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HashMap put() Method in Java

  • Difficulty Level : Basic
  • Last Updated : 03 May, 2021

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.
 


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