ConcurrentHashMap put() method in Java

The put() method of class ConcurrentHashmap in Java is used to insert a mapping into this map. It takes the parameter as a (Key, Value) pair. If an existing key is passed with a value, then this method updates the value of that key. Else if a new pair is passed, then the pair gets inserted as a whole.

Syntax:

public V put(K key, V value)

Parameters: This method accepts two mandatory parameters:

• key: – key with which the specified value is to be associated
• value: – value to be associated with the specified key

Return Value: This method returns the previous value associated with key, or null if there was no mapping for key.

Exception: This method throws NullPointerException if the specified key or value is null

Below is the example to illustrate put() method:

Program 1: When the Key, Value passed is new.

// Java program to demonstrate
// put() method

import java.util.concurrent.ConcurrentHashMap;
import java.util.*;

public class ConcurrentHashMapExample {

    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map<String, String>
            my_cmmap = new ConcurrentHashMap<String, String>();

        // Adding elements to the map
        // using put() method
        my_cmmap.put("1", "1");
        my_cmmap.put("2", "1");
        my_cmmap.put("3", "1");
        my_cmmap.put("4", "1");
        my_cmmap.put("5", "1");
        my_cmmap.put("6", "1");

        // Printing the map
        System.out.print(my_cmmap);
    }
}

Output:

{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

Program 2: When an existing Key, Value is passed.

// Java program to demonstrate
// put() method

import java.util.concurrent.ConcurrentHashMap;
import java.util.*;

public class ConcurrentHashMapExample {

    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map<String, String>
            my_map = new ConcurrentHashMap<String, String>();

        // Variable to get the returned value by put()
        String returnedValue;

        // Adding elements to the map
        // using put() method
        // and printing the returned value each time
        returnedValue = my_map.put("Geek", "100");
        System.out.println("Map: " + my_map);
        System.out.println("Returned Value: " + returnedValue);
        System.out.println();

        returnedValue = my_map.put("Geek", "200");
        System.out.println("Map: " + my_map);
        System.out.println("Returned Value: " + returnedValue);
        System.out.println();

        returnedValue = my_map.put("Geek", "300");
        System.out.println("Map: " + my_map);
        System.out.println("Returned Value: " + returnedValue);
        System.out.println();

        returnedValue = my_map.put("Geek", "400");
        System.out.println("Map: " + my_map);
        System.out.println("Returned Value: " + returnedValue);
        System.out.println();

        returnedValue = my_map.put("Geek", "500");
        System.out.println("Map: " + my_map);
        System.out.println("Returned Value: " + returnedValue);
        System.out.println();

        System.out.print(my_map);
    }
}

Output:

Map: {Geek=100}
Returned Value: null

Map: {Geek=200}
Returned Value: 100

Map: {Geek=300}
Returned Value: 200

Map: {Geek=400}
Returned Value: 300

Map: {Geek=500}
Returned Value: 400

{Geek=500}

Program 3: To demonstrate NullPointerException

// Java program to demonstrate
// put() method

import java.util.concurrent.ConcurrentHashMap;
import java.util.*;

public class ConcurrentHashMapExample {

    public static void main(String[] args)
    {
        // Creating ConcurrentHashMap
        Map<String, String>
            my_cmmap = new ConcurrentHashMap<String, String>();

        // Adding elements to the map
        // using put() method
        my_cmmap.put("1", "1");
        my_cmmap.put("2", "1");
        my_cmmap.put("3", "1");
        my_cmmap.put("4", "1");
        my_cmmap.put("5", "1");
        my_cmmap.put("6", "1");

        // Printing the map
        System.out.println(my_cmmap + "\n");

        // When null key is passed
        try {
            System.out.println("When (null, \"10\") "
                               + "is passed as parameter:");
            my_cmmap.put(null, "10");
        }
        catch (Exception e) {
            System.out.println("Exception: " + e + "\n");
        }

        // When null value is passed
        try {
            System.out.println("When (\"10\", null) "
                               + "is passed as parameter:");
            my_cmmap.put("10", null);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e + "\n");
        }
    }
}

Output:

{1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

When (null, "10") is passed as parameter:
Exception: java.lang.NullPointerException

When ("10", null) is passed as parameter:
Exception: java.lang.NullPointerException