Java HashMap replace() Method

Last Updated : 22 Jan, 2025

The replace() method of the HashMap class in Java is used to replace the value associated with a specific key if the key is already present in the map.

Note: If the key does not exist, the method does nothing and the map remains unchanged.

Example 1: This example demonstrates replacing the value of an existing key in the HashMap.

Java

// Java program to demonstrate the working of replace()
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args) {
      
        // Create a HashMap
        HashMap<Integer, String> hm = new HashMap<>();
        hm.put(1, "Geek1");
        hm.put(2, "Geek2");
        hm.put(3, "Geek3");

        System.out.println("Original map: " + hm);

        // Replace the value for key 2
        String oldValue = hm.replace(2, "Geek10");

        System.out.println("Replaced value: " + oldValue);
        System.out.println("Updated map: " + hm);
    }
}

Output

Original map: {1=Geek1, 2=Geek2, 3=Geek3}
Replaced value: Geek2
Updated map: {1=Geek1, 2=Geek10, 3=Geek3}

Syntax of HashMap replace() Method

public V replace(K key, V newValue)

Parameters:

key: The key whose associated value is to be replaced.
value: The new value to associate with the specified key.

Return Type: This method returns the old value associated with the key or returns null if the key does not exist in the map.

Example 2: This example demonstrates that if the key does not exist in the HashMap, the replace() method does nothing and returns null.

Java

// Jav Program to handle non-existent key
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args) {
      
        // Create a HashMap
        HashMap<Integer, String> hm = new HashMap<>();
        hm.put(1, "Geek1");
        hm.put(4, "Geek4");
      
        System.out.println("Original map: " + hm);

        // Trying to replace value for 
        // a non-existing key (key 2)
        String oldValue = hm.replace(2, "Geek10");

        System.out.println("Replaced value: " + oldValue);
        System.out.println("Updated map: " + hm);
    }
}

Output

Original map: {1=Geek1, 4=Geek4}
Replaced value: null
Updated map: {1=Geek1, 4=Geek4}

Example 3: This example demonstrates using the overloaded replace() method to conditionally replace a value only if the existing value matches a specific value.

Java

// Java Program to demonstrate 
// conditional value replacement
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args) {
      
        // Create a HashMap
        HashMap<Integer, String> hm = new HashMap<>();
        hm.put(1, "Geek1");
        hm.put(2, "Geek2");

        System.out.println("Original map: " + hm);

        // Replace value for key 2 only if 
        // the current value is "Geek2"
        boolean b = hm.replace(2, "Geek2", "Geek10");

        System.out.println("Was value replaced? " + b);
        System.out.println("Updated map: " + hm);
    }
}

Output

Original map: {1=Geek1, 2=Geek2}
Was value replaced? true
Updated map: {1=Geek1, 2=Geek10}

Example 4: This example shows checking if a key exists before calling replace().

Java

// Checking before replacement
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args) {
      
        // Create a HashMap
        HashMap<Integer, String> hm = new HashMap<>();
        hm.put(1, "Geek1");
        hm.put(3, "Geek3");
      
        System.out.println("Original map: " + hm);

        // Check if key exists
        if (hm.containsKey(3)) {
            String oldValue = hm.replace(3, "Geek30");
            System.out.println("Replaced value: " + oldValue);
        } else {
            System.out.println("Key 3 does not exist.");
        }

        System.out.println("Updated map: " + hm);
    }
}

Output

Original map: {1=Geek1, 3=Geek3}
Replaced value: Geek3
Updated map: {1=Geek1, 3=Geek30}

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