Set (Java 2 Platform SE 5.0) (original) (raw)

java.util

Interface Set

All Superinterfaces:

Collection, Iterable

All Known Subinterfaces:

SortedSet

All Known Implementing Classes:

AbstractSet, CopyOnWriteArraySet, EnumSet, HashSet, JobStateReasons, LinkedHashSet, TreeSet

public interface Set

extends Collection

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such thate1.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 andhashCode 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, typicallyNullPointerException 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

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[]	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

size

int size()

Returns the number of elements in this set (its cardinality). If this set contains more than Integer.MAX_VALUE elements, returnsInteger.MAX_VALUE.

Specified by:

[size](../../java/util/Collection.html#size%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Returns:

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

isEmpty

boolean isEmpty()

Returns true if this set contains no elements.

Specified by:

[isEmpty](../../java/util/Collection.html#isEmpty%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Returns:

true if this set contains no elements.

contains

boolean contains(Object o)

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

Specified by:

[contains](../../java/util/Collection.html#contains%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

o - element whose presence in this set is to be tested.

Returns:

true if this set contains the specified element.

Throws:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the type of the specified element is incompatible with this set (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null and this set does not support null elements (optional).

iterator

Iterator<E> iterator()

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).

Specified by:

[iterator](../../java/util/Collection.html#iterator%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Specified by:

[iterator](../../java/lang/Iterable.html#iterator%28%29) in interface [Iterable](../../java/lang/Iterable.html "interface in java.lang")<[E](../../java/util/Set.html "type parameter in Set")>

Returns:

an iterator over the elements in this set.

toArray

Object[] toArray()

Returns an array containing all of the elements in this set. Obeys the general contract of the Collection.toArray method.

Specified by:

[toArray](../../java/util/Collection.html#toArray%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Returns:

an array containing all of the elements in this set.

toArray

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. Obeys the general contract of the Collection.toArray(Object[]) method.

Specified by:

[toArray](../../java/util/Collection.html#toArray%28T[]%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

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.

Returns:

an array containing the elements of this set.

Throws:

[ArrayStoreException](../../java/lang/ArrayStoreException.html "class in java.lang") - the runtime type of a is not a supertype of the runtime type of every element in this set.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified array is null.

add

boolean add(E o)

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 elemente 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, includingnull, and throwing an exception, as described in the specification for Collection.add. Individual set implementations should clearly document any restrictions on the elements that they may contain.

Specified by:

[add](../../java/util/Collection.html#add%28E%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

o - element to be added to this set.

Returns:

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

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the add method is not supported by this set.

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the class of the specified element prevents it from being added to this set.

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null and this set does not support null elements.

[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - if some aspect of the specified element prevents it from being added to this set.

remove

boolean remove(Object o)

Removes the specified element from this set if it is present (optional operation). More formally, removes an element e such that(o==null ? e==null : o.equals(e)), if the set contains such an element. Returns true if the set contained the specified element (or equivalently, if the set changed as a result of the call). (The set will not contain the specified element once the call returns.)

Specified by:

[remove](../../java/util/Collection.html#remove%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

o - object to be removed from this set, if present.

Returns:

true if the set contained the specified element.

Throws:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the type of the specified element is incompatible with this set (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified element is null and this set does not support null elements (optional).

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the remove method is not supported by this set.

containsAll

boolean containsAll(Collection<?> c)

Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.

Specified by:

[containsAll](../../java/util/Collection.html#containsAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

c - collection to be checked for containment in this set.

Returns:

true if this set contains all of the elements of the specified collection.

Throws:

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the types of one or more elements in the specified collection are incompatible with this set (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection contains one or more null elements and this set does not support null elements (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection isnull.

See Also:

contains(Object)

addAll

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). 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.

Specified by:

[addAll](../../java/util/Collection.html#addAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

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](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the addAll method is not supported by this set.

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the class of some element of the specified collection prevents it from being added to this set.

[IllegalArgumentException](../../java/lang/IllegalArgumentException.html "class in java.lang") - if some aspect of some element of the specified collection prevents it from being added to this set.

See Also:

add(Object)

retainAll

boolean retainAll(Collection<?> 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_intersection_ of the two sets.

Specified by:

[retainAll](../../java/util/Collection.html#retainAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

c - collection that defines which elements this set will retain.

Returns:

true if this collection changed as a result of the call.

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the retainAll method is not supported by this Collection.

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the types of one or more elements in this set are incompatible with the specified collection (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if this set contains a null element and the specified collection does not support null elements (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection isnull.

See Also:

remove(Object)

removeAll

boolean removeAll(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 asymmetric set difference of the two sets.

Specified by:

[removeAll](../../java/util/Collection.html#removeAll%28java.util.Collection%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Parameters:

c - collection that defines which elements will be removed from this set.

Returns:

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

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the removeAll method is not supported by this Collection.

[ClassCastException](../../java/lang/ClassCastException.html "class in java.lang") - if the types of one or more elements in this set are incompatible with the specified collection (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if this set contains a null element and the specified collection does not support null elements (optional).

[NullPointerException](../../java/lang/NullPointerException.html "class in java.lang") - if the specified collection isnull.

See Also:

remove(Object)

clear

void clear()

Removes all of the elements from this set (optional operation). This set will be empty after this call returns (unless it throws an exception).

Specified by:

[clear](../../java/util/Collection.html#clear%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Throws:

[UnsupportedOperationException](../../java/lang/UnsupportedOperationException.html "class in java.lang") - if the clear method is not supported by this set.

equals

boolean equals(Object o)

Compares the specified object with this set for equality. Returnstrue 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.

Specified by:

[equals](../../java/util/Collection.html#equals%28java.lang.Object%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Overrides:

[equals](../../java/lang/Object.html#equals%28java.lang.Object%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Parameters:

o - Object to be compared for equality with this set.

Returns:

true if the specified Object is equal to this set.

See Also:

Object.hashCode(), Hashtable

hashCode

int hashCode()

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 hashcode of a null element is defined to be zero. This ensures that s1.equals(s2) implies thats1.hashCode()==s2.hashCode() for any two setss1 and s2, as required by the general contract of the Object.hashCode method.

Specified by:

[hashCode](../../java/util/Collection.html#hashCode%28%29) in interface [Collection](../../java/util/Collection.html "interface in java.util")<[E](../../java/util/Set.html "type parameter in Set")>

Overrides:

[hashCode](../../java/lang/Object.html#hashCode%28%29) in class [Object](../../java/lang/Object.html "class in java.lang")

Returns:

the hash code value for this set.

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.