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public interface Set
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
This interface is a member of the
Java Collections Framework.
addAll(Collection extends E> c)
boolean contains(Object o)
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
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);
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.
boolean remove(Object o)
boolean containsAll(Collection> c)
boolean addAll(Collection extends E> c)
boolean retainAll(Collection> c)
boolean removeAll(Collection> c)
boolean equals(Object o)