JBox2D Setup problems

Newbie & Debugging Questions
#1

I just started learning JBox2d and am getting used to its’ code. I’m pretty sure I set up the OpenGL correctly, but I’m getting these errors:

Exception in thread "main" java.lang.NullPointerException
	at ludumdare.BasicGravity.initObjects(BasicGravity.java:77)
	at ludumdare.BasicGravity.<init>(BasicGravity.java:25)
	at ludumdare.BasicGravity.main(BasicGravity.java:98)

Code(Single class) :

import static org.lwjgl.opengl.GL11.*;
import org.jbox2d.dynamics.BodyType;
import org.jbox2d.collision.shapes.CircleShape;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.FixtureDef;
import org.jbox2d.dynamics.World;
import org.lwjgl.LWJGLException;
import org.lwjgl.opengl.Display;
import org.lwjgl.opengl.DisplayMode;

public class BasicGravity {

	
	private World world;
	
	public BasicGravity()  throws LWJGLException {
		setUpDisplay();
		setUpWorld();
		initGL();
		initObjects();
		render();
	
		
	}
	
	private void render() {
		
		while (!Display.isCloseRequested()) {
		glMatrixMode(GL_MODELVIEW);
		glClear(GL_COLOR_BUFFER_BIT);
		glLoadIdentity();
		
		
		
		Display.update();
		Display.sync(30);
		}
		Display.destroy();
		System.exit(0);
		
	}
	
	private void initGL() {
		glMatrixMode(GL_PROJECTION);
		glLoadIdentity();
		glOrtho(0, Display.getWidth(), 0, Display.getHeight(), -1, 1);
		glMatrixMode(GL_MODELVIEW);
		
	}
	
	private void setUpWorld() {
		Vec2 gravity = new Vec2(0.0f, -10.0f);
		 boolean doSleep = true;
		World world = new World(gravity);
	}
	
	private void initObjects() {
		
		BodyDef bf = new BodyDef();
		bf.position.set(100, 100);
		bf.type = BodyType.DYNAMIC;
		
		CircleShape cs = new CircleShape();
		cs.m_radius = 0.5f;
		
		FixtureDef fd = new FixtureDef();
		fd.shape = cs;
		fd.density = 0.5f;
		fd.friction = 0.3f;        
		fd.restitution = 0.5f;
		
		Body body =  world.createBody(bf);
		body.createFixture(fd);

		
		
	}
	
	
	 private static void setUpDisplay() {
	        try {
	            Display.setDisplayMode(new DisplayMode(1000, 700));
	            Display.setTitle("Graity");
	            Display.create();
	        } catch (LWJGLException e) {
	            e.printStackTrace();
	        }
	    }
	
	public static void main(String[] args) {
		
		try {
			new BasicGravity();
		} catch (LWJGLException e) {}
	}

}

I’m pretty sure I just need to add more JBox2d code, hope you can help.

#2

Looking at your setUpWorld() method, you have

World world = new World(gravity);

This is wrong, because this creates the local variable “world”, but doesn’t update the class variable “world”. Instead just do

 world = new World(gravity);

then the class variable world won’t be null and there will be no exception.

#3

Ah, silly mistake. Now, where my render method is I call this :

//Where I draw the body
		glPushMatrix();
        Vec2 bodyPosition = body.getPosition().mul(30);
        glTranslatef(bodyPosition.x, bodyPosition.y, 0);
        glRotated(Math.toDegrees(body.getAngle()), 0, 0, 1);
        glRectf(-0.75f * 30, -0.75f * 30, 0.75f * 30, 0.75f * 30);
		glPopMatrix();

And get this error :

Exception in thread "main" java.lang.NullPointerException
	at ludumdare.BasicGravity.render(BasicGravity.java:40)
	at ludumdare.BasicGravity.<init>(BasicGravity.java:26)
	at ludumdare.BasicGravity.main(BasicGravity.java:104)

I didn’t use any local variables like you explained.