Synchronized HashMap Example in Java

HashMap is a non-synchronized collection class. If we need to perform thread-safe operations on it then we must need to synchronize it explicitly.

This example shows how to make a HashMap Synchronized in Java using synchronizedMap method of the Collections class.

Iterator should be used in a synchronized block even if we have synchronized the HashMap explicitly.

Map map = Collections.synchronizedMap(new HashMap());
...
// This doesn't need to be in synchronized block
Set set = map.keySet();
// Synchronizing on map, not on set
synchronized (map) {
    // Iterator must be in synchronized block
    Iterator iterator = set.iterator(); 
    while (iterator.hasNext()) {
        ...
    }
}

Map map = Collections.synchronizedMap(new HashMap());

...

// This doesn't need to be in synchronized block

Set set = map.keySet();

// Synchronizing on map, not on set

synchronized (map) {

// Iterator must be in synchronized block

Iterator iterator = set.iterator();

while (iterator.hasNext()) {

...

}

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