Java Queue Interface

Java Queue Interface

In this tutorial, we will find out about the Java Queue interface and its methods.

The Queue interface of the Java assortments system gives the functionality of the queue data structure. It broadens the Collection interface.

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Classes that Implement Queue

Since the Queue is an interface, we cannot provide the direct implementation of it.

In order to use the functionalities of Queue, we need to use classes that implement it:

ArrayDeque
LinkedList
PriorityQueue

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Interfaces that extend Queue

The Queue interface is also extended by various subinterfaces:

• Deque
• BlockingQueue
• BlockingDeque

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Working of Queue Data Structure

In queues, elements are stored and accessed in First In, First Out manner. That is, elements are added from the behind and removed from the front.

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How to use a Queue?

In Java, we must import java.util.Queue package in order to use Queue.

// LinkedList implementation of Queue
Queue<String> animal1 = new LinkedList<>();

// Array implementation of Queue
Queue<String> animal2 = new ArrayDeque<>();

// Priority Queue implementation of Queue
Queue<String> animal 3 = new PriorityQueue<>();

Here, we have created objects animal1, animal2 and animal3 of classes LinkedList, ArrayDeque and PriorityQueue respectively. These objects can use the functionalities of the Queue interface.

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Methods of Queue

The Queue interface incorporates all the strategies for the Collection interface. It is on the grounds that Collection is the super interface of Queue.

A portion of the commonly used strategies for the Queue interface are:

• add() – Inserts the predefined component into the queue. If the task is effective, add() returns true, if not it tosses a special case.
• offer() – Inserts the predefined component into the line. If the task is fruitful, offer() returns true, if not it returns false.
• element() – Returns the head of the queue. Throws an exception if the queue is empty.
• peek() – Returns the head of the queue. Returns null if the queue is empty.
• remove() – Returns and removes the head of the queue. Throws an exception if the queue is empty.
• poll() – Returns and removes the head of the queue. Returns null if the queue is empty.

1. Implementing the LinkedList Class

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<>();

        // offer elements to the Queue
        numbers.offer(1);
        numbers.offer(2);
        numbers.offer(3);
        System.out.println("Queue: " + numbers);

        // Access elements of the Queue
        int accessedNumber = numbers.peek();
        System.out.println("Accessed Element: " + accessedNumber);

        // Remove elements from the Queue
        int removedNumber = numbers.poll();
        System.out.println("Removed Element: " + removedNumber);

        System.out.println("Updated Queue: " + numbers);
    }
}

Output

Queue: [1, 2, 3]
Accessed Element: 1
Removed Element: 1
Updated Queue: [2, 3]

To learn more, visit Java LinkedList.

2. Implementing the PriorityQueue Class

import java.util.Queue;
import java.util.PriorityQueue;

class Main {

    public static void main(String[] args) {
        // Creating Queue using the PriorityQueue class
        Queue<Integer> numbers = new PriorityQueue<>();

        // offer elements to the Queue
        numbers.offer(5);
        numbers.offer(1);
        numbers.offer(2);
        System.out.println("Queue: " + numbers);

        // Access elements of the Queue
        int accessedNumber = numbers.peek();
        System.out.println("Accessed Element: " + accessedNumber);

        // Remove elements from the Queue
        int removedNumber = numbers.poll();
        System.out.println("Removed Element: " + removedNumber);

        System.out.println("Updated Queue: " + numbers);
    }
}

Output

Queue: [1, 5, 2]
Accessed Element: 1
Removed Element: 1
Updated Queue: [2, 5]

To learn more, visit Java PriorityQueue.

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In the next tutorials, we will learn about different subinterfaces of the Queue interface and its implementation in detail.

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Java Queue