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.

@author
Josh Bloch
@author
Neal Gafter
@version
1.35, 02/19/04
@since
1.2
Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.)

Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.

If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns.

Parameters
oelement whose presence in this collection is to be ensured.
Return
true if this collection changed as a result of the call
Throws
UnsupportedOperationExceptionadd is not supported by this collection.
ClassCastExceptionclass of the specified element prevents it from being added to this collection.
NullPointerExceptionif the specified element is null and this collection does not support null elements.
IllegalArgumentExceptionsome aspect of this element prevents it from being added to this collection.
Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.)
Parameters
celements to be inserted into this collection.
Return
true if this collection changed as a result of the call
Throws
UnsupportedOperationExceptionif this collection does not support the addAll method.
ClassCastExceptionif the class of an element of the specified collection prevents it from being added to this collection.
NullPointerExceptionif the specified collection contains one or more null elements and this collection does not support null elements, or if the specified collection is null.
IllegalArgumentExceptionsome aspect of an element of the specified collection prevents it from being added to this collection.
See Also
Removes all of the elements from this collection (optional operation). This collection will be empty after this method returns unless it throws an exception.
Throws
UnsupportedOperationExceptionif the clear method is not supported by this collection.
Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).
Parameters
oelement whose presence in this collection is to be tested.
Return
true if this collection contains the specified element
Throws
ClassCastExceptionif the type of the specified element is incompatible with this collection (optional).
NullPointerExceptionif the specified element is null and this collection does not support null elements (optional).
Returns true if this collection contains all of the elements in the specified collection.
Parameters
ccollection to be checked for containment in this collection.
Return
true if this collection contains all of the elements in the specified collection
Throws
ClassCastExceptionif the types of one or more elements in the specified collection are incompatible with this collection (optional).
NullPointerExceptionif the specified collection contains one or more null elements and this collection does not support null elements (optional).
NullPointerExceptionif the specified collection is null.
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.
Parameters
oObject to be compared for equality with this set.
Return
true if the specified Object is equal to this set.
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 that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of the Object.hashCode method.
Return
the hash code value for this set.
Returns true if this collection contains no elements.
Return
true if this collection contains no elements
Returns an iterator over the elements in this collection. There are no guarantees concerning the order in which the elements are returned (unless this collection is an instance of some class that provides a guarantee).
Return
an Iterator over the elements in this collection
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call).
Parameters
oelement to be removed from this collection, if present.
Return
true if this collection changed as a result of the call
Throws
ClassCastExceptionif the type of the specified element is incompatible with this collection (optional).
NullPointerExceptionif the specified element is null and this collection does not support null elements (optional).
UnsupportedOperationExceptionremove is not supported by this collection.
Removes all this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
Parameters
celements to be removed from this collection.
Return
true if this collection changed as a result of the call
Throws
UnsupportedOperationExceptionif the removeAll method is not supported by this collection.
ClassCastExceptionif the types of one or more elements in this collection are incompatible with the specified collection (optional).
NullPointerExceptionif this collection contains one or more null elements and the specified collection does not support null elements (optional).
NullPointerExceptionif the specified collection is null.
Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
Parameters
celements to be retained in this collection.
Return
true if this collection changed as a result of the call
Throws
UnsupportedOperationExceptionif the retainAll method is not supported by this Collection.
ClassCastExceptionif the types of one or more elements in this collection are incompatible with the specified collection (optional).
NullPointerExceptionif this collection contains one or more null elements and the specified collection does not support null elements (optional).
NullPointerExceptionif the specified collection is null.
Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
Return
the number of elements in this collection
Returns an array containing all of the elements in this collection. If the collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection 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.

Return
an array containing all of the elements in this collection
Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.)

If this collection 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 l is a List known to contain only strings. The following code can be used to dump the list into a newly allocated array of String:

     String[] x = (String[]) v.toArray(new String[0]);
 

Note that toArray(new Object[0]) is identical in function to toArray().

Parameters
athe array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
Return
an array containing the elements of this collection
Throws
ArrayStoreExceptionthe runtime type of the specified array is not a supertype of the runtime type of every element in this collection.
NullPointerExceptionif the specified array is null.