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

25 Sep, 2019
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());    }}
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());    }}
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());    }}
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-
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