Implementation of Queues using Linked List in Java

// Node class
class Node {

    int data;
    Node next;

    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

// Queue using Linked List
class LinkedListQueue {

    private Node front;
    private Node rear;

    // Constructor
    public LinkedListQueue() {
        front = rear = null;
    }

    // Enqueue (Insert)
    public void enqueue(int value) {

        Node newNode = new Node(value);

        // If queue is empty
        if (rear == null) {
            front = rear = newNode;
        } else {
            rear.next = newNode;
            rear = newNode;
        }

        System.out.println(value + " inserted into queue");
    }

    // Dequeue (Delete)
    public int dequeue() {

        if (front == null) {
            System.out.println("Queue is empty (Underflow)");
            return -1;
        }

        int removedValue = front.data;
        front = front.next;

        // If queue becomes empty
        if (front == null) {
            rear = null;
        }

        return removedValue;
    }

    // Peek (View front element)
    public int peek() {

        if (front == null) {
            System.out.println("Queue is empty");
            return -1;
        }

        return front.data;
    }

    // Check if queue is empty
    public boolean isEmpty() {
        return front == null;
    }

    // Display queue
    public void display() {

        if (front == null) {
            System.out.println("Queue is empty");
            return;
        }

        Node temp = front;

        System.out.print("Queue: ");
        while (temp != null) {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }

        System.out.println();
    }

    // Main method
    public static void main(String[] args) {

        LinkedListQueue queue = new LinkedListQueue();

        // Enqueue operations
        queue.enqueue(10);
        queue.enqueue(20);
        queue.enqueue(30);

        // Display
        queue.display();

        // Peek
        System.out.println("Front element: " + queue.peek());

        // Dequeue
        System.out.println("Removed: " + queue.dequeue());
        System.out.println("Removed: " + queue.dequeue());

        // Display again
        queue.display();

        // More deletions
        System.out.println("Removed: " + queue.dequeue());
        System.out.println("Removed: " + queue.dequeue()); // Underflow

        // Check empty
        System.out.println("Is queue empty? " + queue.isEmpty());
    }
}