ConcurrentLinkedQueue in Java

Last Updated : 12 Feb, 2025

In Java, the ConcurrentLinkedQueue is the part of the java.util.concurrent package and implements a FIFO(First-In-First-Out) queue. It is a thread-safe, non-blocking, and scalable queue designed for use in highly concurrent environments. The queue uses a lock-free algorithm, ensuring that multiple threads can safely enqueue and dequeue elements without the need for external synchronization.

The queue allows multiple threads to perform operations without locking
It can be accessed by multiple threads concurrently without causing data corruption
The queue maintains the order of elements as they are added

Example: This example demonstrates creating a thread-safe ConcurrentLinkedQueue, adding elements to it and printing the queue.

Java

// Java Program to demonstrate the working of
// ConcurrentLinkedQueue
import java.util.concurrent.ConcurrentLinkedQueue;

public class Geeks {
    public static void main(String[] args)
    {
        // Create a ConcurrentLinkedQueue
        ConcurrentLinkedQueue<Integer> q
            = new ConcurrentLinkedQueue<>();

        // Adding elements to the queue
        q.offer(1);
        q.offer(2);
        q.offer(3);

        System.out.println("Queue after adding elements: "
                           + q);
    }
}

Output

Queue after adding elements: [1, 2, 3]

Hierarchy of ConcurrentLinkedQueue

java.lang.Object
↳ java.util.AbstractCollection<E>
↳ java.util.AbstractQueue<E>
↳ Class ConcurrentLinkedQueue<E>

ConcurrentLinkedQueue-in-Java-with-Examples

Declaration of ConcurrentLinkedQueue

In Java, the declaration of ConcurrentLinkedQueue can be done as:

ConcurrentLinkedQueue<Type> queueName = new ConcurrentLinkedQueue<>();

Constructors

Constructor	Description
ConcurrentLinkedQueue()	It is used to construct an empty queue.
ConcurrentLinkedQueue(Collection<E> c)	It is used to construct a queue with the elements of the Collection passed as the parameter.

Example 1: This example demonstrates creating and initializing a ConcurrentLinkedQueue both with individual elements and by copying another queue using a constructor.

Java

// Java Program demonstrating both with individual elements
// and by copying another queue using a constructor
import java.util.concurrent.*;

class Geeks {

    public static void main(String[] args)
    {
        // Create a ConcurrentLinkedQueue
        // using ConcurrentLinkedQueue() constructor
        ConcurrentLinkedQueue<Integer> q
            = new ConcurrentLinkedQueue<Integer>();

        q.add(10);
        q.add(20);
        q.add(30);
        q.add(40);

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue: " + q);

        // Create a ConcurrentLinkedQueue
        // using ConcurrentLinkedQueue(Collection c)
        // constructor
        ConcurrentLinkedQueue<Integer> q1
            = new ConcurrentLinkedQueue<Integer>(q);

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue1: " + q1);
    }
}

Output

ConcurrentLinkedQueue: [10, 20, 30, 40]
ConcurrentLinkedQueue1: [10, 20, 30, 40]

Example 2: This example demonstrates the use of ConcurrentLinkedQueue methods like peek(), poll() and size() to interact with and manipulate the queue.

Java

// Java Program to demosntrates the 
// working of peek(), poll(), size()
import java.util.concurrent.*;

class Geeks {

    public static void main(String[] args)
    {
        // Create a ConcurrentLinkedQueue
        // using ConcurrentLinkedQueue()
        // constructor
        ConcurrentLinkedQueue<Integer>
                q = new ConcurrentLinkedQueue<Integer>();

        q.add(10);
        q.add(20);
        q.add(30);
        q.add(40);

        System.out.println("ConcurrentLinkedQueue: "
                + q);

        // Displaying the first element
        // using peek() method
        System.out.println("First Element is: "
                + q.peek());

        // Remove and display the first element
        // using poll() method
        System.out.println("Head Element is: "
                + q.poll());

