Queue Implementation in Java using Array With Examples [Latest]

Queue is a special type of data structure, which is designed to hold elements before processing and process elements in a FIFO (first-in-first-out) manner. It’s part of java collections framework. In this tutorial, we will learn Queue implementation in Java using an array.

Basic Queue Functions

A Queue must have the following functions:

• enqueue(obj) – insert element to the queue.
• dequeue() – remove and return the least recent item from the queue.
• isEmpty() – returns true if the queue is empty, else false.

Queue Implementation in Java

We can implement basic Queue functions using an array.

Here is the complete code to implement a Queue in Java.

package com.journaldev.java;

public class MyQueue {

	public static final int DEFAULT_SIZE = 5;

	private Object data[];

	private int index;

	public MyQueue() {
		data = new Object[DEFAULT_SIZE];
	}

	public MyQueue(int size) {
		data = new Object[size];
	}

	public boolean isEmpty() {
		return index == 0;
	}

	public void enqueue(Object obj) throws Exception {
		if (index == data.length - 1) {
			throw new Exception("Queue is full. Dequeue some objects");
		}
		this.data[index] = obj;
		this.index++;
	}

	public Object dequeue() throws Exception {
		if (isEmpty())
			throw new Exception("Queue is empty");
		Object obj = this.data[0];
		for (int i = 0; i < this.index - 1; i++) {
			data[i] = data[i + 1];
		}
		this.index--;
		return obj;

	}
}

Important Points

• There are two constructors – one to create a Queue with default size, the other one is used to specify the queue size.
• We are using a private integer variable “index” to manage the queue elements.
• We are using an Object array so that we can hold any type of object in the Queue. We can use generics here too but I am avoiding that to keep the program simple.
• If the queue is full, the enqueue() will throw an exception with proper message.
• If the queue is empty and we call the dequeue() function, an exception is raised.

Implementation Limitations
The Queue methods are not synchronized and it’s not thread-safe. There is a chance of data inconsistency if you use this implementation in a multi-threaded environment.

Test Program to Check Queue Implementation

Let’s test our queue implementation with some calls to enqueue() and dequeue() functions.

MyQueue queue = new MyQueue();
queue.enqueue("1");
System.out.println(queue.dequeue());

queue.enqueue("2");
queue.enqueue("3");
queue.enqueue("4");
System.out.println(queue.dequeue());

queue.enqueue("5");
queue.enqueue("6");
// queue.enqueue("7");
// queue.enqueue("8");

Output:

1
2

If you uncomment the last two lines, the queue will get full and an exception will be raised.

Exception in thread "main" java.lang.Exception: Queue is full. Dequeue some objects
	at com.journaldev.java.MyQueue.enqueue(MyQueue.java:25)
	at com.journaldev.java.MyQueue.main(MyQueue.java:56)

Conclusion

A Queue is one of the simplest data structures. We can implement it using an array or a list.