Java

Java RSS

Java Null Pointer Exception

Thread Solved

•

Join Date: Feb 2007

Posts: 27

Reputation: mr.sweetchuck is an unknown quantity at this point

Solved Threads: 1

mr.sweetchuck mr.sweetchuck is offline

Offline

Light Poster

Java Null Pointer Exception

Mar 1st, 2007

Hi I'm trying to write a multi-formed J2ME app. I keep getting a null pointer exception in the startApp() method.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
 
public void startApp()
{
    display.setCurrent(currentForm);
}
 
public void startApp()
{
    display.setCurrent(currentForm);
}

I have currentForm created above and the first is mapped to it, (with the plan of mapping each new from to currentForm.

Help with Code Tags

Java Syntax (Toggle Plain Text)

private Form currentForm, login; //etc

Help with Code Tags

Java Syntax (Toggle Plain Text)

currentForm = login;

I have login set up, so I'm really confused why I getting this error.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
loginTF = new TextField[2];
loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
login = new Form("login", loginTF);
login.addCommand(submit);
login.addCommand(exit);
login.addCommand(clear);
login.setCommandListener(this);
loginTF = new TextField[2];
loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
login = new Form("login", loginTF);
login.addCommand(submit);
login.addCommand(exit);
login.addCommand(clear);
login.setCommandListener(this);

Everything above has been declared as usual. Any suggestions as to what I'm doing wrong?

Thanks, chuck

Nel sogni, come in amore, non ci sono cose impossible

•

Join Date: Feb 2006

Posts: 2,280

Reputation: masijade has much to be proud of

Solved Threads: 242

masijade

Offline

Nearly a Posting Maven

Re: Java Null Pointer Exception

Mar 1st, 2007

Maybe the NullPointerException refers to "display".

Java Programmer and Sun Systems Administrator

----------------------------------------------

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.
--Brian Kernighan

•

Join Date: Feb 2007

Posts: 27

Reputation: mr.sweetchuck is an unknown quantity at this point

Solved Threads: 1

mr.sweetchuck mr.sweetchuck is offline

Offline

Light Poster

Re: Java Null Pointer Exception

Mar 1st, 2007

Nope, sadly not. It's set up as usual:

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
Display display;
//...
display = Display.getDisplay(this);
Display display;
//...
display = Display.getDisplay(this);

Thanks, any other ideas?

Last edited by mr.sweetchuck; Mar 1st, 2007 at 1:56 pm.

Nel sogni, come in amore, non ci sono cose impossible

•

Join Date: Feb 2006

Posts: 2,280

Reputation: masijade has much to be proud of

Solved Threads: 242

masijade

Offline

Nearly a Posting Maven

Re: Java Null Pointer Exception

Mar 2nd, 2007

Some more code is needed. According to the method showed (in a cupsule like it was) display definately would be null, since there is no evidence otherwise. Post the entire class.

•

Join Date: Feb 2007

Posts: 27

Reputation: mr.sweetchuck is an unknown quantity at this point

Solved Threads: 1

mr.sweetchuck mr.sweetchuck is offline

Offline

Light Poster

Re: Java Null Pointer Exception

Mar 2nd, 2007

It's a pretty big class, but I'll try to cut out the irrevelant parts.

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
 
public class MyClient extends MIDlet implements CommandListener
{
/*********************Widget declarations***************************************/
    private Display display;
    private Form currentForm, main, login; //more, but not relivant
    private StringItem options[];
    private TextField loginTF[], cleanUpTF[], patientBill[], wardInfo[], patientDetails[];
    private TextField securityDetails[], securityTF[], medAlertTF[];
    private Command exit, select, clear, submit, remove, mainMenu, first;
    private Command previous, next, last, generateBill, findPatient, update, report;
/****************************Constructor****************************************/
    public void myClient()
    {
        display = Display.getDisplay(this);
/**************************Commands*********************************************/
        //comands initialised
 
/********************Login Screen***********************************************/
        loginTF = new TextField[2];
        loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
        loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
        login = new Form("login", loginTF);
        login.addCommand(submit);
        login.addCommand(exit);
        login.addCommand(clear);
        login.setCommandListener(this);
 
        currentForm = login;
    }
/*********************Displaying The First Screen*******************************/
    public void startApp()
    {
        display.setCurrent(currentForm);
    }
/*********************Pausing The Application***********************************/
    public void pauseApp()
    {}
/**************************Destroying The Application***************************/
    public void destroyApp(boolean unconditional)
    {}
 