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue: "
                + q);

        // Get the size using size() method
        System.out.println("Size: "
                + q.size());
    }
}

Output

ConcurrentLinkedQueue: [10, 20, 30, 40]
First Element is: 10
Head Element is: 10
ConcurrentLinkedQueue: [20, 30, 40]
Size: 3

Performing Various Operations on ConcurrentLinkedQueue

1. Adding Elements: We use add() and addAll() method to insert elements in a ConcurrentLinkedQueue.

Example: This example demonstrates how to add individual elements and a collection of elements to a ConcurrentLinkedQueue using add() and addAll().

Java

// Java Program to demonstrate adding
// elements to ConcurrentLinkedQueue 
// using add() and addAll() method
import java.util.*;
import java.util.concurrent.*;

public class Geeks {

    public static void main(String[] args)
    {
        // Create an instance of ConcurrentLinkedQueue
        ConcurrentLinkedQueue<String> q
                = new ConcurrentLinkedQueue<String>();

        // Add String to queue using add method
        q.add("Java");
        q.add("C++");
        q.add("Python");
        q.add("Js");

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue: " + q);

        // create a ArrayList of Strings
        ArrayList<String> al = new ArrayList<String>();

        // add String to ArrayList
        al.add("Geek1");
        al.add("Geek2");
        al.add("Geek3");
        al.add("Geek4");
        al.add("Geek5");

        // Displaying the existing Collection
        System.out.println("Collection to be added: " + al);

        // apply addAll() method and passed
        // the arraylist as parameter
        boolean b = q.addAll(al);

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("Collection added: " + b);

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue: " + q);
    }
}

Output:

2. Removing Elements: We can use remove() to remove elements from the ConcurrentLinkedQueue.

Example: This example demonstrates how to remove a specific element from a ConcurrentLinkedQueue using the remove() method.

Java

// Java Program to Demonstrate the 
// working of remove() method
import java.util.concurrent.*;

public class Geeks {

    public static void main(String[] args)
    {
        // Create an instance of ConcurrentLinkedQueue
        ConcurrentLinkedQueue<Integer> q
            = new ConcurrentLinkedQueue<Integer>();

        // Add Numbers to queue using add(e) method
        q.add(10);
        q.add(20);
        q.add(30);
        q.add(40);

        System.out.println("ConcurrentLinkedQueue: " + q);

        // apply remove() for Number 78249
        boolean b = q.remove(20);

        System.out.println(
            "number 20 remove successfully? : " + b);

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("Updated ConcurrentLinkedQueue: "
                           + q);
    }
}

Output

ConcurrentLinkedQueue: [10, 20, 30, 40]
number 20 remove successfully? : true
Updated ConcurrentLinkedQueue: [10, 30, 40]

3. Iterating Elements: We can use the iterator() method of ConcurrentLinkedQueue which return an iterator that allows traversing through the elements of the queue in the FIFO order.

Example: This example demonstrates how to use the iterator() method of ConcurrentLinkedQueue to traverse and print its elements in FIFO order.

Java

// Java Program Demonstrate the 
// working of iterator() method
import java.util.*;
import java.util.concurrent.*;

public class Geeks {

    public static void main(String[] args)
    {
        // Create an instance of ConcurrentLinkedQueue
        ConcurrentLinkedQueue<String> q
            = new ConcurrentLinkedQueue<String>();

        // Add String to queue using add(e) method
        q.add("Java");
        q.add("C++");
        q.add("Python");
        q.add("js");

        // Displaying the existing ConcurrentLinkedQueue
        System.out.println("ConcurrentLinkedQueue: " + q);

        // Call iterator() method
        Iterator i = q.iterator();

        // Print elements of iterator
        System.out.println(
            "The String Values of iterator are:");
        while (i.hasNext()) {
            System.out.print(i.next() + " ");
        }
    }
}

Output

ConcurrentLinkedQueue: [Java, C++, Python, js]
The String Values of iterator are:
Java C++ Python js

4. Accessing Elements: We can use peek() and element() to access the elements of ConcurrentLinkedQueue.

Example: This example demonstrates how to access the head element of a ConcurrentLinkedQueue using element() and peek() method.

