Hi here the question the interviewer asked me-

 package jp.co.worksap.intern;
    /**
    *The Stack class represents a last-in-first-out(LIFO) stack of objects
    *And this class can look at the object which has the highest (lowest or so) value.
    *So every object onto the stack must be comparable to each other.
    *@param<E.
    */
    public interface ISortableStack<E extends Comparable<E>>{
    /**

    *Pushes an item onto the top of this stack.
    *If the Element e is null, throws NullPointerException.
    *
    *
    @param e
    *@throws NullPointerException
    */
    public void push(E e);

    /**
    *Removes the object at the top of this stack and returns that object as the value of this function.
    *If the stack is empty, throws EmptyStackException.
    *@return
    *@throws EmptyStackException
    */
    public E pop();

    /**
    *Locks at the object which has the middle value among the all objects without removing it from the stack.
    *Returns the object which has the value of following order <code>(size()/2)+1</code>
    *<pre>
    *e.g.
    *When the stack has the following values (1, 2, 5, 4, 2, 6)
    *this method returns 4 and doesn't remove the object.
    *</pre>
    *
    *If the stack is empty, throws EmptyStackException.
    *@return
    *@throws EmptyStackException
    */
    public E peekMidElement();

    /**
    *Looks at the object which has the highest value among the all objects without removing it from the stack.
    *Returns the object which has the value of the following order <code>size()</code>
    *<pre>
    *e.g.
    *When the stack has the following values (1,2,5,4,2,6)
    *this method returns 6 and doesn't remove that object
    *</pre>
    *
    *If the stack is empty, throws EmptyStackException.
    *@return
    *@throws EmptyStackException
    */
    public E peekHighestElement();

    /**
    *Looks at the object which has the lowest value among the all objects without removing it from the stack.
    *Returns the object which has the value of the following order <code>1</code>
    *<pre>
    *e.g.
    *When the stack has the following values (1,2,5,4,2,6)
    *this method returns 1 and doesn't remove the object.
    *</pre>
    *
    *If the stack is empty, throws EmptyStackException.
    *@return
    *@throws EmptyStackException
    */
    public E peekLowestElement();

    /**
    *Returns the number of objects in this stack.
    *@return
    */
    public int size();

}

Now i have implemented the Interface in Eclipse as per the statement shown below code

public class MyStackClass<E extends Comparable<E>> implements ISortableStack<E>{

    private int A;
    private Node first;

    private class Node{

        private E e;
        private Node next;
    }

    public MyStackClass() {
        first = null;
        A = 0;
    }

    private ArrayList<E> listOne = new ArrayList<E>();


    public void push(E e) {
        // TODO Auto-generated method stub
        Node oldfirst = first;
        first = new Node();
        first.e = e;
        first.next = oldfirst;
        A++;
    }

    @Override
    public E pop() {
        // TODO Auto-generated method stub
        if (isEmpty()) throw new RuntimeException("Stack underflow");
        E e = first.e;                  // save e to return
        first = first.next;             // delete first node
        A--;
        return e;                       // return the saved e
    }

    @Override
    public E peekMidElement() {
        // TODO Auto-generated method stub
        if(listOne.size() <= 0){
            throw new EmptyStackException();
            }
            return listOne.get(listOne.size()/2);
    }

    @Override
    public E peekHighestElement() {
        // TODO Auto-generated method stub
        if(listOne.size() <= 0){
            throw new EmptyStackException();
            }

            return listOne.get(listOne.size() - 1);
    }

    @Override
    public E peekLowestElement() {
        // TODO Auto-generated method stub
        if(listOne.size() <= 0){
            throw new EmptyStackException();
            }

            return listOne.get(0);
    }

    @Override
    public int size() {
        // TODO Auto-generated method stub
        return A;
    }


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

}

Now when i call main method the ISortableStack array list is unable to instantiate shown below.It throws error at ISortableStack as:Cannot make a reference to a non static type . and is not able to instatiate the ISORTABLESTACK.

import java.io.*;

public class DisplayStack {

E ch;
public static void main(String[] args) {
    // TODO Auto-generated method stub


        ISortableStack<Character> s = new ISortableStack<Character>(); //Cannot instantiate ISORTABLESTACK
        char character;
        DisplayStack demo = new DisplayStack();
        while ((character = (char)System.in.read())!= '\n') //Cannot make reference to a non static type
        if (!s.full()) 
               s.push(character);
        while (!s.empty())
           System.out.print(s.pop());
        System.out.println();

}

}

Please help me to get this code work.