator

public Iterator<E> iterator()

Returns an iterator over the set. The iterator has no specific order, unless further specified.

Returns: a set iterator

remove

public boolean remove(Object o)

Removes the specified element from this set (optional operation). If an element e exists, o == null ? e == null : o.equals(e), it is removed from the set.

Parameters: o the object to remove

Returns: true if the set changed (an object was removed)

Throws: UnsupportedOperationException if this operation is not allowed ClassCastException if the type of o is not a valid type for this set. NullPointerException if o is null and this set doesn't allow the removal of a null value.

removeAll

public boolean removeAll(Collection<?> c)

Removes from this set all elements contained in the specified collection (optional operation). If the argument is a set, this returns the asymmetric set difference of the two sets.

Parameters: c the collection to remove from this set

Returns: true if this set changed as a result

Throws: UnsupportedOperationException if this operation is not allowed NullPointerException if c is null ClassCastException if the type of any element in c is not a valid type for this set. NullPointerException if some element of c is null and this set doesn't support removing null values.

retainAll

public boolean retainAll(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 intersection of the two sets.

Parameters: c the collection to keep

Returns: true if this set was modified

size

public int size()

Returns the number of elements in the set. If there are more than Integer.MAX_VALUE mappings, return Integer.MAX_VALUE. This is the cardinality of the set.

Returns: the number of elements

toArray

public Object[] toArray()

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.

Returns: an array of this set's elements

toArray

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

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.

Parameters: a the array to determine the return type; if it is big enough it is used and returned

Returns: 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 NullPointerException if a is null