Changing JLabel text from other Class
Marco
I have 2 classes one is the application and the other one the login
screen, when i start the application i create a JFrame with the login
screen. My login screen checks the username and password in a mysql db
and when correct it returns the userID to the application (because the
application needs to know this). This al works with the following
code :
in the login screen class 'Login' i have the code
[code]
Agenda ag = new Agenda();
...
if(userOK){
System.out.println("userOK gets executed."); //to check if the
credentials where ok
jLabel1.setBackground(Color.green);
jLabel1.setText("You can enter"); //jLabel1 on the login
screen itself
System.out.printf("userid %d\n",userID); //to check the value of the
userID (this works)
ag.setUserID(userID); //method in the application class
'Agenda' to set the userID there
}
[/code]
in the application class 'Agenda' i have the setUserID method:
[code]
public void setUserID(int uID){
System.out.println("setUserID"); //to check if the method gets
called = ok
userIDapp=uID;
System.out.printf("userid %d\n",userIDapp);//to check if the uID
argument gets assigned to the locale variable userIDapp = ok
jLabel2.setText(""+ userID); //when i try to set the text of
jLabel2 a JLabel in the 'Agenda' class i get an error also when i just
put jLabel2.setText("test");
}
[/code]
the error:
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at agenda.Agenda.setUserID(Agenda.java:93)
line 93 =
[code]
jLabel2.setText(""+ userID);
[/code]
I'm still learning so if i make big mistakes here forgive me and teach
me better ways
thanks
Marco
Casper Bang
A good one would be Sun's Swing forum: http://forums.sun.com/forum.jspa?forumID=57
Secondly, your code is practically impossible to read because of all
the comments. When you post sample code, remember to trim it down to
just the essentials where the problem is reproducible.
Third, when encountering an NPE (Null Pointer Exception), one of the
referenced objects must not have been instantiated. Start tracing
backwards from there and deduce where Null could've been introduced,
either by logically tracking code paths or by using a debugger.
The culprit in this case is userID, based on the fact that the
exception is thrown from the event dispatching thread which is invoked
when you call setText. You have an awful lot of similar variables, uID
as the parameter, userIDapp as a copy of uID that you print out and
then finally the userID which you probably never set.
/Casper
Alexey Zinger
2001 Honda CBR600F4i (CCS)
2002 Suzuki Bandit 1200S
1992 Kawasaki EX500
http://azinger.blogspot.com
http://bsheet.sourceforge.net
http://wcollage.sourceforge.net
From: Casper Bang <caspe...@gmail.com>
To: The Java Posse <java...@googlegroups.com>
Sent: Mon, March 29, 2010 6:15:55 PM
Subject: [The Java Posse] Re: Changing JLabel text from other Class
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Casper Bang
On Mar 30, 12:43 am, Alexey Zinger <inline_f...@yahoo.com> wrote:
> My money is jLabel2 is null. Even if userID is null, jLabel2.setText("" + userID) should still work. Not to mention that calling JLabel.setText with null is allowed.
>
> Alexey
> 2001 Honda CBR600F4i (CCS)
> 2002 Suzuki Bandit 1200S
> 1992 Kawasaki EX500http://azinger.blogspot.comhttp://bsheet.sourceforge.nethttp://wcollage.sourceforge.net
>
> ________________________________
> From: Casper Bang <casper.b...@gmail.com>
> To unsubscribe from this group, send email to javaposse+...@googlegroups.com.
Jan Goyvaerts
a regular Swing beginner's error is to work outside the EDT (event dispatch thread). If you're changing the label - or anything visual for that matter - outside the EDT weird things can happen. Look into SwingUtilities to check if the current thread is the EDT. Otherwise schedule it to run in the EDT.
MAYBE that's your problem.
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Marco
i used following code to check if i was on the EDT
if(javax.swing.SwingUtilities.isEventDispatchThread()){
System.out.println("is EDT");
System.out.println( Thread.currentThread().getName() );
jLabel2.setText("test");
}else{
System.out.println("is not EDT");
}
the console returns "is EDT" and the name "AWT-EventQueue-1" but still
gives an error on the 'jLabel.setText("test");' line
in the run method of the 'Agenda' class i use 'jLabel2.setText(""+
userID);' this works and gives no errors. It sets jLabel2 to the
original userID value of 0.
it only generates an error used in the setUserID method that gets
called from the 'Login' class
following lines instantiate (?) jLabel2
private javax.swing.JLabel jLabel2; (ivar)
jLabel2 = new javax.swing.JLabel(); (in the initComponents() method
that gets called in the initial run method of the 'Agenda' class)
the above code was generated by Netbeans.
