import java.util.EmptyStackException;

/**
 * A simple linked list based integer stack.
 *
 * @author John Kloss
 */

public class IntStackTwo {

    /**
     * Simple node class used as the linked list components for the
     * stack.
     */
    class Node {

	int  value;      // value this node contains
	Node next;       // node below this one in the stack

	/**
	 * Creates a new Node with integer value, <code>value</code> and
	 * a pointer to Node, <code>next</code>.
	 *
	 * @param value       The integer value this Node should contain.
	 * @param next        The Node which lies (abstractly) below this
	 *                    Node in the stack.
	 */
	Node(int value, Node next) {
	    this.value = value;
	    this.next  = next;
	}
    }

    Node stackPtr = null;     // pointer to the top of the stack

    
    /**
     * Creates an empty integer stack.
     */
    public IntStackTwo() {
	// do nothing
    }

    
    /**
     * Returns whether this stack is empty.
     *
     * @return           <code>true</code> if this stack is empty; <code>false
     *                   </code> otherwise.
     */
    public boolean isEmpty() {
	return (stackPtr == null);
    }

    
    /**
     * Pushes an integer onto the top of this stack.
     *
     * @param value      The integer to push onto this stack.
     */
    public void push(int value) {
	stackPtr = new Node(value, stackPtr);
    }

    
    /**
     * Pops an integer from the top of this stack.
     *
     * @return           The last integer pushed onto this stack.
     *
     * @exception        EmptyStackException 
     *                        Thrown if this stack contains no integers.
     */
    public int pop() {

	////////////////////////////////////////
	// Note: EmptyStackException is defined
	//       in java.util.
	
	if (stackPtr == null)
	    throw new EmptyStackException();
	else {
	    int retValue = stackPtr.value;
	    stackPtr     = stackPtr.next;

	    return retValue;
	}
    }


    ////////////////////////////////////////
    // Test our stack implementation

    public static void main(String[] args) {
	
	IntStackTwo s = new IntStackTwo();

	for (int i = 0; i < 20; i++) {
	    System.out.println("Pushing: " + i);
	    s.push(i);
	}

	while (s.isEmpty() == false) 
	    System.out.println("Popping: " + s.pop());
    }
}