BlockingDeque in Java

In Java, the BlockingDeque interface is a part of the Java Collections Framework. It is an interface that extends Deque (double-ended queue) and provides methods for thread-safe operation. It allows threads to safely add and remove elements from both ends of the queue.

• BlockingDeque supports thread-safe operations for adding and removing elements from both ends.
• Methods like putFirst(), putLast(), takeFirst(), and takeLast() block the thread until space or an element is available.
• Methods like pollFirst() and pollLast() remove elements without blocking, returning null if empty.
• It allows adding/removing elements from both the front and rear of the deque.
• BlockingDeque is commonly implemented by LinkedBlockingQueue and ArrayBlockingQueue. The LinkedBlockingQueue provides a deque with an unbounded capacity, while ArrayBlockingQueue has a fixed capacity.

Example: This example demonstrates how to insert elements into a BlockingDeque and print its contents.

The elements in the BlockingDeque:
[10, 20, 30, 40, 50]

Java BlockingDeque Hierarchy

This below diagram demonstrates the BlockingDeque Hierarchy:

Declaration of BlockingDeque

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

BlockingDeque<Type> deque = new LinkedBlockingDeque<>();

Note: "Type" is the type of the element we want to store (e.g. Integer, String).

Classes in BlockingDeque

BlockingDeque is implemented by the class LinkedBlockingQueue, which uses a linked list to store elements. We can create a bounded or unbounded deque. If no capacity is specified, it defaults to Integer.MAX_VALUE.

To use BlockingDeque import it like this:

import java.util.concurrent.BlockingDeque;

or

import java.util.concurrent.*;

Example: This example demonstrates how to use LinkedBlockingQueue to add, iterate, remove an element, and print the contents of the deque.

The LinkedBlockingDeque contains:
134 245 23 122 90 

The element 23 has been removed

The LinkedBlockingDeque after remove operation contains:
[134, 245, 122, 90]

Performing Various Operations on BlockingDeque

1. Adding Elements: We can use add() to insert elements to the BlockingDeque.

Example: This example demonstrates how to add elements to a LinkedBlockingQueue and copy elements from one deque to another using addAll() method.

Contents of b1: [10, 20, 30, 40, 50]

Contents of b2: [10, 20, 30, 40, 50]

2. Accessing Elements: We can use methods like contains(), element(), peek(), poll() to access the elements of BlockingDeque.

Example:

The LinkedBlockingDeque, b contains:
[22, 125, 723, 172, 100]
The LinkedBlockingDeque, b contains 22
The head of b: 22
The tail of b: 100

3. Removing Elements: We can use remove() method to remove elements from the BlockingDeque.

Example: This example demonstrates how to remove specific elements, and handle the removal of a non-existent element from a LinkedBlockingDeque.

The LinkedBlockingDeque, b contains:
[75, 86, 13, 44, 10]

Element 1 not found in the deque.

After removal the LinkedBlockingDeque, b contains:
[75, 13, 10]

4. Iterating Elements: We can use the iterator() method to iterate through the elements of  LinkedBlockingDeque.

Example: This example demonstrates how to iterate over a LinkedBlockingDeque using an iterator to print its elements.

The LinkedBlockingDeque contains:
10 20 30 40 50 

Methods

The BlockingDeque interface has a various method which has to be defined by every implementing class. A deque can be implemented as a queue and as a stack, therefore the BlockingDeque interface provides methods to do the same. The table below provides a list of all the methods and their function.

Note: Here, E is the type of elements in the collection and TimeUnit is an enum that represents the time durations.

Methods Declared in Interface java.util.concurrent.BlockingQueue

Methods Declared in Interface java.util.Collection

Methods Declared in Interface java.util.Deque

Methods Declared in Interface java.lang.Iterable

The following are the methods provided by the BlockingDeque for insertion, removal, and examine operations on the deque. Each of the four sets of methods behaves differently if the requested operation is not satisfied immediately. 

• Throws Exception: An exception will be thrown, if the requested operation is not satisfied immediately.
• Special value: A special value is returned if the operation is not satisfied immediately.
• Blocks: The method call is blocked if the attempted operation is not satisfied immediately and it waits until it gets executed.
• Times out: A special value is returned telling whether the operation succeeded or not. If the requested operation is not possible immediately, the method call blocks until it is, but waits no longer than the given timeout. 

Methods on First Element (Head of Deque)

Methods on Last Element (Tail of Deque)

We know that we can insert, remove, and examine elements from both directions in the BlockingDeque. Since BlockingDeque extends BlockingQueue, the methods for insertion, removal, and examination operations acting on a direction on BlockingDeque are similar to BlockingQueue. The following comparison explains the same.