An object that maps keys to values. A map cannot contain duplicate keys;
each key can map to at most one value.
This interface takes the place of the Dictionary class, which was
a totally abstract class rather than an interface.
The Map interface provides three collection views, which
allow a map's contents to be viewed as a set of keys, collection of values,
or set of key-value mappings. The order of a map is defined as
the order in which the iterators on the map's collection views return their
elements. Some map implementations, like the TreeMap class, make
specific guarantees as to their order; others, like the HashMap
class, do not.
Note: great care must be exercised if mutable objects are used as map keys.
The behavior of a map is not specified if the value of an object is changed
in a manner that affects equals comparisons while the object is a
key in the map. A special case of this prohibition is that it is not
permissible for a map to contain itself as a key. While it is permissible
for a map to contain itself as a value, extreme caution is advised: the
equals and hashCode methods are no longer well defined on a such a map.
All general-purpose map implementation classes should provide two
"standard" constructors: a void (no arguments) constructor which creates an
empty map, and a constructor with a single argument of type Map,
which creates a new map with the same key-value mappings as its argument.
In effect, the latter constructor allows the user to copy any map,
producing an equivalent map of the desired class. There is no way to
enforce this recommendation (as interfaces cannot contain constructors) but
all of the general-purpose map implementations in the JDK comply.
Returns true if this map maps one or more keys to the
specified value. More formally, returns true if and only if
this map contains at least one mapping to a value v such that
(value==null ? v==null : value.equals(v)). This operation
will probably require time linear in the map size for most
implementations of the Map interface.
Parameters:
value - value whose presence in this map is to be tested.
Returns:
true if this map maps one or more keys to the
specified value.
Returns the value to which this map maps the specified key. Returns
null if the map contains no mapping for this key. A return
value of null does not necessarily indicate that the
map contains no mapping for the key; it's also possible that the map
explicitly maps the key to null. The containsKey
operation may be used to distinguish these two cases.
Parameters:
key - key whose associated value is to be returned.
Returns:
the value to which this map maps the specified key, or
null if the map contains no mapping for this key.
Throws:
ClassCastException - if the key is of an inappropriate type for
this map.
Associates the specified value with the specified key in this map
(optional operation). If the map previously contained a mapping for
this key, the old value is replaced.
Parameters:
key - key with which the specified value is to be associated.
value - value to be associated with the specified key.
Returns:
previous value associated with specified key, or null
if there was no mapping for key. A null return can
also indicate that the map previously associated null
with the specified key, if the implementation supports
null values.
Removes the mapping for this key from this map if present (optional
operation).
Parameters:
key - key whose mapping is to be removed from the map.
Returns:
previous value associated with specified key, or null
if there was no mapping for key. A null return can
also indicate that the map previously associated null
with the specified key, if the implementation supports
null values.
Copies all of the mappings from the specified map to this map
(optional operation). These mappings will replace any mappings that
this map had for any of the keys currently in the specified map.
Returns a set view of the keys contained in this map. The set is
backed by the map, so changes to the map are reflected in the set, and
vice-versa. If the map is modified while an iteration over the set is
in progress, the results of the iteration are undefined. The set
supports element removal, which removes the corresponding mapping from
the map, via the Iterator.remove, Set.remove,
removeAllretainAll, and clear operations.
It does not support the add or addAll operations.
Returns a collection view of the values contained in this map. The
collection is backed by the map, so changes to the map are reflected in
the collection, and vice-versa. If the map is modified while an
iteration over the collection is in progress, the results of the
iteration are undefined. The collection supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll and clear operations.
It does not support the add or addAll operations.
Returns:
a collection view of the values contained in this map.
Returns a set view of the mappings contained in this map. Each element
in the returned set is a Map.Entry. The set is backed by the
map, so changes to the map are reflected in the set, and vice-versa.
If the map is modified while an iteration over the set is in progress,
the results of the iteration are undefined. The set supports element
removal, which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove, removeAll,
retainAll and clear operations. It does not support
the add or addAll operations.
Compares the specified object with this map for equality. Returns
true if the given object is also a map and the two Maps
represent the same mappings. More formally, two maps t1 and
t2 represent the same mappings if
t1.entrySet().equals(t2.entrySet()). This ensures that the
equals method works properly across different implementations
of the Map interface.
o - object to be compared for equality with this map.
Returns:
true if the specified object is equal to this map.
hashCode
public int hashCode()
Returns the hash code value for this map. The hash code of a map
is defined to be the sum of the hashCodes of each entry in the map's
entrySet view. This ensures that t1.equals(t2) implies
that t1.hashCode()==t2.hashCode() for any two maps
t1 and t2, as required by the general
contract of Object.hashCode.
Java, Java 2D, and JDBC are a trademarks or registered trademarks of Sun Microsystems, Inc. in the US and other countries. Copyright 1993-1999 Sun Microsystems, Inc. 901 San Antonio Road, Palo Alto, California, 94303, U.S.A. All Rights Reserved.
