Java Queue Interface

Queue is a generic interface which extends Collection interface. Queue is used to store the elements in FIFO(First In First Out) manner.
Declaration
interface Queue<<E>>

Queue interface have the following methods.

Method	Description
`E element()`	Returns the element at the head of the queue without removing the element. Throws Exception if the queue is empty.
`boolean offer(E obj)`	Adds `obj` to the queue. Returns `true` if `obj`was added else returns`false`.
`E peek()`	Returns the element at the head of the queue without removing the element. Returns `null` if queue is empty.
`E poll()`	Returns the element at the head of the queue, removing the element. Returns `null` if queue is empty.
`E remove()`	Removes the element at the head of the queue, returning the element in process. Throws an exception if queue is empty.

The classes which implement the Queue interface are AbstractQueue, ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue

QueueDemo

CODE

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class QueueDemo
{
    public static void main(String arg[])
    {
        Queue<Integer> abq = new ArrayBlockingQueue<Integer>(3);
        abq.add(1);
        abq.add(2);
        abq.add(3);
        System.out.println("Till now we have added 3 numbers");
        System.out.println("Now adding fourth element violating queue behaviour");
        System.out.println("Added fourth element : " + abq.offer(4)); // LINE A
        System.out.println("Elements in ArrayBlockingQueue : " + abq);
        abq.clear(); // Clears the elements in Queue // LINE B
        System.out.println("Remove first element : " + abq.poll()); // LINE C    
    }
}

OUTPUT

Till now we have added 3 numbers
Now adding fourth element violating queue behaviour
Added fourth element : false
Elements in ArrayBlockingQueue : [1, 2, 3]
Remove first element : null

DESCRIPTION

In the above program we have created a ArrayBlockingQueue with the reference abq and restricted its capacityt to 3 and added three elements later at LINE A we tried to add fourth element using offer method which returned false since the capacity is restricted. At LINE B we have cleared the elements in the queue using clear method and tried to reomve the first element using poll method in the queue which returned false since the queue is empty

THINGS TO TRY

Replace the code at LINE A with the code below
System.out.println("Added fourth element : " + abq.add(4));
Since add method throws java.lang.IllegalStateException when the size of a ArrayBlockingQueue is violated.

Replace the code at LINE C with the code below
System.out.println("Remove first element : " + abq.remove());
Since remove method throws java.util.NoSuchElementException when an attempt is made to remove an element from an empty queue the above code throws a compilation error.

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