The **Queue interface** is available in `java.util` package and extends the *collection* interface. A **queue**<sup>1</sup> is an ordered list of objects with its use limited to insert elements at the end of a list and deleting elements from the start of a list. It follows the First-In-First-Out (FIFO) principle. 

Below is a visualization of the insertion of objects in a queue. Objects are always inserted at the tail of the queue.

Below is a visualization of the removal of objects in a queue. Objects are always removed from the head of the queue.

The most common classes that implement Queue interface in Java are: 
* PriorityQueue
* LinkedList

# Commonly used methods
* **add()**: is used to add an element at the tail of the queue. The element to be inserted is passed as the parameter. The method throws an exception if the operation is unsuccessful, and returns `true` if the operation is successful.

* **offer()**: is used to add an element at the tail of the queue. The element to be inserted is passed as the parameter. The method returns `false` if the operation is unsuccessful, and returns `true` if the operation is successful.

* **remove()**: returns the head of the queue and removes it. The method throws an exception if the queue is empty. 

* **poll()**: the `remove()` method returns the head of the queue and removes it. It returns `null` if the queue is empty.  

* **peek()**: returns the head of the queue without removing it. It returns `null` if the queue is empty.

* **element()**: the `peek()`  method returns the head of the queue without removing it. It throws an exception if the queue is empty.

* **size()**: returns the number of elements in the queue.

* **isEmpty()**: returns `true` if the queue is empty, otherwise it returns `false`.

## Other implementing classes

* AbstractQueue
* ArrayBlockingQueue
* ArrayDeque
* ConcurrentLinkedDeque 
* ConcurrentLinkedQueue
* DelayQueue
* LinkedBlockingDeque, 
* LinkedBlockingQueue
* LinkedTransferQueue,
* PriorityBlockingQueue, 
* PriorityQueue
* SynchronousQueue


### References
<sup>1</sup> https://www.educative.io/answers/what-is-a-queue

What is the Java Queue interface?

The Queue interface is available in java.util package and extends the collection interface. A queue1 is an ordered list of objects with its use limited to insert elements at the end of a list and deleting elements from the start of a list. It follows the First-In-First-Out (FIFO) principle. Below is a visualization of the insertion of objects in a queue. Objects are always inserted at the tail of the queue. 