???
greetings
Marco
On 30 mrt, 09:17, Jan Goyvaerts <java.arti...@gmail.com> wrote:
> I haven't got time yet to read the whole conversation, but...
>
> a regular Swing beginner's error is to work outside the EDT (event dispatch
> thread). If you're changing the label - or anything visual for that matter -
> outside the EDT weird things can happen. Look into SwingUtilities to check
> if the current thread is the EDT. Otherwise schedule it to run in the EDT.
>
> MAYBE that's your problem.
>
>
>
> On Tue, Mar 30, 2010 at 00:43, Alexey Zinger <inline_f...@yahoo.com> wrote:
> > My money is jLabel2 is null. Even if userID is null, jLabel2.setText("" +
> > userID) should still work. Not to mention that calling JLabel.setText with
> > null is allowed.
>
> > Alexey
> > 2001 Honda CBR600F4i (CCS)
> > 2002 Suzuki Bandit 1200S
> > 1992 Kawasaki EX500
> >http://azinger.blogspot.com
> >http://bsheet.sourceforge.net
> >http://wcollage.sourceforge.net
>
> > ------------------------------
> > *From:* Casper Bang <casper.b...@gmail.com>
> > *To:* The Java Posse <java...@googlegroups.com>
> > *Sent:* Mon, March 29, 2010 6:15:55 PM
> > *Subject:* [The Java Posse] Re: Changing JLabel text from other Class
> > javaposse+...@googlegroups.com<javaposse%2Bunsubscribe@googlegroups .com>
Alexey Zinger
Subject: [The Java Posse] Re: Changing JLabel text from other Class
Marco
you are right, at the point i want to change the jLabel2 it is 'null',
but it isn't 'null' when i first set it to '0'. Is this because it
gets instantiated in an other method (gets done automatically by
netbeans)?
when i put
jLabel2 = new JLabel();
in front of the setText i don't get the error but the text on my
original jLabel2 doesn't change (probably because i'm referring to the
wrong instance.), how can i reach the correct instance or how can i
make shure jLabel2 doesn't become 'null' ?
keep in mind I am a beginner, thanks for the patience.
greetings
Marco
On 30 mrt, 18:10, Alexey Zinger <inline_f...@yahoo.com> wrote:
> Why don't you stick a little debug output of jLabel2 value somewhere just before you call jLabel2.setText.
>
> ________________________________
> From: Marco <marco.sto...@gmail.com>
> > > javaposse+...@googlegroups.com<javaposse%2Bunsubscribe@googlegroups .com>
> > > .
> > > For more options, visit this group at
> > >http://groups.google.com/group/javaposse?hl=en.
>
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David Stevens
You are getting rather confused about some fundamental things.
An object first has to be declared...
JLabel myLabel; // at this point the variable 'myLabel' is null
... and then created using the 'new' operator...
myLabel = new JLabel("Hello World");
These two things can be done in one line of code...
JLabel myLabel = new JLabel("Hello World");
A really good place to start learning Java is: http://java.sun.com/docs/books/tutorial/
There is a wealth of information there contained in 'trails'.
And a really good forum to ask questions like this is: http://www.coderanch.com/forums
There is an easy way to post formatted code there - look for the
'Beginning Java' forum.
Hope this helps,
David
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Alexey
Java references, object instantiation, etc. Just a few things off the
top of my head:
1. jLabel2 is an object reference and therefore can never be set to
0. It can be null, which is the state it's in when you're trying to
call setText on it. Trace your code backwards and I guarantee you
won't find a place where it's guaranteed to be set. Keep in mind that
this by itself leaves it as null until you assign it to something
else:
JLabel jLabel2;
Don't forget to keep track of objects enclosing those references. Do
you have multiple? In your example, are you instantiating jLabel2 in
one instance, but trying to use that reference belonging to another
object? Your code snippets don't make that clear.
2. Be sure you understand the difference between an object and a
reference pointing to that object. Consider the following:
Example1:
JLabel label1 = new JLabel();
JLabel label2 = label1;
label1.setText("All work and no play...");
label1 = null;
Example2:
JLabel label1 = new JLabel();
label1.setText("All work and no play...");
JLabel label2 = new JLabel(label1.getText());
label1.setText("Makes Jack a dull boy");
Play around with this and stick it in a simple GUI, reorder the lines,
add stuff. Be sure you're able to look at code like this and know
what its side effects and results are before you run it.