WeakHashMapis a kind ofMapimplemented based on theHashTable. TheWeakHashMaponly storesWeakReferencetype as their keys. Once the key is no longer used, the respective mapping will be automatically removed when the garbage collection is done.
The putIfAbsent method adds the key-value pair to the WeakHashMap object if the key is not present in the map. If the key is already present, then it skips the operation.
public V putIfAbsent(K key,V value)
If the key is already present in the map, then the value associated with the key is returned, and no insertion will be done.
If the key is not present in the map, the key-value pair is inserted, and null is returned.
The null return can also denote that the map previously associated the null value with a key.
The code below demonstrates how to use the putIfAbsent method.
import java.util.WeakHashMap;class Main {public static void main( String args[] ) {// create a WeakHashMapWeakHashMap<Integer, String> numbers = new WeakHashMap<>();// add key-value pairs to the WeakHashMapnumbers.put(1, "one");numbers.put(2, "Two");numbers.put(3, "three");System.out.println("The map is => " + numbers);System.out.println("\nTrying to add (1, \"ONE\") in the map ");String value = numbers.putIfAbsent(1, "ONE");// key - 1 is already present in the map// so the value associated with the key is returnedSystem.out.println("The value is => " + value);System.out.println("\nTrying to add (4, \"Four\") in the map ");value = numbers.putIfAbsent(4, "FOUR");// key - 4 is not already present in the map// so a new entry with the key as 4 and value as FOUR will be added// and null is returnedSystem.out.println("The value is => " + value);System.out.println("The map is => " + numbers);}}
We created a WeakHashMap with three entries.
We attempted to add a new entry to the WeakHashMap with:
key 1. In this case, the old value will be returned and the insertion will be skipped.key 4. In this case, the new key-value entry will be inserted and null is returned.