Java HashMap replace() Method
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 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.
// 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 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().
// 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}
