Java HashMap merge() Method

Last Updated : 15 Jan, 2025

In Java, the merge() method of the HashMap class is used to either add new key-value pairs or update the existing one in the map by merging values using a specified function.

If the key is not present in the map, the merge() method adds the key with the given value.
If the key already exists in the map, the method uses a BiFunction to combine the old value and the new value into one and then updates the key with the combined value.

Note: It either adds the new entry or updates an existing one by merging values together

Example 1: The below Java program demonstrates the use of merge() method to add new key-value pairs and update the existing ones by combining old and new values using the Lambda function.

// Java program to demonstrate adding and updating 
// key-value pairs using merge()
import java.util.HashMap;

public class Main {
    public static void main(String[] args)
    {
        // creating a HashMap to store key-value pairs
        HashMap<String, Integer> hm = new HashMap<>();
        hm.put("A", 10);
        hm.put("B", 20);

        // use merge() for a key that 
        // is not present in the map
        hm.merge("C", 30,
            (oldValue, newValue) -> oldValue + newValue);

        // use merge() for a key that 
        // is already present in the map
        hm.merge("A", 15,
            (oldValue, newValue) -> oldValue + newValue);

        System.out.println(hm);
    }
}

Output

{A=25, B=20, C=30}

Syntax of HashMap merge() Method

default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

Parameters: This method takes three parameters.

Key: The key whose value is to be merged
Value: The new value to be merged with the existing one.
remappingFunction: It is a function that takes two arguments the current value and the new value and then calculate the new value

Return Type: Return the new value associated with the key or null if the new value is null.

Example 2: The below Java program demonstrates the use of merge() to update the existing value or insert the new key-value pair using a custom BiFunction.

// Java program to demonstrate merging 
// string values using merge()
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args)
    {
      
        HashMap<String, String> hm = new HashMap<>();
        hm.put("A", "Hello");
        hm.put("B", "World");

        // use merge() for a key that 
        // is already present in the map
        hm.merge("A", "Java",
                (oldValue,newValue) -> oldValue + " " + newValue);

        // use merge() for the key that
        // is not present in the map
        hm.merge("C", "Programming",
            (oldValue, newValue) -> oldValue + newValue);

        System.out.println(hm);
    }
}