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NullPointerException on DoubleBuffering


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#1 epicpunnum   Members   -  Reputation: 458

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Posted 13 November 2012 - 08:24 PM

So I am currently working on a Swing-based level editor for my current game. The Editor that holds everything is a JFrame, holding multiple JScrollPanes for each general purpose (tiles, object properties, room being edited, etc.) To display the content on, I placed a Panel inside of each JScrollpane. However, for the room being edited, I felt it would be necessary to include a child of Panel that uses Runnable to continuously update the Grid object as well as all of the communication.

Oddly though, I have been receiving a NullPointerException in my double-buffering approach. According to my stack trace, the problem lies in the Graphics.drawImage() method, but as you can see my code should be preventing my BufferedImage from being null.
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Panel;
import java.awt.image.BufferedImage;
/**
* Class used to hold room design in progress
* Uses Runnable to keep updated
*
* @author Ian_Donovan
* @date
*/
public class RoomPanel extends Panel implements Runnable{
private static final long serialVersionUID = 1L;
private int w=1,h=1;
private Grid grid = new Grid(800,640,32,32);
public RoomPanel(int w, int h){
  super();
  this.w=w;
  this.h=h;
  this.setSize(w,h);
  start();
}

//runnable stuff below
public void update(){
  BufferedImage bfi = new BufferedImage(w,h,BufferedImage.TYPE_INT_ARGB);
  Graphics bfg;
  boolean buffered = true;
  bfg = bfi.getGraphics();
  if (bfg==null || bfi == null){
   buffered = false;
   bfg=this.getGraphics();
  }
  bfg.setColor(Color.BLACK);
  bfg.fillRect(0, 0, w, h);
  grid.draw(bfg);
  if (buffered){
   Graphics finalGraphics = this.getGraphics();
   finalGraphics.drawImage(bfi,0,0,null);
   finalGraphics.dispose();
  }
  bfg.dispose();
}
//RUNNABLE THREADING WORKS FINE (Coppied from working code of other projects)
}
For those looking to test it, Grid.draw() only draws a small rectangle for now, so I felt as if it was unnecessary to include.

If someone could explain this error, I would be very grateful :)
Thank you so much in advance.

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#2 TheChubu   Crossbones+   -  Reputation: 4761

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Posted 13 November 2012 - 09:00 PM

This block:
if (bfg==null || bfi == null){
buffered = false;
bfg=this.getGraphics();
}

If im correct, when bfi is null, you don't do anything to fix it right? So if it's null, it keeps being null. Then you do:

if (buffered){
Graphics finalGraphics = this.getGraphics();
finalGraphics.drawImage(bfi,0,0,null);
finalGraphics.dispose();
}

And send bfi as parameter in drawImage. If it was null, you'd be sending a null right there.

Moreover, you do this: bfg = bfi.getGraphics(); before checking if bfi is null. If bfi is null from the get go, it'd trow a NullPointer exception right there.

BUT, you actually call a BufferedImage constructor so it shouldn't be null at all (i guess that if the JVM failed to allocate memory for an object you'd have bigger issues than a nullpointer :D ).

Then this line: finalGraphics.drawImage(bfi,0,0,null); That null right there is supposed to work anyways? I also read that you should use createGraphics instead of getGraphics in the documentation.

Aaaand thats all I can point out. I have no experience with Graphics in Java :D (well, I actually have but it was just for drawing some squares and I forgot all of it :P)

"I AM ZE EMPRAH OPENGL 3.3 THE CORE, I DEMAND FROM THEE ZE SHADERZ AND MATRIXEZ"

 

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#3 n3oplasm   Members   -  Reputation: 132

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Posted 14 November 2012 - 02:39 PM

The call to this.getGraphics() will return null if the component is not visible (i.e. the panel has been added to a JFrame which is not currently set to visible at the time the thread is running). My guess is that finalGraphics is null at the time finalGraphics.drawImage(bfi,0,0,null); is being called. As far as I know, a call to getGraphics (or the preferred createGraphics as TheChubu pointed out) of a BufferedImage object should not return null.

Also, unless there's a reason you aren't currently, I'd consider using the BufferStrategy class that the Java library provides for double-buffering. See the links below for examples of usage in an active-rendering loop. Hope that helps!

http://docs.oracle.com/javase/tutorial/extra/fullscreen/bufferstrategy.html
http://www.gamedev.net/page/resources/_/technical/general-programming/java-games-active-rendering-r2418




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