Queue Overview

Queue is the linear data structure that follows a first-in-first-out (FIFO) order. It is a pre-defined Interface present in java.util package, Introduced from JDK1.5.

The primary implementation classes of the queue is

PriorityQueue
LinkedList

Important Methods of Queue

enQue() - This method will add an object into Queue at the rear end.
deQue() - This method will remove the object from the Queue from the front end.
isFull() - returns true if the Queue is full.
isEmpty() - returns true if Queue is empty.
display() - returns all the objects present in Queue.

Features

Priority Queue is an Implementation class of Queue Interface.
Present in java.util pack and introduced from JDK1.5
Only comparable type objects can be inserted in the Priority Queue.

Methods of Priority Queue

Method	Description
boolean add(object);	It is used to insert the element into this queue and return true upon success.
Object remove();	It is used to remove the head object of this queue.
Object poll();	It is used to remove the object object from the head, or return null if Queue is empty.
Object peak();	It is used to retrieve or read the head object of this Queue. Or return null if the retrieveQueue is empty.
Object element();	It is used to retrieves, but doesn't remove, the head of this Queue.

Example:

package com.quipoin;

import java.util.PriorityQueue;

public class QueueDemo {
	public static void main(String[] args) {
		PriorityQueue<Integer> queue=new PriorityQueue<>();
		queue.add(10);
		queue.add(20);
		queue.add(30);
		queue.add(40);
		queue.add(50);

		System.out.println(queue);

		queue.poll();//retrieves and removes head element
		System.out.println(queue);
		System.out.println("--------------------");
		queue.clear();//This method will clear the elements from the Queue.
		System.out.println(queue);
	}
}

Output:

[10, 20, 30, 40, 50]
[20, 40, 30, 50]

Key Point

Queue follows FIFO principle.
Priority Queue orders elements based on natural ordering or a comparator.
Queue methods help efficiently manage elements.
PriorityQueue not allow null elements.
It is not thread-safe.

Two Minute Drill

Use Queue when you need an ordered data structure following FIFO.
Use PriorityQueue when priority-based ordering is required.