ConcurrentHashMap putIfAbsent() is returning null

Question

The following program is printing null. I am not able to understand why.

public class ConcurrentHashMapTest {     public static final ConcurrentMap<String, String> map = new ConcurrentHashMap<>(5, 0.9f, 2);      public static void main(String[] args) {         map.putIfAbsent("key 1", "value 1");         map.putIfAbsent("key 2", "value 2");          String value = get("key 3");         System.out.println("value for key 3 --> " + value);     }      private static String get(final String key) {         return map.putIfAbsent(key, "value 3");     } } 

Could someone help me understand the behavior?

Answer 1

Problem is that by definition putIfAbsent return old value and not new value (old value for absent is always null). Use computeIfAbsent - this will return new value for you.

private static String get(final String key) {     return map.computeIfAbsent(key, s -> "value 3"); } 

Answer 2

ConcurrentMap.putIfAbsent returns the previous value associated with the specified key, or null if there was no mapping for the key. You did not have a value associated with "key 3". All correct.

NOTE: Not just for ConcurrentMap, this applies to all implementations of Map.