Uni student needs helps

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
What I am trying to do is output each entry in the stack that is created previously using the draw method until the stack is empty. I have tried to do this but I get null pointer exceptions. What I need to understand is what part of my thought process am I omitting? Once again any responses are always appreciated.
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 final static int increment=20;
private String aString, anotherString;
private Button anEnterButton, aReverseButton;
private TextField aTextField;
private Stack aStack = new Stack();
private int numberLine;
     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 ) {
          this.numberLine=40;
          g.drawString(anotherString, 10,this.numberLine);
          this.numberLine=this.numberLine+Exam28.increment;
          repaint();
     public void actionPerformed(ActionEvent event){
//enters the text from the textfield and puts it into the stack
     String enterButton = event.getActionCommand();
     if (enterButton.equals(Exam28.enter)){
          this.aString = aTextField.getText();
          this.aStack.push(aString);
//how do I empty the stack and print it's output from here using the paint method?
//And print each output of the stack on a different line?
     String reverseButton=event.getActionCommand();
     if (reverseButton.equals(Exam28.reverse)){
          if (this.aStack.empty()){
     else
         anotherString =(String)this.aStack.pop();

Answer 1

In your reverseButton action, just call repaint ...only.
Inside your paint method ...do all the logic to empty and print the stack like this:
    while(!aStack.isEmpty()) {
      anotherString =(String)this.aStack.pop();
      g.drawString(anotherString, 10,this.numberLine);
      this.numberLine=this.numberLine+Exam28.increment;
    this.numberLine=40; // reset the counter for next time... so you can do it all again.You have to do it all at once, right inside paint, because a new call to paint will clear any previous output.