Java Queue isEmpty(), size(), element() and peek() Example

Let's demonstrate Queue interface methods - isEmpty(), size(), element(),peek() with examples.

• Check if a Queue is empty.
• Find the size of a Queue.
• Search for an element in a Queue.
• Get the element at the front of the Queue without removing it.

import java.util.LinkedList;
import java.util.Queue;

/**
 * Demonstrate Queue interface methods with LinkedList implementation.
 * @author javaguides.net
 *
 */

public class QueueSizeSearchFrontExample {
    public static void main(String[] args) {
        Queue<String> elementQueue = new LinkedList<>();

        elementQueue.add("element1");
        elementQueue.add("element2");
        elementQueue.add("element3");
        elementQueue.add("element4");

        System.out.println("WaitingQueue : " + elementQueue);

        // Check is a Queue is empty
        System.out.println("is waitingQueue empty? : " + elementQueue.isEmpty());

        // Find the size of the Queue
        System.out.println("Size of waitingQueue : " + elementQueue.size());

        // Check if the Queue contains an element
        String name = "Johnny";
        if(elementQueue.contains(name)) {
            System.out.println("WaitingQueue contains " + name);
        } else {
            System.out.println("Waiting Queue doesn't contain " + name);
        }

        // Get the element at the front of the Queue without removing it using element()
        // The element() method throws NoSuchElementException if the Queue is empty
        String firstElementInTheWaitingQueue =  elementQueue.element();
        System.out.println("Waiting Queue (element()) : " + firstElementInTheWaitingQueue);

        // Get the element at the front of the Queue without removing it using peek()
        // The peek() method is similar to element() except that it returns null if the Queue is empty
        firstElementInTheWaitingQueue = elementQueue.peek();
        System.out.println("Waiting Queue : " + firstElementInTheWaitingQueue);

    }
}

Output:

WaitingQueue : [element1, element2, element3, element4]
is waitingQueue empty? : false
Size of waitingQueue : 4
Waiting Queue doesn't contain Johnny
Waiting Queue (element()) : element1
Waiting Queue : element1

Reference

https://www.javaguides.net/2018/08/collections-framework-queue-interface.html