E
- the type of elements maintained by this setpublic interface Set<E> extends Collection<E>
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.
Collection
,
List
,
AbstractSet
Modifier and Type | Method and Description |
---|---|
boolean |
add(E e)
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.
|
boolean add(E e)
The stipulation above does not imply that sets must accept all
elements; sets may refuse to add any particular element, including
null, and throw 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.
add
in interface Collection<E>
e
- element to be added to this setUnsupportedOperationException
- if the add operation
is not supported by this setClassCastException
- if the class of the specified element
prevents it from being added to this setNullPointerException
- if the specified element is null and this
set does not permit null elementsIllegalArgumentException
- if some property of the specified element
prevents it from being added to this setboolean addAll(Collection<? extends E> c)
addAll
in interface Collection<E>
c
- collection containing elements to be added to this setUnsupportedOperationException
- if the addAll operation
is not supported by this setClassCastException
- if the class of an element of the
specified collection prevents it from being added to this setNullPointerException
- if the specified collection contains one
or more null elements and this set does not permit null
elements, or if the specified collection is nullIllegalArgumentException
- if some property of an element of the
specified collection prevents it from being added to this setadd(Object)
void clear()
clear
in interface Collection<E>
UnsupportedOperationException
- if the clear method
is not supported by this setboolean contains(Object o)
contains
in interface Collection<E>
o
- element whose presence in this set is to be testedClassCastException
- if the type of the specified element
is incompatible with this set
(optional)NullPointerException
- if the specified element is null and this
set does not permit null elements
(optional)boolean containsAll(Collection<?> c)
containsAll
in interface Collection<E>
c
- collection to be checked for containment in this setClassCastException
- if the types of one or more elements
in the specified collection are incompatible with this
set
(optional)NullPointerException
- if the specified collection contains one
or more null elements and this set does not permit null
elements
(optional),
or if the specified collection is nullcontains(Object)
boolean equals(Object o)
equals
in interface Collection<E>
equals
in class Object
o
- object to be compared for equality with this setObject.hashCode()
,
HashMap
int hashCode()
Object.hashCode()
.hashCode
in interface Collection<E>
hashCode
in class Object
Object.equals(Object)
,
equals(Object)
boolean isEmpty()
isEmpty
in interface Collection<E>
Iterator<E> iterator()
boolean remove(Object o)
remove
in interface Collection<E>
o
- object to be removed from this set, if presentClassCastException
- if the type of the specified element
is incompatible with this set
(optional)NullPointerException
- if the specified element is null and this
set does not permit null elements
(optional)UnsupportedOperationException
- if the remove operation
is not supported by this setboolean removeAll(Collection<?> c)
removeAll
in interface Collection<E>
c
- collection containing elements to be removed from this setUnsupportedOperationException
- if the removeAll operation
is not supported by this setClassCastException
- if the class of an element of this set
is incompatible with the specified collection
(optional)NullPointerException
- if this set contains a null element and the
specified collection does not permit null elements
(optional),
or if the specified collection is nullremove(Object)
,
contains(Object)
boolean retainAll(Collection<?> c)
retainAll
in interface Collection<E>
c
- collection containing elements to be retained in this setUnsupportedOperationException
- if the retainAll operation
is not supported by this setClassCastException
- if the class of an element of this set
is incompatible with the specified collection
(optional)NullPointerException
- if this set contains a null element and the
specified collection does not permit null elements
(optional),
or if the specified collection is nullremove(Object)
int size()
size
in interface Collection<E>
Object[] toArray()
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.
This method acts as bridge between array-based and collection-based APIs.
toArray
in interface Collection<E>
<T> T[] toArray(T[] a)
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 null. (This is useful in determining the length of this set only if the caller knows that this set does not contain any null elements.)
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.
Like the 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.
Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String:
String[] y = x.toArray(new String[0]);Note that toArray(new Object[0]) is identical in function to toArray().
toArray
in interface Collection<E>
T
- the runtime type of the array to contain the collectiona
- 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.ArrayStoreException
- if the runtime type of the specified array
is not a supertype of the runtime type of every element in this
setNullPointerException
- if the specified array is nullCopyright (c) 2014, Oracle and/or its affiliates. All rights reserved. Use of this specification is subject to license terms.