The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Map
implementations are grouped into general-purpose, special-purpose, and concurrent implementations.
The three general-purpose
Map
implementations are
HashMap
,
TreeMap
and
LinkedHashMap
. If you need SortedMap
operations or key-ordered Collection
-view iteration, use TreeMap
; if you want maximum speed and don't care about iteration order, use HashMap
; if you want near-HashMap
performance and insertion-order iteration, use LinkedHashMap
. In this respect, the situation for Map
is analogous to Set
. Likewise, everything else in the
Set Implementations section also applies to Map
implementations.
LinkedHashMap
provides two capabilities that are not available with LinkedHashSet
. When you create a LinkedHashMap
, you can order it based on key access rather than insertion. In other words, merely looking up the value associated with a key brings that key to the end of the map. Also, LinkedHashMap
provides the removeEldestEntry
method, which may be overridden to impose a policy for removing stale mappings automatically when new mappings are added to the map. This makes it very easy to implement a custom cache.
For example, this override will allow the map to grow up to as many as 100 entries and then it will delete the eldest entry each time a new entry is added, maintaining a steady state of 100 entries.
private static final int MAX_ENTRIES = 100; protected boolean removeEldestEntry(Map.Entry eldest) { return size() > MAX_ENTRIES; }
There are three special-purpose Map implementations
EnumMap
,
WeakHashMap
and
IdentityHashMap
. EnumMap
, which is internally implemented as an array
, is a high-performance Map
implementation for use with enum keys. This implementation combines the richness and safety of the Map
interface with a speed approaching that of an array. If you want to map an enum to a value, you should always use an EnumMap
in preference to an array.
WeakHashMap
is an implementation of the Map
interface that stores only weak references to its keys. Storing only weak references allows a key-value pair to be garbage-collected when its key is no longer referenced outside of the WeakHashMap
. This class provides the easiest way to harness the power of weak references. It is useful for implementing "registry-like" data structures, where the utility of an entry vanishes when its key is no longer reachable by any thread.
IdentityHashMap
is an identity-based Map
implementation based on a hash table. This class is useful for topology-preserving object graph transformations, such as serialization or deep-copying. To perform such transformations, you need to maintain an identity-based "node table" that keeps track of which objects have already been seen. Identity-based maps are also used to maintain object-to-meta-information mappings in dynamic debuggers and similar systems. Finally, identity-based maps are useful in thwarting "spoof attacks" that are a result of intentionally perverse equals
methods because IdentityHashMap
never invokes the equals
method on its keys. An added benefit of this implementation is that it is fast.
The
java.util.concurrent
package contains the
ConcurrentMap
interface, which extends Map
with atomic putIfAbsent
, remove
, and replace
methods, and the
ConcurrentHashMap
implementation of that interface.
ConcurrentHashMap
is a highly concurrent, high-performance implementation backed up by a hash table. This implementation never blocks when performing retrievals and allows the client to select the concurrency level for updates. It is intended as a drop-in replacement for Hashtable
: in addition to implementing ConcurrentMap
, it supports all the legacy methods peculiar to Hashtable
. Again, if you don't need the legacy operations, be careful to manipulate it with the ConcurrentMap
interface.