Need Help regarding separate GUI and logic class

Question

Hi,
I was given this assignment and I'm stuck big time.
Basically I have a GUI class for a stopwatch interface.
public class WatchGui implements Gui{
    JButton b1, b2;
    JLabel minutes, symbol1, seconds, symbol2, hundredths;
    StopWatch stopwatch;
    public WatchGui(){
         stopwatch = new StopWatch();
         b1 = new JButton("RUN");
        minutes = new JLabel("00");
        ActionListener al = new MyActionListener(stopwatch);
        b1.addActionListener(al);
    //  Called to connect a watch to the GUI
    public void connect(Watch w) {
    public void setDisplay(String mm) {
         minutes.setText(mm);
    public void addComponentsToPane(Container pane) {
        pane.setLayout(null);
        pane.add(b1);
        pane.add(b2);
        pane.add(minutes);
        Insets insets = pane.getInsets();
    public void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("XXX");
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //Set up the content pane.
        WatchGui Pane = new WatchGui();
        Pane.addComponentsToPane(frame.getContentPane());
        //Size and display the window.
        Insets insets = frame.getInsets();
        frame.setVisible(true);
}I have a watch class which function as the engine for the stop watch
import java.awt.event.*;
import javax.swing.*;
class StopWatch implements Watch {
     WatchGui watchgui;
     public StopWatch() {
          running = false;
          count = 0;
    public void connect(Gui g) {
         watchgui = (WatchGui) g;
         watchgui.createAndShowGUI();
    public void runStop() {
         mm = String.valueOf("100");
     watchgui.setDisplay(mm);
class MyActionListener implements ActionListener {
     StopWatch stopwatch;
     public MyActionListener(StopWatch stopwatch) {
          this.stopwatch = stopwatch;
     public void actionPerformed (ActionEvent e) {
          stopwatch.runStop();
}To get the StopWatch to work, I have this driver class
import java.lang.reflect.Constructor;
public class Driver
    public static void main(String[] args)
      throws Exception
     if( args.length!=1 ){
         System.err.println("Usage: Driver WATCHNAME");
         System.exit(1);
         return;
     String watchName= args[0];
     Class cl= Class.forName(watchName,true,Thread.currentThread().getContextClassLoader());
     Constructor c[]= cl.getConstructors();
     if( c.length==0 ){
         System.err.println("There is NO constructor in your watch class");
         System.exit(1);
         return;
     if( c.length>1 ){
         System.err.println( "There is more than one constructor in your class");
         System.exit(1);
     //Construct the components
     Watch w=(Watch)c[0].newInstance(new Object[0]);
     Gui g= new WatchGui();
     //Connect them to each other
     g.connect(w);
     w.connect(g);
     //Reset the components()
     g.reset();
     w.reset();
     //And away we go...
     try{
         for(;;){
          Thread.sleep(10); //milliseconds
          w.tick();
     }catch(InterruptedException ie){
         System.exit(1);
}My currently problem is that when i click on the button in the GUI, it will go to MyActionListener class and call the runStop method.
public void runStop() {
     mm = String.valueOf("100");
     watchgui.setDisplay(mm);
The problem is that when watchgui.setDisplay(mm) is called, exception is thrown instead. I'm suspecting it has something to do with the initialization of watchgui. Can anyone advice?

