큐(Queue)

package 큐구현;

import java.util.NoSuchElementException;

class Queue<T>{

    class Node<T>{
        private Node<T> next;
        private T data;

        public Node(T data){
            this.data = data;
        }
    }

    private Node<T> start;
    private Node<T> end;


    public void add(T item){
        Node<T> node = new Node<T>(item);

        if(end!=null){
            end.next = node;
        }
        end = node;
        if(start==null){
            start = node;
        }
    }

    public T pull(){

        if(start==null){
            throw new NoSuchElementException();
        }

        T data = start.data;
        start = start.next;

        //조심
        if(start==null){
            end = null;
        }
        return data;
    }

    public T peek(){
        if(start==null){
            throw new NoSuchElementException();
        }
        return start.data;
    }

    
    public boolean isEmpty(){
        return (start==null);
    }

}


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

        Queue<Integer> q = new Queue<>();

        q.add(1);
        q.add(2);
        q.add(3);
        q.add(4);
        
        System.out.println(q.pull());
        System.out.println(q.pull());
        System.out.println(q.peek());
        System.out.println(q.pull());
        System.out.println(q.isEmpty());
        System.out.println(q.pull());
        System.out.println(q.isEmpty());
    }
}