How to use the HashMap merge() function in Java

An instance of the Java HashMap calls the merge() function, which inserts a new key-value pair in the HashMap.

To use the merge() function with HashMap, the program needs to import the java.util.HashMap module:

import java.util.HashMap;

Prototype

map, an instance of the Java Hashmap, can call the merge() function as illustrated below:

Parameters

The merge() function accepts the following three parameters:

  • key: The key for the new key-value pair that is inserted.
  • value: The value for the new key-value pair that is inserted.
  • remappingFunction: The function for mapping the value if an existing value is already associated with the key.

Return values

The merge() function returns the value associated with the key. merge() returns null if no value is associated with the key.

If the remappingFunction results in a null value, the key-value pair is removed from the HashMap.

Example

The following example demonstrates how to use the merge() function with a Java HashMap.

  • The program below creates a HashMap results and indicates in line 7 that the first element of a pair is a string and the second element is an integer.
  • It then populates the results HashMap through the put() method and displays the HashMap.

Example 1

  • The program merges a new key-value pair. The remappingFunction is a Java lambda expression; it adds the old value and the new value and associates it to the key Sara if Sara is already a key in the HashMap. Since the key Sara is not present in the HashMap, the associated value is 90.

Example 2

  • In this example, the key Sara is already present in the HashMap. Therefore, the remappingFunction adds the old marks, which are 95, and new marks, which are 76, and associates the result with the key. Hence, the new value associated with Sara is 166.
import java.util.HashMap;
class ExampleHashMap {
    public static void main(String[] args) {
        // create a new hashmap
        HashMap<String, Integer> results = new HashMap<>();
        System.out.println("A HashMap for mapping marks to students");
       
        // add elements to the hashmap
        results.put("Ali", 89);
        results.put("Ahmed", 95);
        results.put("Sana", 70);
        results.put("Amna", 100);
        //display the hashmap
        System.out.println("Updated HashMap: " + results);
        // Example 1
        int val = results.merge("Sara", 90, (val1, val2) -> val1+val2);
        System.out.println("The new value is " + val);
        //display the hashmap
        System.out.println("Updated HashMap: " + results);
        // Example 2
        val = results.merge("Sara", 76, (val1, val2) -> val1+val2);
        System.out.println("The new value is" + val);
        //display the hashmap
        System.out.println("Updated HashMap: " + results);
    }
}
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