Java Program to Implement the queue data structure

In this article, we will understand how to implement the queue data structure. A Queue is a linear structure which follows a particular order in which the operations are performed. The order is First In First Out (FIFO).

Below is a demonstration of the same −

Suppose our input is −

Input Queue: [150, 300, 450, 600]

The desired output would be −

After removing an element, the elements of the queue are: [300, 450, 600]

Algorithm

Step 1 - START
Step 2 - Declare namely
Step 3 - Add elements to it using the ‘offer’ method.
Step 4 - Display the queue content
Step 5 - Use the ‘poll’ method to delete the element from the queue.
Step 6 - Display the elements of the queue after calling the ‘poll’ method.
Step 7 - Display the result
Step 8 - Stop

Example 1

Here, , we use built-in defined functions to execute all stack operations.

import java.util.Queue;
import java.util.LinkedList;
public class Demo {
   public static void main(String[] args) {
      System.out.println("The required packages have been imported");
      Queue<Integer> input_queue = new LinkedList<>();
      input_queue.offer(150);
      input_queue.offer(300);
      input_queue.offer(450);
      input_queue.offer(600);
      System.out.println("The queue is defined as: " + input_queue);
      int removedNumber = input_queue.poll();
      System.out.println("After removing an element, the elements of the queue are: " +input_queue);
   }
}

Output

The required packages have been imported
The queue is defined as: [150, 300, 450, 600]
After removing an element, the elements of the queue are: [300, 450, 600]

Example 2

Here, we use user defined functions to execute all stack operations.

public class Queue {
   int SIZE = 5;
   int items[] = new int[SIZE];
   int front, rear;
   Queue() {
      front = -1;
      rear = -1;
   }
   boolean isFull() {
      if (front == 0 && rear == SIZE - 1) {
         return true;
      }
      return false;
   }
   boolean isEmpty() {
      if (front == -1)
         return true;
      else
         return false;
   }
   void enQueue(int element) {
      if (isFull()) {
         System.out.println("\nThe queue is full");
      }
      else {
         if (front == -1) {
            front = 0;
         }
         rear++;
         items[rear] = element;
         System.out.println("\nThe element " + element + " is inserted");
      }
   }
   int deQueue() {
      int element;
      if (isEmpty()) {
         System.out.println("\nThe queue is empty");
         return (-1);
      }
      else {
         element = items[front];
         if (front >= rear) {
            front = -1;
            rear = -1;
         }
         else {
            front++;
         }
         System.out.println("\nThe element " +element + " is deleted");
         return (element);
      }
   }
   void display() {
      int i;
      if (isEmpty()) {
         System.out.println("The queue is empty ");
      }
      else {
         System.out.println("\nThe elements of the queue are: ");
         for (i = front; i <= rear; i++)
            System.out.print(items[i] + " ");
      }
   }
   public static void main(String[] args) {
      Queue input_queue = new Queue();
      for(int i = 1; i < 6; i ++) {
         input_queue.enQueue(i * 100);
      }
      System.out.println("The queue is defined as: " + input_queue);
      input_queue.enQueue(6);
      input_queue.display();
      input_queue.deQueue();
      input_queue.display();
   }
}

Output

The element 100 is inserted

The element 200 is inserted

The element 300 is inserted

The element 400 is inserted

The element 500 is inserted
The queue is defined as: Queue@2a139a55

The queue is full

The elements of the queue are:
100 200 300 400 500
The element 100 is deleted

The elements of the queue are:
200 300 400 500