Java Reference
In-Depth Information
analogous names. This helps you remember the methods and what they
do.
Generally, you can expect a
Map
to be optimized for finding values listed
under keys. For example, the method
containsKey
will usually be much
more efficient than
containsValue
on larger maps. In a
HashMap
, for ex-
ample,
containsKey
is
0
(1), whereas
containsValue
is
0
(
n
) the key is found
by hashing, but the value must be found by searching through each ele-
ment until a match is found.
Map
is not a collection, but there are methods that let you view the map
using collections:
public Set<K>
keySet()
Returns a
Set
whose elements are the keys of this map.
public Collection<V>
values()
Returns a
Collection
whose elements are the values of this
map.
public Set<Map.Entry<K,V>>
enTRySet()
Returns a
Set
whose elements are
Map.Entry
objects that rep-
resent single mappings in the map. As you will see soon,
Map.Entry
is a nested interface with methods to manipulate the
entry.
The collections returned by these methods are backed by the
Map
, so
removing an element from one of these collections removes the cor-
responding key/value pair from the map. You cannot add elements to
these collectionsthey do not support the optional methods for adding to
a collection. If you iterate through the key and value sets in parallel you
cannot rely on getting key/value pairsthey may return values from their
respective sets in any order. If you need such pairing, you should use
entrySet
.
