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Null Graphics Object

Posted on 2004-09-14
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Last Modified: 2010-03-31
Hey Experts!

What does it mean when the Graphics object that is related to a certain JPanel is null at a certain point of your program when you want to use it?

for example:

TestJPanel.getGraphics.drawline(...); //gives a null pointer exception becasue of the null graphics object
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Question by:mte01
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by:objects
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probably because there is not one available (eg. panel isn't visible)
you really shouldn't use getGraphics() and only use the Graphics object passed to you in the paint methods.
Why do you need it outside the paint() methods?

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by:mte01
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for(int j=0,curheight=0;j<descno;j++)
        {
          LCCol curcol = (LCCol)curLCTab.getRowAt(i).elementAt(inc+j);
          lblspecs[f][j] = new JLabel(curcol.getColLabelRep()+": "+curcol.getColValue());
          jPanel5rep2[f].add(lblspecs[f][j]);
        }
        jPanel10rep2.add(jPanel5rep2[f],new XYConstraints(15,panelheight,-1,-1));
        panelheight += jPanel5rep2[f].getPreferredSize().getHeight() + 10;
        this.getGraphics().drawLine(0,panelheight+(int)jPanel3.getPreferredSize().getHeight(),150,panelheight+(int)jPanel3.getPreferredSize().getHeight());
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by:mte01
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Sorry ignore the previous code......
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by:mte01
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                 for(int j=0,curheight=0;j<descno;j++)
        {
          LCCol curcol = (LCCol)curLCTab.getRowAt(i).elementAt(inc+j);
          lblspecs[f][j] = new JLabel(curcol.getColLabelRep()+": "+curcol.getColValue());
          jPanel5rep2[f].add(lblspecs[f][j]);
        }
        jPanel10rep2.add(jPanel5rep2[f],new XYConstraints(15,panelheight,-1,-1));
        jPanel5rep2[f].getGraphics().drawLine(0,panelheight+(int)jPanel3.getPreferredSize().getHeight(),150,panelheight+(int)jPanel3.getPreferredSize().getHeight());

This is part of the function that I am using..........I think the panel is visible when I am using the Graphics object........Moreover, the program is very large, and I want to draw lines at the end of certain JPanels in certain functions in my program (that's why it would be very difficult to use paint....
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by:CI-Ia0s
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It means that TestJPanel has no graphics object or is not currently displayable. Make sure that you have declared and initialized TestJPanel and that TestJPanel is not null.
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by:CI-Ia0s
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I'd suggest having a for loop where you declare the entire array of panels, if that's possible...
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objects earned 500 total points
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> and I want to draw lines at the end of certain JPanels in certain functions in my program

problem with that is that your painting will be lost if your window is obscured , or the window is resized, or any other event occurs that requires the window to be repainted.

I'd suggest instead painting to an offscreen image, and painting that image to the screen in your paint method.
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by:CI-Ia0s
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Declare and construct, I should say...
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by:CI-Ia0s
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Or you could do what objects says, cause he's always right (and it *does* make more sense). ;)
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by:objects
Comment Utility
> cause he's always right

dunno about that :)
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by:mte01
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>> I'd suggest having a for loop where you declare the entire array of panels, if that's possible...

I am doing that but I forgot to include that part in the code:

for(int i=0,f=0;i<rowno;i++)
      {
      if(gvec.contains(new Integer(i)))
      {
        jPanel5rep2[f] = new JPanel(new GridLayout(descno,1));
        for(int j=0,curheight=0;j<descno;j++)
        {
          LCCol curcol = (LCCol)curLCTab.getRowAt(i).elementAt(inc+j);
          lblspecs[f][j] = new JLabel(curcol.getColLabelRep()+": "+curcol.getColValue());
          jPanel5rep2[f].add(lblspecs[f][j]);
        }
        jPanel10rep2.add(jPanel5rep2[f],new XYConstraints(15,panelheight,-1,-1));
        panelheight += jPanel5rep2[f].getPreferredSize().getHeight() + 10;
        jPanel5rep2[f].getGraphics().drawLine(0,panelheight+(int)jPanel3.getPreferredSize().getHeight(),150,panelheight+(int)jPanel3.getPreferredSize().getHeight());
        f++;
      }
               ................
              .................

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by:mte01
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>> your painting will be lost if your window is obscured , or the window is resized, or any other event occurs that requires the window to be repainted.

I drew a line using the getGraphics method (in another function) of the applet that I am using (this.getGraphics()...), and I tested the program using the appletviewer and in a Web Browser, and I changed the size of the window, minimized it, navigated away from it and returned back......etc, but there were no problems.....
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by:objects
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It may work sometimes, but there is no guarantee. As soon as the panel needs repainting for whatever reason its paint methods will get called and they paint whatever is there.
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by:mte01
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You can see the full source at the following adress: http://students.aub.edu.lb/~mte01/lcfilemain.zip

The getGraphics() method is used inside "Applet5.java"....If you feel that the paint method can be used properly for repainting the JPanels (and maybe the JApplet)......please do post your method
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by:objects
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its a bit hard to work out exactly what your code is doing.
looks like you could simplify things by creating some seperate classes for some of your panels

a side observation is you don't need to sychronise any methods dealing with your gui as they should only ever get called from the one thread.
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by:mte01
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>>its a bit hard to work out exactly what your code is doing.

I know, and I'll be asking many questions about it later :), an you'll be getting many points from them :)......I'll be dropping a few notes of explanation for you about what the whole application is trying to do sometimes later

>> you could simplify things by creating some seperate classes for some of your panels

Yes I thought that would solve the problem.....it's just that I have many Panels to draw, and in each panel I want to draw a certain line at a certain place (some at the beginning, some at the end, some small, other medium or big....etc)....so I would have to write a class for almost each type of panel I am using which would be really cumbersome

>>you don't need to sychronise any methods dealing with your gui as they should only ever get called from the one thread.

Yes I read some documentation about that, and write now I am only letting the main thread call these functions (not any created thread)....despite that, sometimes I get an error of "current thread not main thread" upon reaching a line of code that creates a certain GUI object......synchronized solved this problem :) !!
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by:mte01
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I'll keep this question posted for like a day to see if anyone still can have a solution for my problem...
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