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

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