Example 1: Java program to implement Stack
public class Queue {
int SIZE = 5;
int items[] = new int[SIZE];
int front, rear;
Queue() {
front = -1;
rear = -1;
}
// check if the queue is full
boolean isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
// check if the queue is empty
boolean isEmpty() {
if (front == -1)
return true;
else
return false;
}
// insert elements to the queue
void enQueue(int element) {
// if queue is full
if (isFull()) {
System.out.println("Queue is full");
}
else {
if (front == -1) {
// mark front denote first element of queue
front = 0;
}
rear++;
// insert element at the rear
items[rear] = element;
System.out.println("Insert " + element);
}
}
// delete element from the queue
int deQueue() {
int element;
// if queue is empty
if (isEmpty()) {
System.out.println("Queue is empty");
return (-1);
}
else {
// remove element from the front of queue
element = items[front];
// if the queue has only one element
if (front >= rear) {
front = -1;
rear = -1;
}
else {
// mark next element as the front
front++;
}
System.out.println( element + " Deleted");
return (element);
}
}
// display element of the queue
void display() {
int i;
if (isEmpty()) {
System.out.println("Empty Queue");
}
else {
// display the front of the queue
System.out.println("\nFront index-> " + front);
// display element of the queue
System.out.println("Items -> ");
for (i = front; i <= rear; i++)
System.out.print(items[i] + " ");
// display the rear of the queue
System.out.println("\nRear index-> " + rear);
}
}
public static void main(String[] args) {
// create an object of Queue class
Queue q = new Queue();
// try to delete element from the queue
// currently queue is empty
// so deletion is not possible
q.deQueue();
// insert elements to the queue
for(int i = 1; i < 6; i ++) {
q.enQueue(i);
}
// 6th element can't be added to queue because queue is full
q.enQueue(6);
q.display();
// deQueue removes element entered first i.e. 1
q.deQueue();
// Now we have just 4 elements
q.display();
}
}
Output
Queue is empty Insert 1 Insert 2 Insert 3 Insert 4 Insert 5 Queue is full Front index-> 0 Items -> 1 2 3 4 5 Rear index-> 4 1 Deleted Front index-> 1 Items -> 2 3 4 5 Rear index-> 4
In the above example, we have implemented the queue data structure in Java.
To learn the working about the queue, visit Queue Data Structure.
Example 2: Implement stack using Queue interface
Java provides a built Queue interface that can be used to implement a queue.
import java.util.Queue;
import java.util.LinkedList;
class Main {
public static void main(String[] args) {
// Creating Queue using the LinkedList class
Queue<Integer> numbers = new LinkedList<>();
// enqueue
// insert element at the rear of the queue
numbers.offer(1);
numbers.offer(2);
numbers.offer(3);
System.out.println("Queue: " + numbers);
// dequeue
// delete element from the front of the queue
int removedNumber = numbers.poll();
System.out.println("Removed Element: " + removedNumber);
System.out.println("Queue after deletion: " + numbers);
}
}
Output
Queue: [1, 2, 3] Removed Element: 1 Queue after deletion: [2, 3]
In the above example, we have used the Queue interface to implement the queue in Java. Here, we have used the LinkedList class that implements the Queue interface.
- numbers.offer() - insert elements to the rear of the queue
- numbers.poll() - remove an element from the front of the queue
Notice, we have used the angle brackets <Integer> while creating the queue. It represents that the queue is of the generic type.
We can also use other interfaces and classes instead of Queue and LinkedList. For example,