Answer 1

import java.awt.*;
import java.awt.event.ActionListener;
import javax.swing.*;
public class WatchGui implements Gui{
     JButton b1, b2;
    JLabel minutes, symbol1, seconds, symbol2, hundredths;
    StopWatch stopwatch;
    public WatchGui(){
         stopwatch = new StopWatch();
         b1 = new JButton("RUN/STOP");
        b2 = new JButton("  RESET  ");
        minutes = new JLabel("00");
        symbol1 = new JLabel(":");
        seconds = new JLabel("00");
        symbol2 = new JLabel(".");
        hundredths = new JLabel("00");
        minutes.setFont(new Font("Arial", Font.BOLD, 40));
        symbol1.setFont(new Font("Arial", Font.BOLD, 40));
        seconds.setFont(new Font("Arial", Font.BOLD, 40));
        symbol2.setFont(new Font("Arial", Font.BOLD, 20));
        hundredths.setFont(new Font("Arial", Font.BOLD, 20));
        ActionListener al = new MyActionListener(stopwatch);
        b1.addActionListener(al);
    //  Called to connect a watch to the GUI
    public void connect(Watch w) {
    //Called to reset the GUI
    public void reset() {
         minutes.setText("00");
         seconds.setText("00");
         hundredths.setText("00");
    //Called whenever the value displayed by the watch changes
    public void setDisplay(String mm, String ss, String hh) {
         minutes.setText(mm);
         seconds.setText(ss);
         hundredths.setText(hh);
    public void addComponentsToPane(Container pane) {
        pane.setLayout(null);
        pane.add(b1);
        pane.add(b2);
        pane.add(minutes);
        pane.add(symbol1);
        pane.add(seconds);
        pane.add(symbol2);
        pane.add(hundredths);
        Insets insets = pane.getInsets();
        Dimension size = b1.getPreferredSize();
        b1.setBounds(50 + insets.left, 20 + insets.top,
                     size.width, size.height);
        size = b2.getPreferredSize();
        b2.setBounds(150 + insets.left, 20 + insets.top,
                     size.width, size.height);
        size = minutes.getPreferredSize();
        minutes.setBounds(75 + insets.left, 70 + insets.top,
                size.width, size.height);
        size = symbol1.getPreferredSize();
        symbol1.setBounds(120 + insets.left, 70 + insets.top,
                size.width, size.height);
        size = seconds.getPreferredSize();
        seconds.setBounds(135 + insets.left, 70 + insets.top,
                size.width, size.height);
        size = symbol2.getPreferredSize();
        symbol2.setBounds(182 + insets.left, 87 + insets.top,
                size.width, size.height);
        size = hundredths.getPreferredSize();
        hundredths.setBounds(190 + insets.left, 87 + insets.top,
                size.width, size.height);
    public void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("StopWatch");
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //Set up the content pane.
        WatchGui Pane = new WatchGui();
        Pane.addComponentsToPane(frame.getContentPane());
        //Size and display the window.
        Insets insets = frame.getInsets();
        frame.setSize(300 + insets.left + insets.right,
                      150 + insets.top + insets.bottom);
        frame.setVisible(true);
import java.awt.event.*;
import javax.swing.*;
class StopWatch implements Watch {
     WatchGui watchgui;
     long startTime;
     boolean running;
     javax.swing.Timer timer;
     String mm, ss, hh;
     int count, tmp;
     public StopWatch() {
          running = false;
          count = 0;
          timer = new javax.swing.Timer(1000, new ActionListener()
               public void actionPerformed(ActionEvent e)
                     count++;
                     mm = String.valueOf(count);
                     ss = String.valueOf(count);
                     hh = String.valueOf(count);
                     watchgui.setDisplay(mm, ss, hh);
    //Called to connect the GUI to watch
    public void connect(Gui g) {
         watchgui = (WatchGui) g;
         watchgui.createAndShowGUI();
          //watchgui.setDisplay(mm, ss, hh);
    //Called to initialise the watch
    public void reset() {
         watchgui.reset();
    //Called to deliver a TICK to the watch
    public void tick() {
    //Called whenever the run/stop button is pressed
    public void runStop() {
         int a = 33;
         mm = "" + a;
          ss = "" + a;
          hh = "" + a;
          watchgui.setDisplay(mm, ss, hh);
         if (running == false) {
              count = 0;
              timer.start();
              running = true;
         }else {
              timer.stop();
              running = false;
    //Called whenever the lap/reset button is pressed
    public void lapReset() {}
class MyActionListener implements ActionListener {
     StopWatch stopwatch;
     public MyActionListener(StopWatch stopwatch) {
          this.stopwatch = stopwatch;
     public void actionPerformed (ActionEvent e) {
          stopwatch.runStop();
import java.lang.reflect.Constructor;
public class Driver
    public static void main(String[] args)
      throws Exception
     if( args.length!=1 ){
         System.err.println("Usage: Driver WATCHNAME");
         System.exit(1);
         return;
     String watchName= args[0];
     Class cl= Class.forName(watchName,true,Thread.currentThread().getContextClassLoader());
     Constructor c[]= cl.getConstructors();
     if( c.length==0 ){
         System.err.println("There is NO constructor in your watch class");
         System.exit(1);
         return;
     if( c.length>1 ){
         System.err.println( "There is more than one constructor in your class");
         System.exit(1);
     //Construct the components
     Watch w=(Watch)c[0].newInstance(new Object[0]);
     Gui g= new WatchGui();
     //Connect them to each other
     g.connect(w);
     w.connect(g);
     //Reset the components()
     g.reset();
     w.reset();
     //And away we go...
     try{
         for(;;){
          Thread.sleep(10); //milliseconds
          w.tick();
     }catch(InterruptedException ie){
         System.exit(1);
public interface Gui
    //Called to connect a watch to the GUI
    public void connect(Watch w);
    //Called to reset the GUI
    public void reset();
    //Called whenever the value displayed by the watch changes
    public void setDisplay(String mm, String ss, String hh);
interface Watch
    //Called to connect the GUI to watch
    public void connect(Gui g);
    //Called to initialise the watch
    public void reset();
    //Called to deliver a TICK to the watch
    public void tick();
    //Called whenever the run/stop button is pressed
    public void runStop();
    //Called whenever the lap/reset button is pressed
    public void lapReset();
}