Queue Interface

The Queue Interface is part of the java.util package and extends the Collection interface. Queues are data structures that work in the FIFO (First In First Out) sequence. Queues are ordered lists and all elements are inserted at the end of the queue. The deletion of elements is done from the front of the queue.

Declaration

import java.util.*;
public interface Queue extends Collection

Queue Objects

Queue <Obj> queue = new PriorityQueue <obj>(); where obj defines the type of objects or elements contained in the queue.

Example

import java.util.LinkedList;
import java.util.Queue;
public class QueueDemo
{
public static void main(String[] args)
{
Queue<Integer> q = new LinkedList<>();
//Adding elements
for (int i=0; i<5; i++)
q.add(i);
System.out.println(“Elements of the Queue” + q);
//removing an element
int removable = q.remove();
System.out.println(“removed element” + removable);
System.out.println(q);
//To peek
int head = q.peek();
System.out.println(“head of the queue” + head);
int size = q.size();
System.out.println(“Size of the Queue” + size);
}
}

OUTPUT

Elements of the Queue [0, 1, 2, 3, 4] removed element – 0
[1, 2, 3, 4] head of the queue – 1
Size of the queue – 1

Deque Interface

Deque is the subtype of the queue interface. This is a type of queue that allows users to remove and add elements from both sides of the queue. Deques can serve both algorithms like FIFO (First In First Out) and LIFO (Last In First Out).

Declaration

public interface Deque extends Queue

Example

import java.util.*;
public class DequeExample
{
public static void main(String[] args)
{
Deque<String> deque = new LinkedList<String>();
deque.add("Element 1 (Tail)");
deque.addFirst("Element 2 (Head)");
deque.addLast("Element 3 (Tail)");
deque.push("Element 4 (Head)");
deque.offer("Element 5 (Tail)");
deque.offerFirst("Element 6 (Head)");
System.out.println(deque + "\n"); 
deque.removeFirst();
deque.removeLast();
System.out.println("Deque after removing " + "first and last: " + deque);
}
}

 

OUTPUT

[Element 6 (Head), Element 4 (Head), Element 2 (Head), Element 1 (Tail), Element 3 (Tail), Element 5 (Tail)] Deque after removing first and last: [Element 4 (Head), Element 2 (Head), Element 1 (Tail), Element 3 (Tail)]