Setpublic interface Set implements CollectionA 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 Summary |
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| public boolean | add(E e)Adds the specified element to this set if it is not already present
(optional operation). More formally, adds the specified element
e to this set if the set contains no element e2
such that
(e==null ? e2==null : e.equals(e2)).
If this set already contains the element, the call leaves the 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 throw an exception, as described in the
specification for {@link Collection#add Collection.add}.
Individual set implementations should clearly document any
restrictions on the elements that they may contain.
| | public boolean | addAll(java.util.Collection 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 undefined if the specified
collection is modified while the operation is in progress.
| | public void | clear()Removes all of the elements from this set (optional operation).
The set will be empty after this call returns.
| | public boolean | contains(java.lang.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)).
| | public boolean | containsAll(java.util.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.
| | public boolean | equals(java.lang.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.
| | 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 hash code 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
{@link Object#hashCode}.
| | public boolean | isEmpty()Returns true if this set contains no elements.
| | public java.util.Iterator | 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).
| | public boolean | remove(java.lang.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
this set contains such an element. Returns true if this set
contained the element (or equivalently, if this set changed as a
result of the call). (This set will not contain the element once the
call returns.)
| | public boolean | removeAll(java.util.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.
| | public boolean | retainAll(java.util.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.
| | 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.
| | public java.lang.Object[] | toArray()Returns an array containing all of the elements in this set.
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.
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.
| | public 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.
If the set 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 set.
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 {@link #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().
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