What is the EnumMap.putIfAbsent method in Java?

EnumMap is similar to Map, except that EnumMap only takes the Enum type as key. Also, all the keys must be from a single enum type. For further details, refer here.

The putIfAbsent method adds the key-value pair to the EnumMap object if the key is not present in the map. If the key is already present, then it skips the operation.

Syntax

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, then 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.

Code

The code below demonstrates how to use the putIfAbsent method.

import java.util.EnumMap;
class ComputeIfPresent {
    enum Subject {
      MATHS, SCIENCE, PROGRAMMING, ECONOMICS
    };
    public static void main( String args[] ) {
        EnumMap<Subject, Integer> map = new EnumMap<>(Subject.class);
        map.put(Subject.MATHS, 50);
        map.put(Subject.SCIENCE, 60);
        map.put(Subject.PROGRAMMING, 70);
        System.out.println("The map is => " + map);
        System.out.println("\nTrying to add (Subject.MATHS, 10)");
        Integer value = map.putIfAbsent(Subject.MATHS, 10);
        
        // key -  MATHS is already present in the map 
        // so the value associated with the key is returned 
        System.out.println("The value is => " + value);
        System.out.println("\nTrying to add (Subject.ECONIMICS, 80)");
        value = map.putIfAbsent(Subject.ECONOMICS, 80);
        
        // key - ECONOMICS is not present
        // so a new entry will be added and null is returned
        System.out.println("The value is => " + value);
        System.out.println("The map is => " + map);
    }
}

In the code above, we:

  • Created an Enum for the Subjects with the name Subject.

  • Created an EnumMap with the name map.

  • Added three entries to the map.

  • We use the putIfAbsent method to add a new entry to the EnumMap with:

    1. The already existing key Subject.MATHS. In this case, the old value will be returned, and the insertion will be skipped.
    2. The new key Subject.ECONOMICS. In this case, the new key-value entry will be inserted, and null is returned.

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