ConcurrentMap

java.util.concurrent.ConcurrentMap

The 

java.util.concurrent.ConcurrentMap

 interface represents a Map which
is capable of handling concurrent access (puts and gets) to it.

The 

ConcurrentMap

 has a few extra atomic methods in addition to the methods it inherits from its superinterface, 

java.util.Map

.

ConcurrentMap Implementations

Since 

ConcurrentMap

 is an interface, you need to use one of its implementations in order to use it. The

java.util.concurrent

 package contains the following implementations of the 

ConcurrentMap

 interface:
ConcurrentHashMap

ConcurrentHashMap

The 

ConcurrentHashMap

 is very similar to the 

java.util.HashTable

 class, except that 

ConcurrentHashMap

 offers better concurrency than 

HashTable

 does. 

ConcurrentHashMap

 does
not lock the 

Map

 while you are reading from it. Additionally, 

ConcurrentHashMap

 does not lock the entire 

Map

 when writing to it. It only locks the part
of the

Map

 that is being written to, internally.

Another difference is that 

ConcurrentHashMap

 does not throw 

ConcurrentModificationException

 if the

ConcurrentHashMap

 is changed while being iterated.
The 

Iterator

 is not designed to be used by more than one thread though.

Checkout the official JavaDoc for more details about 

ConcurrentMap

 and 

ConcurrentHashMap

.

ConcurrentMap Example

Here is an example of how to use the 

ConcurrentMap

 interface. The example uses a 

ConcurrentHashMap

implementation:

ConcurrentMap concurrentMap = new ConcurrentHashMap();

concurrentMap.put("key", "value");

Object value = concurrentMap.get("key");