HashMap put() Method in Java

The java.util.HashMap.put() method of HashMap is used to insert a mapping into a map. This means we can insert a specific key and the value it is mapping to into a particular map. If an existing key is passed then the previous value gets replaced by the new value. If a new pair is passed, then the pair gets inserted as a whole.

Syntax:

Hash_Map.put(key, value)

Parameters: The method takes two parameters, both are of the Object type of the HashMap. 

• key: This refers to the key element that needs to be inserted into the Map for mapping.
• value: This refers to the value that the above key would map into.

Return Value: If an existing key is passed then the previous value gets returned. If a new pair is passed, then NULL is returned.
Below programs are used to illustrate the working of java.util.HashMap.put() Method: 
Program 1: When passing an existing key. 

Java

// Java code to illustrate the put() method
import java.util.*;

public class Hash_Map_Demo {
    public static void main(String[] args)
    {

        // Creating an empty HashMap
        HashMap<Integer, String> hash_map = new HashMap<Integer, String>();

        // Mapping string values to int keys
        hash_map.put(10, "Geeks");
        hash_map.put(15, "4");
        hash_map.put(20, "Geeks");
        hash_map.put(25, "Welcomes");
        hash_map.put(30, "You");

        // Displaying the HashMap
        System.out.println("Initial Mappings are: " + hash_map);

        // Inserting existing key along with new value
        String returned_value = (String)hash_map.put(20, "All");

        // Verifying the returned value
        System.out.println("Returned value is: " + returned_value);

        // Displaying the new map
        System.out.println("New map is: " + hash_map);
    }
}

Output:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: Geeks
New map is: {20=All, 25=Welcomes, 10=Geeks, 30=You, 15=4}

Program 2: When passing a new key. 

Java

// Java code to illustrate the put() method
import java.util.*;

public class Hash_Map_Demo {
    public static void main(String[] args)
    {

        // Creating an empty HashMap
        HashMap<Integer, String> hash_map = new HashMap<Integer, String>();

        // Mapping string values to int keys
        hash_map.put(10, "Geeks");
        hash_map.put(15, "4");
        hash_map.put(20, "Geeks");
        hash_map.put(25, "Welcomes");
        hash_map.put(30, "You");

        // Displaying the HashMap
        System.out.println("Initial Mappings are: " + hash_map);

        // Inserting existing key along with new value
        String returned_value = (String)hash_map.put(50, "All");

        // Verifying the returned value
        System.out.println("Returned value is: " + returned_value);

        // Displaying the new map
        System.out.println("New map is: " + hash_map);
    }
}

Output:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: null
New map is: {50=All, 20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}

Note: The same operation can be performed with any type of Mappings with variation and combination of different data types.