BlockingDeque

Table of Contents

• BlockingDeque Usage
  • BlockingDeque methods
• BlockingDeque Extends BlockingQueue
• BlockingDeque Implementations
• BlockingDeque Code Example

The BlockingDeque interface in the java.util.concurrent class represents a deque which is thread safe to put into, and take instances from. In this text I will show you how to use this BlockingDeque.

The BlockingDeque class is a Deque which blocks threads tring to insert or remove elements from the deque, in case it is either not possible to insert or remove elements from the deque.

A deque is short for "Double Ended Queue". Thus, a deque is a queue which you can insert and take elements from, from both ends.

BlockingDeque Usage

A BlockingDeque could be used if threads are both producing and consuming elements of the same queue. It could also just be used if the producting thread needs to insert at both ends of the queue, and the consuming thread needs to remove from both ends of the queue. Here is an illustration of that:

A BlockingDeque - threads can put and take from both ends of the deque.

A thread will produce elements and insert them into either end of the queue. If the deque is currently full, the inserting thread will be blocked until a removing thread takes an element out of the deque. If the deque is currently empty, a removing thread will be blocked until an inserting thread inserts an element into the deque.

BlockingDeque methods

A BlockingDeque has 4 different sets of methods for inserting, removing and examining the elements in the deque. Each set of methods behaves differently in case the requested operation cannot be carried out immediately. Here is a table of the methods:

	Throws Exception	Special Value	Blocks	Times Out
Insert	addFirst(o)	offerFirst(o)	putFirst(o)	offerFirst(o, timeout, timeunit)
Remove	removeFirst(o)	pollFirst(o)	takeFirst(o)	pollFirst(timeout, timeunit)
Examine	getFirst(o)	peekFirst(o)

	Throws Exception	Special Value	Blocks	Times Out
Insert	addLast(o)	offerLast(o)	putLast(o)	offerLast(o, timeout, timeunit)
Remove	removeLast(o)	pollLast(o)	takeLast(o)	pollLast(timeout, timeunit)
Examine	getLast(o)	peekLast(o)

The 4 different sets of behaviour means this:

1. Throws Exception: 
   If the attempted operation is not possible immediately, an exception is thrown.
2. Special Value: 
   If the attempted operation is not possible immediately, a special value is returned (often true / false).
3. Blocks: 
   If the attempted operation is not possible immedidately, the method call blocks until it is.
4. Times Out: 
   If the attempted operation is not possible immedidately, the method call blocks until it is, but waits no longer than the given timeout. Returns a special value telling whether the operation succeeded or not (typically true / false).

BlockingDeque Extends BlockingQueue

The BlockingDeque interface extends the BlockingQueue interface. That means that you can use aBlockingDeque as a BlockingQueue. If you do so, the various inserting methods will add the elements to the end of the deque, and the removing methods will remove the elements from the beginning of the deque. The inserting and removing methods of the BlockingQueue interface, that is.

Here is a table of what the methods of the BlockingQueue does in a BlockingDeque implementation:

BlockingQueue	BlockingDeque
add()	addLast()
offer() x 2	offerLast() x 2
put()	putLast()
remove()	removeFirst()
poll() x 2	pollFirst()
take()	takeFirst()
element()	getFirst()
peek()	peekFirst()

BlockingDeque Implementations

Since BlockingDeque is an interface, you need to use one of its many implementations to use it. Thejava.util.concurrent package has the following implementations of the BlockingDeque interface:

• LinkedBlockingDeque

BlockingDeque Code Example

Here is a small code example of how to use the BlockingDeque methods:

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

deque.addFirst("1");
deque.addLast("2");

String two = deque.takeLast();
String one = deque.takeFirst();