}
/******************************End of Class*************************************/
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class MyClient extends MIDlet implements CommandListener
{
/*********************Widget declarations***************************************/
    private Display display;
    private Form currentForm, main, login; //more, but not relivant
    private StringItem options[];
    private TextField loginTF[], cleanUpTF[], patientBill[], wardInfo[], patientDetails[];
    private TextField securityDetails[], securityTF[], medAlertTF[];
    private Command exit, select, clear, submit, remove, mainMenu, first;
    private Command previous, next, last, generateBill, findPatient, update, report;
/****************************Constructor****************************************/
    public void myClient()
    {
        display = Display.getDisplay(this);
/**************************Commands*********************************************/
        //comands initialised

/********************Login Screen***********************************************/
        loginTF = new TextField[2];
        loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
        loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
        login = new Form("login", loginTF);
        login.addCommand(submit);
        login.addCommand(exit);
        login.addCommand(clear);
        login.setCommandListener(this);

        currentForm = login;
    }
/*********************Displaying The First Screen*******************************/
    public void startApp()
    {
        display.setCurrent(currentForm);
    }
/*********************Pausing The Application***********************************/
    public void pauseApp()
    {}
/**************************Destroying The Application***************************/
    public void destroyApp(boolean unconditional)
    {}

}
/******************************End of Class*************************************/

Last edited by mr.sweetchuck; Mar 2nd, 2007 at 1:07 pm. Reason: removing irelivant code

Nel sogni, come in amore, non ci sono cose impossible

•

Join Date: Feb 2006

Posts: 2,280

Reputation: masijade has much to be proud of

Solved Threads: 242

masijade

Offline

Nearly a Posting Maven

Re: Java Null Pointer Exception

Mar 2nd, 2007

What about the Class Display.

Is it at all possible that display = Display.getDisplay(this); returns null?

Please provide this getDisplay method, or if it comes from some thrid party API, then please provide the JavaDoc from this method.

•

Join Date: Feb 2007

Posts: 27

Reputation: mr.sweetchuck is an unknown quantity at this point

Solved Threads: 1

mr.sweetchuck mr.sweetchuck is offline

Offline

Light Poster

Re: Java Null Pointer Exception

Mar 5th, 2007

The Display method has worked for me half a dozen time so far, so it can't be that. (I even copied and pasted it in from a working program)

Nel sogni, come in amore, non ci sono cose impossible

•

Join Date: Feb 2006

Posts: 2,280

Reputation: masijade has much to be proud of

Solved Threads: 242

masijade

Offline

Nearly a Posting Maven

Re: Java Null Pointer Exception

Mar 5th, 2007

And that says nothing. Those classes are those classes, this class is this class. If you do not wish to provide the info we would need to help you, then at least don't waste our time.

•

Join Date: Feb 2007

Posts: 27

Reputation: mr.sweetchuck is an unknown quantity at this point

Solved Threads: 1

mr.sweetchuck mr.sweetchuck is offline

Offline

Light Poster

Re: Java Null Pointer Exception

Mar 5th, 2007

Display comes from:
javax.microedition.lcdui.Display

Which is a default class that the compiler recognises and I have not over-written.

The api is available on:
http://kickjava.com/3205.htm#33462

This is what the api says:

Help with Code Tags

(Toggle Plain Text)

public static Display getDisplay(MIDlet m)

This is what I did:

 Help with Code Tags 
 Java Syntax (Toggle Plain Text) 
class MyClass extends MIDlet
{
...
Display display;
Form currentForm,login;
TextField loginTF[];
...
public void MyClass
{
display = Display.getDisplay(this);
...
loginTF = new TextField[2];
loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
login = new Form("login", loginTF);
...
currentForm = login;
}
...
public void startApp()
{
display.setCurrent(currentForm);
}
class MyClass extends MIDlet
{
...
Display display;
Form currentForm,login;
TextField loginTF[];
...
public void MyClass
{
display = Display.getDisplay(this);
...
loginTF = new TextField[2];
loginTF[0] = new TextField("Staff Number:", "", 10, TextField.NUMERIC);
loginTF[1] = new TextField("Password:", "", 50, TextField.PASSWORD);
login = new Form("login", loginTF);
...
currentForm = login;
}
...
public void startApp()
{
display.setCurrent(currentForm);
}

Nel sogni, come in amore, non ci sono cose impossible

•

Join Date: Feb 2006

Posts: 2,280

Reputation: masijade has much to be proud of

Solved Threads: 242

masijade

Offline

Nearly a Posting Maven

Re: Java Null Pointer Exception

#10

Mar 5th, 2007

According to the documentation for Form and Display, and if this is actually the line throwing the null pointer exception, then display is the only thing that even can be null, since the setCurrent method will accept a null value, so currentForm is allowed to be null.

Take out all the dots and attempt to run the Class exactly as it is portrayed here. If that works, then the problem lies somewhere else in your code. Maybe there is some process somewhere in your class that sets display to null again. I don't know since I can't see the rest of the code.

As I said, however, reading the Display and Form API pages from this lcdui package, indicates that display is the only thing in that line that even could cause a NullPointerExcpetion.