Queue Data Structure - The Coding Shala

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Queue Data Structure

In this post, we will see the basics of the queue. The queue is a Data Structure, also known as the First-in-first-out data structure(FIFO). In a queue, there are two main operations: Enqueue and Dequeue. The insert Operation is called enqueue and the new element is always added at the end of the queue. The delete operation is called dequeue and we only allowed to remove the first element.

Queue Java Program

The following is a simple implementation of the queue: 

import java.util.ArrayList;
import java.util.List;

//Queue implementation

class MyQueue{
 //to store data
 private List<Integer> data;
 private int q_start; //start point of queue
 
 //initial state
 MyQueue(){
  data = new ArrayList<Integer>();
  q_start = 0;
 }
 
 //Enqueue Operation
 //add element at the end of queue are return true if successful
 public boolean EnQueue(int element) {
  data.add(element);
  return true;
 }
 
 //DeQueue operation
 //remove element if there any
 //move start pointer
 public boolean DeQueue() {
  if(isEmpty()) return false;
  q_start++;
  return true;
 }
 
 //check is empty or not
 public boolean isEmpty() {
  if(data.size() <= q_start) return true;
  return false;
 }
 
 //return first element
 public int Front(){
  if(isEmpty()) return -1;
  return data.get(q_start);
 }
 
}


public class Main{
 public static void main(String[] args) {
  MyQueue queue = new MyQueue();
  queue.EnQueue(4);
  queue.EnQueue(5);
  queue.EnQueue(6);
  if(queue.isEmpty() == false) { //check if not empty
   System.out.println("First Element is: "+queue.Front());
  }
  queue.DeQueue();
  if(queue.isEmpty() == false) { //check if not empty
   System.out.println("First Element is: "+queue.Front());
  }
  queue.DeQueue();
  System.out.println("First Element is: "+queue.Front());
  
  queue.DeQueue();
  //now queue is empty  will return -1
  System.out.println("First Element is: "+queue.Front());
  
  if(queue.isEmpty() == false) { //check if not empty
   System.out.println("First Element is: "+queue.Front());
  }else {
   System.out.println("Queue is Empty");
  }
 }
}
Output: 

First Element is: 4
First Element is: 5
First Element is: 6
First Element is: -1
Queue is Empty


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