Interface: java.util.Set<E>
- public interface Set<E>
- extends Collection<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.
Methods
-
addtop
public 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 elementesuch 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 elements that they may contain.
- Specified by:
- add from Collection<E>
-
addAlltop
public 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 from Collection<E>
-
cleartop
public 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 from Collection<E>
-
containstop
public 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 elementesuch that(o==null ? e==null : o.equals(e)).- Specified by:
- contains from Collection<E>
-
containsAlltop
public 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 from Collection<E>
-
equalstop
public boolean equals(Object o)Compares the specified object with this set for equality. Returns true 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 from Collection<E>
-
hashCodetop
public 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 thats1.equals(s2)implies thats1.hashCode()==s2.hashCode()for any two setss1ands2, as required by the general contract of the Object.hashCode method.- Specified by:
- hashCode from Collection<E>
-
isEmptytop
public boolean isEmpty()Returns true if this set contains no elements.- Specified by:
- isEmpty from Collection<E>
-
iteratortop
public 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 from Collection<E>
- iterator from Iterable<E>
-
removetop
public boolean remove(Object o)Removes the specified element from this set if it is present (optional operation). More formally, removes an elementesuch 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 from Collection<E>
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removeAlltop
public 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 from Collection<E>
-
retainAlltop
public 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 from Collection<E>
-
sizetop
public int size()Returns the number of elements in this set (its cardinality). If this set contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.- Specified by:
- size from Collection<E>
-
toArraytop
public 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 from Collection<E>
-
toArraytop
public <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. Obeys the general contract of the Collection.toArray(Object[]) method.- Specified by:
- toArray from Collection<E>