What is the ConcurrentLinkedQueue.removeAll() method in Java?

The removeAll() method will remove all the elements of the passed collection from the ConcurrentLinkedQueue if present.

The ConcurrentLinkedQueue is thread-safe unbounded queue. The elements are ordered by FIFO(First-In-First-Out). The elements are inserted at the tail (end) and retrieved from the head (start) of the queue. The null elements are not allowed as an element of the queue.

Note: We can use the ConcurrentLinkedQueue when multiple threads are sharing a single Queue.

Syntax

boolean removeAll(Collection<?> c)

Argument

This method takes the collection object to be removed from the queue as an argument.

The NullPointerException is thrown if the passed collection contains a null element.

Return Value

This method returns true if the queue changed as a result of the call (any one of the elements from the Collection is present and removed from the queue). Otherwise, false will be returned.

Code

The code below demonstrates how to use the removeAll() method:

import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.ArrayList;
class RemoveAll {
    public static void main( String args[] ) {
        // create a new queu which can have integer elements
        ConcurrentLinkedQueue<Integer> queue = new ConcurrentLinkedQueue<>();
        // add elements to the queue
        queue.add(1);
        queue.add(2);
        queue.add(3);
        // create a new array list 
        ArrayList<Integer> list = new ArrayList<>();
        // add some elements to it 
        list.add(1);
        list.add(4);
        System.out.println("The queue is "+ queue);
        System.out.println("The list is "+ list);
        //remove all the elements of the list from the queue
        System.out.println("\nCalling queue.removeAll(list). Is queue changed - " + queue.removeAll(list));
        System.out.println("\nThe queue is "+ queue);
        
    }
}

Explanation

In the code above:

  • In line 1 and 2: We import the ConcurrentLinkedQueue and ArrayList classes.

  • From line 6-10: We create a new ConcurrentLinkedQueue object with the name queue and add three elements (1,2,3) to the queue object using the add() method.

  • From line 12-15: We create a new ArrayList object with the name list and add two elements (1,4) to list object using the add() method.

  • In line 21: We use the removeAll() method to remove all the elements of list from queue. The element 1 in the list is present in queue, so it is removed. The element 4 is not present in queue, so it is not removed. After calling the removeAll() method, the content of the queue object changes, so true is returned.

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