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Javis: a small this.getParent() problem. JB 5........

Posted on 2002-07-08
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Last Modified: 2008-02-26
Hiyah!

I have a problem with this snippet of code in JBuilder 5, from the program at (http://cs.baylor.edu/~donahoo/NIUNet/javis.html), its in the Javis.Java file...:

Container parent = this;
               
          while ( !( parent instanceof Frame ) )
               parent = parent.getParent();

          // Create the interface object to draw widgets and handle events.
          try {
               m_iface=new SwingInterface( (Frame) parent, getSize(), getCodeBase(), filename );
               // Add the interface to the applet.
               getContentPane().add( m_iface );
               m_run_ok = true;
          }
          catch (Exception e) {
               m_run_ok = false;
               System.err.println( e.getMessage() );
                        System.err.println("Error in Init!!!");
          }

The 'parent = parent.getParent();' bit keeps quitting with a null pointer exception.

What's going on? This code apparently works in other development environments, so why not JB 5?
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Comment
Question by:Lab_Rat
  • 2
  • 2
  • 2
  • +2
8 Comments
 
LVL 86

Expert Comment

by:CEHJ
ID: 7138041
Well, in the contexts you've seen it working before, you were probably looking at Frame-based stuff. In this  case, you're using an applet, which extends Panel. It *is* a container, hence the assignment:

Container parent = this;

works.

This also works:

while ( !( parent instanceof Frame ) )

as an applet is not an instanceof Frame.

Therefore

parent = parent.getParent();

works. But since the applet does not *have* a parent, you get a null pointer.


0
 
LVL 3

Expert Comment

by:yasser_helmy
ID: 7138229
The highest level parent is not a frame. The loop executes until parent is referencing the highest level container. Since this is not a frame, the loop continues once more. But since parent does not have a parent the method getParent() returns null. The loop then continues, but the line:
(parent instanceof Frame)
throws an exception since null cannot be used with the instaceof operator.
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LVL 4

Accepted Solution

by:
kylar earned 50 total points
ID: 7138493
Change your code from this:

while ( !( parent instanceof Frame ) )
              parent = parent.getParent();

to:

while ( ! (( parent instanceof Window) && ( parent instanceof Applet)) )
              parent = parent.getParent();

And that will get you the proper top level window. Alternately you could use SwingUtilities.getWindowAncestor() call. You will also have to modify the constructor:

m_iface=new SwingInterface( (Frame) parent, getSize(), getCodeBase(), filename );

To not pass a frame, but instead to pass whatever is proper (perhaps a container).

Cheers,
Kylar
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Expert Comment

by:objects
ID: 7139122
> But since the applet does not *have* a parent

Applets can have parents, and generally do.

I guess JBuilder mustn't run the applet in a browser, and instead runs it somehow in it's own container.

The problem is that your loops searches the component hierarchy for an instance of a Frame. If it doesn't find one then when it gets to the top of the tree you'll get an npe. So just change your code to test for null:

while ( parent!=null &&  !(parent instanceof Frame) )
              parent = parent.getParent();

or use:

Frame frame = (Frame) SwingUtilities.getAncestorOfClass(Frame.class, this);
0
 
LVL 4

Expert Comment

by:kylar
ID: 7139151
D'Oh.. my code change should be an OR instead of an AND.. change the && to an ||

Cheers
0
 
LVL 1

Author Comment

by:Lab_Rat
ID: 7140118
Thanks to everyone, but especially Kylar and objects!

Not only did you give two answers, one in the original algorithem style, and a new one utilising the API more efficiently, but both examples worked first time, and solved my problem.

Points for Kylar in this answer, and I've posted some points for you too objects on this board.
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LVL 3

Expert Comment

by:yasser_helmy
ID: 7140320
Hi all,
i think there is a problem with 'objects' code. the exception will not be thrown but the parent var will still be null. to fix this, use
Container parent = this;
Container son = this;
  while (parent!=null && !(parent instanceof Frame)){
    son = parent;
    parent = parent.getParent();
  }
  //when you get here, parent will either reference a frame or become null
  if(parent ==null)
    parent = son;

0
 
LVL 92

Expert Comment

by:objects
ID: 7140360
> i think there is a problem with 'objects' code

Watch what you're saying :-)

> the exception will not be thrown but the parent var
> will still be null

That's true if an instance of Frame is not found in the component hierarchy. And is desirable, as I am assuming that the SwingInterface ctor expects a Frame instance.
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