public class Queue<T>
{
    private Node<T> head;
    private Node<T> tail;
    private int size;

    private class Node<T>
    {
        private T thisElement;
        private Node<T> nextElement;

        public Node(T thisElement, Node<T> nextElement)
        {
            this.thisElement = thisElement;
            this.nextElement = nextElement;
        }
    }

    public Queue()
    {
        tail = new Node<T>(null, null);
        head = new Node<T>(null, tail);
        size = 0;
    }

    public boolean isEmpty()
    {
        return size == 0;
    }

    public T peek()
    {
        if(size == 0)
        {
            return null;
        }
        else
        {
            return head.thisElement;
        }
    }

    public boolean offer(T element)
    {
        if(size == 0)
        {
            head.thisElement = element;
            size++;
        }
        else if(size == 1)
        {
            tail.thisElement = element;
            size++;
        }
        else
        {
            Node<T> temp = new Node<T>(element, null);
            tail.nextElement = temp;
            tail = temp;
        }

        return true;
    }

    public T pool()
    {
        if(size == 0)
        {
            return null;
        }
        else if(size == 1)
        {
            return head.thisElement;
        }
        else
        {
            T tempElement = head.thisElement;
            head = head.nextElement;
            return tempElement;
        }
    }

    public static void main(String [] args)
    {
        Queue<String> q = new Queue<String>();
        String a = "1";
        String b = "2";
        String c = "3";
        String d = "4";

        q.offer(a);
        q.offer(b);
        q.offer(c);
        q.offer(d);

        System.out.println(q.pool() + q.pool() + q.pool() + q.pool());


    }
}