LinkedBlockingQueue Class in Java
Last Updated : 20 Feb, 2025
The LinkedBlockingQueue in Java is part of the java.util.concurrent package and implements the BlockingQueue interface. It provides a thread-safe, bounded, or unbounded queue used for managing tasks in a producer-consumer scenario. This queue can be used in multithreaded environments where one thread produces data that other threads consume.
- This class is thread-safe.
- The queue supports blocking operations.
- If the constructor is provided with a capacity, it becomes a bounded queue, meaning the maximum number of elements it can hold is limited.
- If no capacity is provided, the queue is unbounded and can grow as required by the number of elements added.
- The LinkedBlockingQueue follows First In First Out (FIFO) ordering for both the insertion and removal of elements.
Example: This example demonstrates how to insert elements into a LinkedBlockingQueue and print its contents.
Java // Java Program to demonstrate // how LinkedBlockingQueue works import java.util.concurrent.LinkedBlockingQueue; public class Geeks { public static void main(String[] args) throws InterruptedException { // Create a LinkedBlockingQueue // with a capacity of 3 LinkedBlockingQueue<String> q = new LinkedBlockingQueue<>(3); // Insert elements into the queue q.put("Geek 1"); q.put("Geek 2"); q.put("Geek 3"); // Print the elements of the queue System.out.println("LinkedBlockingQueue is: "); for (String i : q) { System.out.println(i); } } } OutputLinkedBlockingQueue is: Geek 1 Geek 2 Geek 3
LinkedBlockingQueue Hierarchy
Declaration of LinkedBlockingQueue
In Java, the declaration of LinkedBlockingQueue can be done as:
LinkedBlockingQueue<Type> queue = new LinkedBlockingQueue<>();
Note: Here the Type is the type of the element the queue will store(e.g String, Integer etc)
Constructors
Constructor | Description |
|---|
LinkedBlockingQueue() | This creates a LinkedBlockingQueue with a capacity of Integer.MAX_VALUE. |
LinkedBlockingQueue(int capacity) | This creates a LinkedBlockingQueue with the given (fixed) capacity |
LinkedBlockingQueue(Collection<? extends E> c) | This creates a LinkedBlockingQueue with a capacity of Integer.MAX_VALUE, initially containing the elements of the given collection, added in traversal order of the collection’s iterator. |
Example 1: This example demonstrates how to create a LinkedBlockingQueue and insert elements to it using the add() method.
Java // Java program to demonstrate // LinkedBlockingQueue() constructor import java.util.concurrent.LinkedBlockingQueue; public class Geeks{ public static void main(String[] args) { // create object of LinkedBlockingQueue // using LinkedBlockingQueue() constructor LinkedBlockingQueue<Integer> l = new LinkedBlockingQueue<Integer>(); // add numbers l.add(1); l.add(2); l.add(3); System.out.println("LinkedBlockingQueue:" + l); } } OutputLinkedBlockingQueue:[1, 2, 3]
Example 2: This example demonstrates how to create a LinkedBlockingQueue with a specified initial capacity and add elements to it.
Java // Java program to demonstrate // LinkedBlockingQueue(int initialCapacity) constructor import java.util.concurrent.LinkedBlockingQueue; public class Geeks { public static void main(String[] args) { // define capacity of LinkedBlockingQueue int c = 15; // create object of LinkedBlockingQueue // using LinkedBlockingQueue(int initialCapacity) // constructor LinkedBlockingQueue<Integer> l = new LinkedBlockingQueue<Integer>(c); // add numbers l.add(1); l.add(2); l.add(3); // print queue System.out.println("LinkedBlockingQueue:" + l); } } OutputLinkedBlockingQueue:[1, 2, 3]
Example 3: This example demonstrates how to create a LinkedBlockingQueue using a collection like a vector to initialize the queue.
Java // Java program to demonstrate // LinkedBlockingQueue(Collection c) constructor import java.util.concurrent.LinkedBlockingQueue; import java.util.*; public class Geeks { public static void main(String[] args) { // Creating a Collection Vector<Integer> v = new Vector<Integer>(); v.addElement(1); v.addElement(2); v.addElement(3); // create object of LinkedBlockingQueue // using LinkedBlockingQueue(Collection c) // constructor LinkedBlockingQueue<Integer> l = new LinkedBlockingQueue<Integer>(v); // print queue System.out.println("LinkedBlockingQueue:" + l); } } OutputLinkedBlockingQueue:[1, 2, 3]
Performing Various Operations on LinkedBlockingQueue
1. Adding Elements: We can use add() method to insert elements in a LinkedBlockingQueue.
Example: This example demonstrates how to create a LinkedBlockingQueue with a specified capacity of 15, add elements to the queueusing the add() and print the content of the queue.
