ConcurrentHashMap remove() method in Java - GeeksforGeeks

05 Feb, 2021
A. remove(Object key)
The remove(Object key) method of class ConcurrentHashmap in Java is used to remove the 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: 
Java
// Java program to demonstrate remove() method import java.util.*;import java.util.concurrent.ConcurrentHashMap; 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);    }}
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 with null key
Java
// Java program to demonstrate remove() method import java.util.*;import java.util.concurrent.ConcurrentHashMap; 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);        }    }}
Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

Exception: java.lang.NullPointerException

 
B. remove(Object key, Object value)
The remove(Object key, Object value) method of class ConcurrentHashmap in Java is used to remove the 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: 
Java
// Java program to demonstrate remove() method import java.util.*;import java.util.concurrent.ConcurrentHashMap; 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);    }}
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 with null key 
Java
// Java program to demonstrate// remove(key, value) null key import java.util.*;import java.util.concurrent.ConcurrentHashMap; 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);        }    }}
Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

Exception: java.lang.NullPointerException

 
Program 3: To demonstrate NullPointerException with the null value
Java
// Java program to demonstrate// remove(key, value) null value import java.util.*;import java.util.concurrent.ConcurrentHashMap; 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);         try {            // Removing the mapping with correct key and            // null value using remove() method            my_cmmap.remove("1", null);            my_cmmap.remove("7", null);        }        catch (Exception e) {            System.out.println("Exception: " + e);        }    }}
Map: {1=1, 2=1, 3=1, 4=1, 5=1, 6=1}

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