A linked list-based implementation of a stack.

ListBasedStack.java

public class ListBasedStack<T> implements Stack<T>
{
    private Node<T> top = null;

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

    public void clear()
    {
        top = null;
    }

    public void push(T data)
    {
        top = new Node<T>(data, top);
    }

    public T pop()
    {
        if(isEmpty()) {
            throw new IllegalStateException("Stack is empty");
        }
        T data = top.data;
        top = top.next;
        return data;
    }

    public T peek()
    {
        if(isEmpty()) {
            throw new IllegalStateException("Stack is empty");
        }
        return top.data;
    }

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

        public Node(T data)
        {
            this(data, null);
        }

        public Node(T data, Node<T> n)
        {
            this.data = data;
            next = n;
        }
    }

    public interface Stack<T> {

        /**
         * Puts an element on the top of the stack
         */
        public void push(T data);

        /**
         * Takes the top element off of the stack
         */
        public T pop();

        /**
         * Takes the top element off of the stack without actually removing it
         */
        public T peek();
        
        public boolean isEmpty();

    }

}