Data Structure: Queue

Queue.java

//Queue
//Author: Milon

import java.util.NoSuchElementException;

//Node class
class Node{
    //Instance variable
    private Object data;
    private Node nextNode;

    //Constructor
    public Node(){
        data = null;
        nextNode = null;
        }

    public Node(Object item, Node next){
        data = item;
        nextNode = next;
        }

    //Set methods
    public void setData(Object item){
        data = item;
        }

    public void setNextNode(Node next){
        nextNode = next;
        }

    //Get methods
    public Object getData(){
        return data;
        }

    public Node getNextNode(){
        return nextNode;
        }

    //Overloaded toString() method
    public String toString(){
        return ("" + data);
        }
    }

//Queue class
public class Queue{
    //Instance variable
    private Node head;
    private Node tail;
    private int size;

    //Constructor
    public Queue(){
        head = tail = null;
        size = 0;
        }

    //Returns is the queue empty
    public boolean isEmpty(){
        return head == null;
        }

    //Returns the size of the Queue
    public int size(){
        return size;
        }

    //Push method
    public void push(Object item){
        head = tail = new Node(item, head);
        ++size;
        }

    //pop method
    public Object pop(){
        if(isEmpty()){
            System.out.println("Stack is empty.");
            //throw new NoSuchElementException;
            return null;
            }
        if(head.getNextNode() == tail){
            Node current = head;
            head = tail = null;
            return current.getData();
            }
        Node current = head;
        while(current.getNextNode().getNextNode() !=null)
            current = current.getNextNode();
        Object obj = current.getNextNode().getData();
        current.setNextNode(null);
        tail = current;
        --size;
        return obj;
        }

    //Overloaded toString method to show the entire list
    public String toString(){
        String str = "[ ";
        if(head != null){
            str += head.getData();
            Node current = head.getNextNode();
            while(current.getNextNode() != null){
                str += ", " + current.getData();
                current = current.getNextNode();
                }
            str += ", " + current.getData();
            }
        str += " ]";
        return str;
        }
    }

QueueImplement.java

//Queue Implementation class
//Author: Milon

import java.util.Scanner;
import java.util.Random;

public class QueueImplement{
    public static void main(String args[]){
        Scanner input = new Scanner(System.in);
        Random rand = new Random();
        Queue queue= new Queue();

        System.out.print("How many number of element do you want: ");
        int n = input.nextInt();

        for(int i=0;i<n;i++)
            queue.push(new Integer(rand.nextInt(100)));

        while(true){
            System.out.println("\n\n");
            System.out.println("* * * * * Queue Implementation * * * * *");
            System.out.println("1\. Show the whole queue.");
            System.out.println("2\. Show does the queue empty.");
            System.out.println("3\. Insert an element.");
            System.out.println("4\. Delete an element.");
            System.out.println("5\. Show the queue size.");
            System.out.println("6\. Exit.");
            System.out.println("\n\n");

            System.out.print("Enter your choice: ");
            int choice = input.nextInt();

            switch(choice){
                case 1:
                        System.out.println("The queue is: " + queue.toString());
                        break;

                case 2:
                        if(queue.isEmpty())
                            System.out.println("The queue is empty.");
                        else
                            System.out.println("The queue is not empty.");
                        break;

                case 3:
                        System.out.print("Inter the element: ");
                        int item = input.nextInt();
                        queue.push(new Integer(item));
                        System.out.println("Item inserted successfully.");
                        break;

                case 4:
                        System.out.println("Item " + queue.pop() + " deleted successfully.");
                        break;

                case 5:
                        if(queue.isEmpty())
                            System.out.println("The queue is empty.");
                        else
                            System.out.println("Size of the queue is " + queue.size());
                        break;

                case 6:
                        System.exit(1);
                        break;

                default:
                        break;
                }
            }
        }
    }