ConcurrentHashMap remove() method in Java

remove(Object key)

The remove(Object key) method of class ConcurrentHashmap in Java is used to remove a mapping from the map. If the key does not exist in the map, then this function does nothing.

Syntax

public V remove(Object key)

Parameters: This method accepts a mandatory parameter key which is the key that needs to be removed



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

Exceptions: This method throws NullPointerException if the specified key is null.

Below are the programs to illustrate remove() method:

Program 1:

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// Java program to demonstrate
// remove() 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("Map: " + my_cmmap);
        System.out.println();
  
        // Removing the mapping
        // with existing key 6
        // using remove() method
        String valueRemoved = my_cmmap.remove("6");
  
        // Printing the map after remove()
        System.out.println("After removing mapping with key 6:");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Value removed: " + valueRemoved);
        System.out.println();
  
        // Removing the mapping
        // with non-existing key 10
        // using remove() method
        valueRemoved = my_cmmap.remove("10");
  
        // Printing the map after remove()
        System.out.println("After removing mapping with key 10:");
        System.out.println("Map: " + my_cmmap);
        System.out.println("Value removed: " + valueRemoved);
        System.out.println();
    }
}

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Output:


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

After removing mapping with key 6:
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Value removed: 1

After removing mapping with key 10:
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Value removed: null





Program 2: To demonstrate NullPointerException











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// Java program to demonstrate 


// remove() 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("Map: " + my_cmmap); 


        System.out.println(); 


  


        try { 


            // Removing the mapping with null key 


            // using remove() method 


            my_cmmap.remove(null); 


        } 


        catch (Exception e) { 


            System.out.println("Exception: " + e); 


        } 


    } 


} 



















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Output:


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

Exception: java.lang.NullPointerException





remove(Object key, Object value)


The remove(Object key, Object value) method of class ConcurrentHashmap in Java is used to remove a mapping from the map. The mapping with the specified (key, value) pair is searched in the map and remove, if found, and return true. If the key does not exist in the map, then this function does nothing and returns false.


Syntax:


public boolean remove(Object key, Object value)


Parameters: This method accepts two parameters




    key – key with which the specified value is associated.

  • value – value expected to be associated with the specified key.
    • 



Return Value: This method returns true if the value was removed. Else it returns false.


Exceptions: This method throws NullPointerException if the specified key is null.


Below are the programs to illustrate remove() method:


Program 1:











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// Java program to demonstrate 


// remove() 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("Map: " + my_cmmap); 


        System.out.println(); 


  


        // Removing the mapping 


        // with existing pair 6, 1 


        // using remove() method 


        boolean valueRemoved = my_cmmap.remove("6", "1"); 


  


        // Printing the map after remove() 


        System.out.println("After removing mapping with pair (6, 1):"); 


        System.out.println("Map: " + my_cmmap); 


        System.out.println("Was value removed: " + valueRemoved); 


        System.out.println(); 


  


        // Removing the mapping 


        // with non-existing pair 1, 2 


        // using remove() method 


        valueRemoved = my_cmmap.remove("1", "2"); 


  


        // Printing the map after remove() 


        System.out.println("After removing mapping with pair (1, 2):"); 


        System.out.println("Map: " + my_cmmap); 


        System.out.println("Was value removed: " + valueRemoved); 


        System.out.println(); 


    } 


} 



















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Output:


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

After removing mapping with pair (6, 1):
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Was value removed: true

After removing mapping with pair (1, 2):
Map: {1=1, 2=1, 3=1, 4=1, 5=1}
Was value removed: false





Program 2: To demonstrate NullPointerException











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                                    code
                                




















// Java program to demonstrate 


// remove() 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("Map: " + my_cmmap); 


        System.out.println(); 


  


        try { 


            // Removing the mapping 


            // with null key 


            // using remove() method 


            my_cmmap.remove(null, "1"); 


        } 


        catch (Exception e) { 


            System.out.println("Exception: " + e); 


        } 


    } 


} 



















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Output:


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

Exception: java.lang.NullPointerException





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