What is the HashMap.merge() method in Java?

The merge method in Java will:

  • Insert a new entry if the mapping for the key is not present.

  • If a mapping for the key is already present, then the value associated with the key will be assigned based on the value returned from the BiFunctionA functional interface that accepts two arguments and produces a result. argument of the merge method.

Syntax

mapObj.merge(key, value, BiFunction);

  • key: The key for which the value is to be added or merged.

  • value: The value to be associated with key. If there is already a value associated with that key in the map, then BiFunction recomputes the value.

  • BiFunction: A functional interface that takes two arguments, oldVal and newVal, and returns a single value.

Example

import java.util.HashMap;
class merge {
    public static void main( String args[] ) {
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        map.put("Maths", 50);
        map.put("Science", 60);
        map.put("Programming", 70);
        
        System.out.println( "The map is - \n" + map);
        map.merge("Economics", 40, (oldVal, newVal) -> { return oldVal + newVal; });
        System.out.println( "\nThe map afer executing - map.merge(Economics, 40, (oldVal, newVal) -> { return oldVal + newVal; }); \n" + map);
        map.merge("Maths", 40, (oldVal, newVal) -> { return oldVal + newVal; });
        System.out.println( "\n\nThe map afer executing - map.merge(Maths, 40, (oldVal, newVal) -> { return oldVal + newVal; }); \n" + map);
    }
}

In the code above, we have:

  • Created a HashMap with the name map and added some entries into it.

  • Called the merge method of the HashMap object for the Economics key.

map.merge("Economics", 40, (oldVal, newVal) -> { return oldVal + newVal; });

The call above to the merge method will check if the Economics key is already present on the map. In our case, the Economics key is not present in the map, so a new entry with the Economics key and value 40 will be added to the map object.

  • We again called the merge method of the HashMap object for the key Maths.
map.merge("Maths", 40, (oldVal, newVal) -> { return oldVal + newVal; });

The above call to the merge method will check if the Maths key is already present on the map. In our case, the Maths key is present in the map, so the BiFunction will be executed and the value returned from the BiFunction is updated for the Maths key. In our BiFunction, we returned the sum of the old value and the new value from the BiFunction.

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