inner class

[kofi ofei]

the following programme below compiles but when i run it nothing is displayed in the applet viewer


import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class InnerClasses extends Applet{
String m;
public InnerClasses(){
MyListener m = new MyListener();
addMouseListener(m);
}
//start an inner class called MyListener
public final class MyListener extends MouseAdapter{
public void mouseClicked(MouseEvent e){
if(e.getClickCount()== 1) m = "Single click";
else if(e.getClickCount()== 2) m = "Double click";
int mod = e.getModifiers();
if((mod & InputEvent.BUTTON1_MASK) !=0) m += "Button1";
else if((mod & InputEvent.BUTTON1_MASK)!=0) m += "Button2";
if(e.isAltDown()) m += "ALT key is down";
if(e.isControlDown()) m += "CONTROL key is down";
if(e.isShiftDown()) m += "SHIFT key is down";
repaint();
}
}
public void paint(Graphics g){
g.setFont(new Font("Serif",Font.PLAIN,18));
g.drawString(m, 10, 80);
}
}

[Mark Spritzler]

Moving to Java In General (Beginner) forum.

Mark

[Marilyn de Queiroz]

I think when the applet starts, it tries to "paint" itself, but at that point in time, 'm' is still null. So the applet throws a NullPointerException and dies.

[Marilyn de Queiroz]

Also, formatting your code will help people who want to try to help you.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
  
public class InnerClasses extends Applet{
    String m = "no click yet";
    public InnerClasses(){
        MyListener m = new MyListener();
        addMouseListener(m);
    }
    
    //start an inner class called MyListener
    public final class MyListener extends MouseAdapter{
        public void mouseClicked(MouseEvent e){
            if(e.getClickCount()== 1) 
                m = "Single click";
            else if(e.getClickCount()== 2) 
                m = "Double click";
            int mod = e.getModifiers();
            if((mod & InputEvent.BUTTON1_MASK) !=0) 
                m += "Button1";
            else if((mod & InputEvent.BUTTON1_MASK)!=0) 
                m += "Button2";
            if(e.isAltDown()) 
                m += "ALT key is down";
            if(e.isControlDown()) 
                m += "CONTROL key is down";
            if(e.isShiftDown()) 
                m += "SHIFT key is down";
            repaint();
        }
    }
      
    public void paint(Graphics g){
        g.setFont(new Font("Serif",Font.PLAIN,18));
        g.drawString(m, 10, 80);
    }
}  

[ May 22, 2005: Message edited by: Marilyn de Queiroz ]