HashMap replace(key, value) method in Java with Examples

The replace(K key, V value) method of Map interface, implemented by HashMap class is used to replace the value of the specified key only if the key is previously mapped with some value.

Syntax:

public V replace(K key, V value)

Parameters: This method accepts two parameters:

• key: which is the key of the element whose value has to be replaced.
• value: which is the new value which has to be mapped with the provided key.

Return Value: This method returns the previous value associated with the specified key. If there is no such key mapped, then it returns null, if the implementation supports null value.

Exceptions: This method will throw:

• NullPointerException if the specified key or value is null, and this map does not permit null keys or values and
• IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map.

Program 1:

// Java program to demonstrate
// replace(K key, V value) method

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // Create a HashMap and add some values
        HashMap<String, Integer> map
            = new HashMap<>();
        map.put("a", 100);
        map.put("b", 300);
        map.put("c", 300);
        map.put("d", 400);

        // print map details
        System.out.println("HashMap: "
                           + map.toString());

        // provide value for the key which has
        // to replace it's current value,
        // using replace(K key, V value) method
        map.replace("b", 200);

        // print new mapping
        System.out.println("New HashMap: "
                           + map.toString());
    }
}

Output:

HashMap: {a=100, b=300, c=300, d=400}
New HashMap: {a=100, b=200, c=300, d=400}

Program 2:

// Java program to demonstrate
// replace(K key, V value) method

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {
        // Create a HashMap and add some values
        HashMap<String, Integer> map
            = new HashMap<>();
        map.put("a", 100);
        map.put("b", 300);
        map.put("c", 300);
        map.put("d", 400);

        // print map details
        System.out.println("HashMap: "
                           + map.toString());

        // provide value for the key which has
        // to replace it's current value, and will
        // store the value in k using the
        // replace(K key, V value) method
        int k = map.replace("b", 200);

        // print the value of k
        System.out.println("Previous value of 'b': "
                           + k);

        // print new mapping
        System.out.println("New HashMap: "
                           + map.toString());
    }
}

Output:

HashMap: {a=100, b=300, c=300, d=400}
Previous value of 'b': 300
New HashMap: {a=100, b=200, c=300, d=400}

Program 3:

// Java program to demonstrate
// replace(K key, V value) method

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {
        // Create a HashMap and add some values
        HashMap<String, Integer> map
            = new HashMap<>();
        map.put("a", 100);
        map.put("b", 300);
        map.put("c", 300);
        map.put("d", 400);

        // print map details
        System.out.println("HashMap: "
                           + map.toString());

        // provide value for the key which is
        // not mapped previously and store the
        // return value in Integer k, using
        // replace(K key, V value) method
        Integer k = map.replace("e", 200);

        // print the value of k
        System.out.println("Value of k, returned "
                           + "for key 'e': " + k);

        // print new mapping
        System.out.println("New HashMap: "
                           + map.toString());
    }
}

Output:

HashMap: {a=100, b=300, c=300, d=400}
Value of k, returned for key 'e': null
New HashMap: {a=100, b=300, c=300, d=400}

References: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#replace-K-V-