Program to use ConcurrentHashMap’s putIfAbsent method



Definition of ConcurrentHashMap’s putIfAbsent method >

public V putIfAbsent(K key, V value)

What do putIfAbsent method do>
If map does not contain specified key, put specified key-value pair in map and return null.
If map already contains specified key, return value corresponding to specified key.


putIfAbsent method is equivalent to writing following code >
synchronized (map){
    if (!map.containsKey(key))
      return map.put(key, value);
     else
      return map.get(key);
   }

Full Program to use putIfAbsent method >
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/** Copyright (c), AnkitMittal JavaMadeSoEasy.com */
public class ConcurrentHashMapTest {
   public static void main(String args[]) {
      ConcurrentMap<Integer, String> concurrentHashMap =
                                       new ConcurrentHashMap<Integer, String>();
      concurrentHashMap.put(1, "javaMadeSoEasy");
      System.out.println("concurrentHashMap : "+concurrentHashMap);
     
      System.out.println("\n putIfAbsent method >> "+
                          concurrentHashMap.putIfAbsent(1, "ankit"));
      System.out.println("concurrentHashMap : "+concurrentHashMap);
      System.out.println("\n putIfAbsent method >> "+
                          concurrentHashMap.putIfAbsent(2, "audi"));
      System.out.println("concurrentHashMap : "+concurrentHashMap);
     
   }
}
/*OUTPUT
concurrentHashMap : {1=javaMadeSoEasy}
putIfAbsent method >> javaMadeSoEasy
concurrentHashMap : {1=javaMadeSoEasy}

putIfAbsent method >> null
concurrentHashMap : {2=audi, 1=javaMadeSoEasy}
*/

concurrentHashMap.putIfAbsent(1, "ankit") > returned javaMadeSoEasy because map was already having that key.

concurrentHashMap.putIfAbsent(2, "audi") > putted specified key-value pair in map and  returned null because map wasn’t having that key.



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