Java // Java Program to Demonstrate adding // elements to the LinkedBlockingQueue import java.util.concurrent.LinkedBlockingQueue; public class Geeks { public static void main(String[] args) { // define capacity of LinkedBlockingQueue int c = 15; // create object of LinkedBlockingQueue LinkedBlockingQueue<Integer> l = new LinkedBlockingQueue<Integer>(c); // add numbers l.add(1); l.add(2); l.add(3); // print queue System.out.println("LinkedBlockingQueue:" + l); } } OutputLinkedBlockingQueue:[1, 2, 3]
2. Remove Elements: We can use remove() method to remove elements from the LinkedBlockingQueue.
Example: This example demonstrates how to remove a specified element from the queue using the remove().
Java // Java Program to demonstrate removing // elements from the LinkedBlockingQueue import java.util.concurrent.LinkedBlockingQueue; public class Geeks { public static void main(String[] args) { // define capacity of LinkedBlockingQueue int c = 15; // create object of LinkedBlockingQueue LinkedBlockingQueue<Integer> l = new LinkedBlockingQueue<Integer>(c); // add numbers l.add(1); l.add(2); l.add(3); // print queue System.out.println("LinkedBlockingQueue:" + l); // remove element 2 l.remove(2); // print queue System.out.println("Updated LinkedBlockingQueue:" + l); } } OutputLinkedBlockingQueue:[1, 2, 3] Updated LinkedBlockingQueue:[1, 3]
3. Iterating Elements: We can use the iterator() method of LinkedBlockingQueue which return an iterator that allows traversing through the elements of the queue.
Example:
Java // Java Program Demonstrate iterating // over LinkedBlockingQueue import java.util.concurrent.LinkedBlockingQueue; import java.util.Iterator; public class Geeks { public static void main(String[] args) { // define capacity of LinkedBlockingQueue int c = 7; // create object of LinkedBlockingQueue LinkedBlockingQueue<String> l = new LinkedBlockingQueue<String>(c); // Add element to LinkedBlockingQueue l.add("1"); l.add("2"); l.add("3"); l.add("4"); // create Iterator of linkedQueue using iterator() method Iterator<String> i = l.iterator(); // print result System.out.println("Elements in the LinkedBlockingQueue: "); while (i.hasNext()) System.out.print(i.next() + " "); } } OutputElements in the LinkedBlockingQueue: 1 2 3 4
4. Accessing Elements: We can use peek() to retrieve the head of the LinkedBlockingQueue without removing it.
Example:
Java // Java Program Demonstrate accessing // elements of LinkedBlockingQueue import java.util.concurrent.LinkedBlockingQueue; public class Geeks { public static void main(String[] args) { // define capacity of LinkedBlockingQueue int c = 7; // create object of LinkedBlockingQueue LinkedBlockingQueue<String> l = new LinkedBlockingQueue<String>(c); // Add element to LinkedBlockingQueue l.add("Java"); l.add("C++"); l.add("Python"); l.add("Js"); // find head of linkedQueue using peek() method String s = l.peek(); // print result System.out.println("LinkedBlockingQueue: " + l); // print head of queue System.out.println("Head of LinkedBlockingQueue is: " + s); // removing one element l.remove(); // again get head of queue s = l.peek(); // print result System.out.println("Removing one element from Queue"); System.out.println("Updated LinkedBlockingQueue is: " + l); // print head of queue System.out.println("Updated Head of LinkedBlockingQueue is: " + s); } } Output:
Methods
Method | Description |
|---|
| clear() | Atomically removes all of the elements from this queue. |
| contains(Object o) | Returns true if this queue contains the specified element. |
| drainTo(Collection<? super E> c) | Removes all available elements from this queue and adds them to the given collection. |
| drainTo(Collection<? super E> c, int maxElements) | Removes at most the given number of available elements from this queue and adds them to the given collection. |
| 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. |
| 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 if it is possible to do so immediately without exceeding the queue’s capacity, returning true upon success and false if this queue is full. |
| offer(E e, long timeout, TimeUnit unit) | Inserts the specified element at the tail of this queue, waiting if necessary up to the specified wait time for space to become available. |
| put(E e) | Inserts the specified element at the tail of this queue, waiting if necessary for space to become available. |
| remainingCapacity() | Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking. |
| 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 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.AbstractCollection
Methods | Description |
|---|
| containsAll(Collection<?> c) | Returns true if this collection contains all of the elements in the specified collection. |
| isEmpty() | Returns true if this collection contains no elements. |
| toString() | Returns a string representation of this collection. |
Methods declared in class java.util.AbstractQueue
Methods | Description |
|---|
| add(E e) | Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success, and throwing an IllegalStateException if no space is currently available. |
| addAll(Collection<? extends E> c) | Adds all of the elements in the specified collection to 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 interface java.util.concurrent.BlockingQueue
Methods | Description |
|---|
| add(E e) | Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success, and throwing an IllegalStateException if no space is currently available. |
| poll(long timeout, TimeUnit unit) | Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available. |
| take() | Retrieves and removes the head of this queue, waiting if necessary until an element becomes available. |
Methods declared in interface java.util.Collection
Methods | Description |
|---|
| addAll(Collection<? extends E> c) | Adds all of the elements in the specified collection to 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. |
| isEmpty() | Returns true if this collection contains no elements. |
| 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
Methods | 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. |
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