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-
