Java Reference
In-Depth Information
// Removes one mapping to the value 2 - usually inefficient and of
// limited use
m
.
values
().
remove
(
2
);
// Remove all mappings to 4
m
.
values
().
removeAll
(
Collections
.
singleton
(
4
));
// Keep only mappings to 2 & 3
m
.
values
().
retainAll
(
Arrays
.
asList
(
2
,
3
));
// Deletions can also be done via iterators
Iterator
<
Map
.
Entry
<
String
,
Integer
>>
iter
=
m
.
entrySet
().
iterator
();
while
(
iter
.
hasNext
())
{
Map
.
Entry
<
String
,
Integer
>
e
=
iter
.
next
();
if
(
e
.
getValue
()
==
2
)
iter
.
remove
();
}
// Find values that appear in both of two maps. In general, addAll()
// and retainAll() with keySet() and values() allow union and
// intersection
Set
<
Integer
>
v
=
new
HashSet
<
Integer
>(
m
.
values
());
v
.
retainAll
(
singleton
.
values
());
// Miscellaneous methods
m
.
clear
();
// Deletes all mappings
m
.
size
();
// Returns number of mappings: currently 0
m
.
isEmpty
();
// Returns true
m
.
equals
(
empty
);
// true: Maps implementations override equals
The
Map
interface includes a variety of general-purpose and special-purpose imple‐
mentations, which are summarized in
Table 8-3
. As always, complete details are in
the JDK's documentation and javadoc. All classes in
Table 8-3
are in the
java.util
package except
ConcurrentHashMap
and
ConcurrentSkipListMap
, which are part of
java.util.concurrent
.
Table 8-3. Map implementations
Class
Representation
Since
null
keys
null
values
Notes
Hashtable
1.2
Yes
Yes
General-purpose implementation.
HashMap
Hashtable
5.0
No
No
General-purpose threadsafe
implementation; see
ConcurrentMap
interface.
Concurren
tHashMap
s
a
Hashtable
6.0
No
No
Specialized threadsafe implementation;
see
ConcurrentNavigableMap
interface.
Concurrent
SkipList
Map
Array
5.0
No
Yes
Keys are instances of an enum.
EnumMap
