Uni student needs help... again 8(

Question

hello,
i need some help in understanding my applet. i have little experience programming and i am required to write the following small applet as part of an exam for this the first thursday in june.
http://www-csfy.cs.ualberta.ca/~c114/LabExams/LElocations/lbe28.html
I also am restricted to using the following classes and methods:
http://www-csfy.cs.ualberta.ca/~c114/LabExams/LElocations/library.html
I have written some of the code for this exercise and it compiles fine, but when I try to push the enter button and push the string object into the stack I get a nullpointerException. Does anyone know why this would be the case? What would be my fix and why?
Once again Thanks in Advance to all who respond.
This is the code that I have written so far:
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
import java.util.*;
public class Exam28 extends Applet implements ActionListener {
private final static String enter = "Enter";
private final static String reverse = "Reverse";
private String aString;
private Button anEnterButton, aReverseButton;
private TextField aTextField;
private Stack aStack;
     public void init() {
          this.aTextField = new TextField(20);
          this.add(aTextField);
          this.anEnterButton = new Button(Exam28.enter);
          this.anEnterButton.addActionListener(this);
          this.add(this.anEnterButton);
          this.aReverseButton = new Button(Exam28.reverse);
          this.aReverseButton.addActionListener(this);
          this.add(this.aReverseButton);
          repaint();
     public void paint( Graphics g ) {
     public void actionPerformed(ActionEvent event){
     String enterButtonPicked=event.getActionCommand();
     if (enterButtonPicked.equals(Exam28.enter)){
          aString=aTextField.getText();
          aStack.push(aString);

Answer 1

you need to initialize your Stack before you push anything on to it.
you could do it when you declare itat the top, like:
private Stack aStack = new Stack();
cheers
jamie