About JApplet ItemListener

Question

I'm trying to run the following code for a simple Applet but i keep getting an error when I invoke the ItemListener. any1 know how I could fix this. When I check on any of the checkboxes " Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException" and the message i'm trying to display does not get displayed. The buttons work fine.

    import java.awt.*;
    import javax.swing.*;
    import java.awt.event.*;

public class Hotel extends JApplet implements ActionListener{
JCheckBox cb1, cb2;
JButton b1, b2;
JLabel lb;
JTextField jt;
double tot,num;

public void init(){
    Container cp=getContentPane();
    cp.setLayout(null);
    cb1=new JCheckBox("Single");
    cb2=new JCheckBox ("Double");
    b1=new JButton ("Amount");
    b2= new JButton("Exit");
    lb= new JLabel("");
    jt=new JTextField();
    jt.setBounds (100,100,70,20);
    cb1.setBounds(100, 50,70,20);
    cb2.setBounds(100,200,70,20);
    b1.setBounds(100,250,100,20);
    b2.setBounds(100,300,70,20);
    lb.setBounds(100,400,200,20);

    cp.add(lb);
    cp.add(cb1);
    cp.add(cb2);
    cp.add(b1);
    cp.add(b2);
    cp.add(jt);
    b1.addActionListener(this);
    b2.addActionListener(new BtnExit());
    cb1.addItemListener(new CBox());
    cb2.addItemListener(new CBox2());

}

      @Override
public void actionPerformed(ActionEvent ae) {
   //this one works fine
    num=Double.parseDouble(jt.getText());
       if (cb1.isSelected()){
           tot=10000*num;
                                     }
       if (cb2.isSelected()){
          tot=15000*num;
           lb.setText("15 000/room");
       }
      lb.setText(String.valueOf(tot));
}
}

class CBox implements ItemListener {

@Override
public void itemStateChanged(ItemEvent ie) {
    //to display the price per room once the user ticks on "Single"
    Hotel h=new Hotel();
    h.lb.setVisible(true);
    h.lb.setText("You will pay 10000/room");
                }


}
class CBox2 implements ItemListener{

@Override
public void itemStateChanged(ItemEvent ie) {
   //to display the price per room once the user ticks on the "Double" checkbox
    Hotel h=new Hotel();
    h.lb.setVisible(true);
    h.lb.setText("You will pay 15 000/room");
}

}

Post the stack trace and point out the line in your code it's coming from. — Philip Whitehouse, Aug 17 '13 at 20:46
1) Why code an applet? If it is due to spec. by teacher, please refer them to [Why CS teachers should stop teaching Java applets](http://programmers.blogoverflow.com/2013/05/why-cs-teachers-should-stop-teaching-java-applets/). 2) Ensure the [Java Console](http://www.java.com/en/download/help/javaconsole.xml) is configured to show for applets & JWS apps. If there is no output at the default level, raise it and try again. 3) Always copy/paste error & exception output. — Andrew Thompson, Aug 17 '13 at 22:24

kiheru · Answer 1 · 2013-08-17T21:12:30.817

You are creating new Hotel instances in the listeners, instead of modifying the old. You should pass the existing instance to the listener:

class CBox implements ItemListener {
    final Hotel hotel;
    CBox(Hotel h) {
        this.hotel = h;
    }

    @Override
    public void itemStateChanged(ItemEvent ie) {
        hotel.lb.setVisible(ie.getStateChange() == ItemEvent.SELECTED);
        hotel.lb.setText("You will pay 10000/room");
    }
}

And used like:

cb1.addItemListener(new CBox(this));

Furthermore, you should likely check the new check box state, rather than setting the label visible unconditionally. Just as an observation about the intended functionality (Implemented in the above code).

Similarly for the other item listener.

Alternatively you could use anonymous classes:

cb1.addItemListener(new ItemListener() {
    @Override
    public void itemStateChanged(ItemEvent ie) {
        lb.setText("You will pay 10000/room");
        lb.setVisible(ie.getStateChange() == ItemEvent.SELECTED);
    }
});

Also, you should really use layout managers. Swing is designed to work with those. null layouts are a disaster waiting to happen.

*"`null` layouts are a disaster waiting to happen."* Much like the Titanic at a good clip on her maiden voyage.. Java GUIs might have to work on a number of platforms, on different screen resolutions & using different PLAFs. As such they are not conducive to exact placement of components. To organize the components for a robust GUI, instead use layout managers, or [combinations of them](http://stackoverflow.com/a/5630271/418556), along with layout padding & borders for [white space](http://stackoverflow.com/q/17874717/418556). — Andrew Thompson, Aug 17 '13 at 22:23

About JApplet ItemListener

1 Answers1