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How to synchronize HashMap in Java with example

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. In this tutorial we will see how to synchronize HashMap.

In this example we have a HashMap it is having integer keys and String type values. In order to synchronize it we are using Collections.synchronizedMap(hashmap)  it returns a thread-safe map backed up by the specified HashMap.

Important point to note in the below example:
Iterator should be used in a synchronized block even if we have synchronized the HashMap explicitly (As we did in the below code).


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()){
Complete Code:

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Iterator;
public class HashMapSyncExample {
    public static void main(String args[]) {
         HashMap hmap= new HashMap();
         hmap.put(2, "Anil");
         hmap.put(44, "Ajit");
         hmap.put(1, "Brad");
         hmap.put(4, "Sachin");
         hmap.put(88, "XYZ");

         Map map= Collections.synchronizedMap(hmap);
         Set set = map.entrySet();
             Iterator i = set.iterator();
             // Display elements
             while(i.hasNext()) {
                Map.Entry me = (Map.Entry);
                System.out.print(me.getKey() + ": ");

1: Brad
2: Anil
4: Sachin
88: XYZ
44: Ajit

asked Jun 11, 2015 in Java Interview Questions by rajesh
0 votes

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