J2ME (Java) - Null Pointer Exception caught in Display Class

Question

I'm currently working with MIDlets (I am using a Visual MIDlet) in Netbeans, and a NullPointerException is being thrown but I do not know why.

Note: The exception is not thrown when the program runs on the emulator, only when the OK Command button is pressed.

Here is the error I get

TRACE: <at java.lang.NullPointerException:   0>, Exception caught in Display class
java.lang.NullPointerException:   0
        at javax.microedition.lcdui.Display$ChameleonTunnel.callScreenListener(), bci=46
        at com.sun.midp.chameleon.layers.SoftButtonLayer.processCommand(), bci=74
        at com.sun.midp.chameleon.layers.SoftButtonLayer.soft1(), bci=37
        at com.sun.midp.chameleon.layers.SoftButtonLayer.keyInput(), bci=36
        at com.sun.midp.chameleon.CWindow.keyInput(), bci=38
        at javax.microedition.lcdui.Display$DisplayEventConsumerImpl.handleKeyEvent(), bci=17
        at com.sun.midp.lcdui.DisplayEventListener.process(), bci=277
        at com.sun.midp.events.EventQueue.run(), bci=179
        at java.lang.Thread.run(Thread.java:619)

I have stripped out all of the code unrelated to the exception, so that you can read it easier. Below is a simplified version if the code I have, which throws the above exception.

package stMidlet;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class StoryMidlet extends MIDlet implements CommandListener {

    private boolean midletPaused = false;
    private Command commandOk1;
    private Form form1;
    private TextField textField1;

    public StoryMidlet() {
        commandOk1 = new Command("Ok", Command.OK, 1);
        textField1 = new TextField("Enter value: ", null, 120, TextField.ANY);
        form1 = new Form(null, new Item[]{textField1});
        form1.addCommand(commandOk1);
Display.getDisplay(this).setCurrent(form1);
    }

/* There were some methods here pre-inserted by netbeans. */

/* I have not changed these, but I can post them if you need me too */

/* initialize() */

/* startMIDlet() */

/* resumeMidlet() */

/* switchDisplayable */


/* getDisplay() */

/* exitMidlet() */

/* startApp() */

/* pauseApp() */

/* destroyApp() */


    public void commandAction(Command c, Displayable d) {
        if (c == commandOk1)
        {
            System.out.println("Test");
        }
    }

}

I have been trying to solve this for at least an hour, with no prevail. The only thing I can think of worth mentioning is:

• Netbeans showed a warning with the line Display.getDisplay(this)..... saying there was a leak in the constructor. I moved it into the initialize() method which sedated the warning, but the exception still occurs.

Any help will be greatly appreciated.

Thanks, Tom.

Answer 1

EDIT: I might need to redact my answer because I think what I said is not relevant, but I'll leave it up on the off-chance that it could help!

It's been a long time since I have worked in J2ME, but having looked at some old code I noticed I never did anything that useful in the constructor. I'm betting your call to Display.getDisplay(this) is causing a NullPtrException because something hasn't been initialized yet. In fact, I'm pretty sure using the this pointer in a constructor is fairly certain to cause this type of error.

Try dealing with Display in the startApp() function, and if this code I'm referring to is correct, you should keep a boolean that marks if your MIDlet has been initialized yet or not.

You can look at some old code of mine here for reference:

http://code.google.com/p/jmingle/source/browse/trunk/src/org/oep/jmingle/JMingle.java#68

Answer 2

Maybe you need to add

form1.setCommandListener(this);

Answer 3

I also noticed it....

public Welcome(String k, ChatApp c) {
        super(k);
        name = new TextField("Name", "", 140, TextField.ANY);
        exit = new Command("Exit", Command.EXIT, 0);
        enter = new Command("Enter", Command.OK, 0);
        midlet = c;
        this.append(name);
        this.addCommand(exit);
        this.addCommand(enter);
    }

Lacked the

this.setCommandListener(this);

and always got the null-pointer exception... This works well but it seems that we forget it too often when our code gets too complex ^^

Answer 4

You must do in order:

• form1.setCommandListener
• form1.addCommand..
• ....
• after all: MIDlet.getDisplay.setCurrent(form1);

If you invoke 'setCurrent' before adding the commands and listener, the form1 screen still appear but when you hit a command, it raises above error.