Java

// Java Program Demonstrate the 
// working of element() and peek()
import java.util.concurrent.*;

public class Geeks {

    public static void main(String[] args)
        throws IllegalStateException
    {
        // Create an instance of ConcurrentLinkedQueue
        ConcurrentLinkedQueue<Integer> q
            = new ConcurrentLinkedQueue<>();

        // Add numbers to end of Queue
        q.add(10);
        q.add(20);
        q.add(30);
        q.add(40);

        System.out.println("Queue: " + q);

        // print head using element()
        System.out.println("Queue's head: " + q.element());

        // print head using peek()
        System.out.println("Queue's head: " + q.peek());
    }
}

Output

Queue: [10, 20, 30, 40]
Queue's head: 10
Queue's head: 10

Methods of ConcurrentLinkedQueue

Method	Description
add(E e)	Inserts the specified element at the tail of this queue.
addAll(Collection<? extends E> c)	Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection’s iterator.
contains(Object o)	Returns true if this queue contains the specified element.
forEach(Consumer<? super E> action)	Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
isEmpty()	Returns true if this queue contains no elements.
iterator()	Returns an iterator over the elements in this queue in the proper sequence.
offer(E e)	Inserts the specified element at the tail of this queue.
remove(Object o)	Removes a single instance of the specified element from this queue, if it is present.
removeAll(Collection<?> c)	Removes all of this collection’s elements that are also contained in the specified collection (optional operation).
removeIf(Predicate<? super E> filter)	Removes all of the elements of this collection that satisfy the given predicate.
retainAll(Collection<?> c)	Retains only the elements in this collection that are contained in the specified collection (optional operation).
size()	Returns the number of elements in this queue.
spliterator()	Returns a Spliterator over the elements in this queue.
toArray()	Returns an array containing all of the elements in this queue, in the proper sequence.
toArray(T[] a)	Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array.

Methods Declared in Class java.util.AbstractQueue

Methods	Description
clear()	Removes all the elements from this queue.
element()	Retrieves, but does not remove, the head of this queue.
remove()	Retrieves and removes the head of this queue.

Methods Declared in Class java.util.AbstractCollection

Method	Description
containsAll(Collection<?> c)	Returns true if this collection contains all of the elements in the specified collection.
toString()	Returns a string representation of this collection.

Methods Declared in Interface java.util.Collection

Method	Description
clear()	Removes all of the elements from this collection (optional operation).
containsAll(Collection<?> c)	Returns true if this collection contains all of the elements in the specified collection.
equals(Object o)	Compares the specified object with this collection for equality.
hashCode()	Returns the hash code value for this collection.
parallelStream()	Returns a possibly parallel Stream with this collection as its source.
stream()	Returns a sequential Stream with this collection as its source.
toArray(IntFunction<T[]> generator)	Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.

Methods Declared in Interface java.util.Queue

Method	Description
element()	Retrieves, but does not remove, the head of this queue.
peek()	Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
poll()	Retrieves and removes the head of this queue, or returns null if this queue is empty.
remove()	Retrieves and removes the head of this queue.

ConcurrentLinkedQueue toArray() Method in Java

RishabhPrabhu

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Practice Tags :

ConcurrentLinkedQueue in Java

Hierarchy of ConcurrentLinkedQueue

Declaration of ConcurrentLinkedQueue

Constructors

Performing Various Operations on ConcurrentLinkedQueue

Methods of ConcurrentLinkedQueue

Methods Declared in Class java.util.AbstractQueue

Methods Declared in Class java.util.AbstractCollection

Methods Declared in Interface java.util.Collection

Methods Declared in Interface java.util.Queue

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