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How to synchronize HashMap in Java with example

By Chaitanya Singh | Filed Under: Java Collections

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.

Example:

In this example we have a HashMap<Integer, String> 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).

Syntax:

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:

package beginnersbook.com;
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<Integer, String> hmap= new HashMap<Integer, String>();
         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();
         synchronized(map){
             Iterator i = set.iterator();
             // Display elements
             while(i.hasNext()) {
                Map.Entry me = (Map.Entry)i.next();
                System.out.print(me.getKey() + ": ");
                System.out.println(me.getValue());
             }
         }
    }
}

Output:

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

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  5. Java – Replace element in a LinkedList example
  6. How to convert ArrayList to string array in java

Comments

  1. munni says

    July 2, 2014 at 1:07 PM

    What’s the use of synchronizing hashmap?
    As sync says only one at a time — which part of example says that only one is getting accessed?
    Basically i wanted to know the difference b/w sync and non sync hashmap using multi threaded example and basically showing the difference

    It would be great it u cud clarify my questions.

    Reply
  2. Chinmayee Das says

    April 3, 2015 at 11:38 AM

    Hello,

    In the above program, how the output came, i am not getting, in which way, its printing the result?? can u please brief about this..

    Thanks
    chinmayee

    Reply
    • Rajeev Shrikar says

      June 10, 2016 at 8:14 AM

      what error you are getting? It executed perfectly for me.
      May be you have kept package name as it is, try removing that line or keep the java file according to the package name.

      Reply
  3. Rajeev Shrikar says

    June 10, 2016 at 8:09 AM

    simple and easily understandable.
    Please show a program in multi threaded environment too!

    Reply

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