[libGDX] [box2D] How to use Contact Listener?

Newbie & Debugging Questions
#1

I have done a test but I receipt a NullPointerException. I don’t now why receipt this, so, I have the contact listener in a class and in the Screen class I put this method: (and connect it to the create method)


public void testContact(){
		BodyDef bdtest = new BodyDef();
		bdtest.type = BodyType.StaticBody;
		bdtest.position.set(new Vector2(5, 5));
		
		btest = world.createBody(bdtest);
		
		CircleShape cs = new CircleShape();
		cs.setRadius(0.5f);
		
		ftest = btest.createFixture(cs, 0.6f);
		
		//NullPointerExeption
		test = new Test(instance, player.fplayer, ftest);
		
		world.setContactListener(test);
	}

This is the NullPointerException:


java.lang.NullPointerException
	at com.unclain.udevelop.prueba1.ScreenTestOne.testContact(ScreenTestOne.java:301)
	at com.unclain.udevelop.prueba1.ScreenTestOne.show(ScreenTestOne.java:125)
	at com.badlogic.gdx.Game.setScreen(Game.java:62)
	at com.unclain.udevelop.prueba1.GameScreen$1.onEvent(GameScreen.java:71)
	at aurelienribon.tweenengine.BaseTween.callCallback(BaseTween.java:380)
	at aurelienribon.tweenengine.BaseTween.updateStep(BaseTween.java:521)
	at aurelienribon.tweenengine.BaseTween.update(BaseTween.java:424)
	at aurelienribon.tweenengine.TweenManager.update(TweenManager.java:166)
	at com.unclain.udevelop.prueba1.GameScreen.render(GameScreen.java:37)
	at com.badlogic.gdx.Game.render(Game.java:46)
	at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:187)
	at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:110)

This is the ContactListener class:


package com.unclain.udevelop.prueba1.contacts;

import com.badlogic.gdx.physics.box2d.Contact;
import com.badlogic.gdx.physics.box2d.ContactImpulse;
import com.badlogic.gdx.physics.box2d.ContactListener;
import com.badlogic.gdx.physics.box2d.Fixture;
import com.badlogic.gdx.physics.box2d.Manifold;
import com.unclain.udevelop.prueba1.GameScreen;
import com.unclain.udevelop.prueba1.ScreenTestOne;
import com.unclain.udevelop.prueba1.UDevelop1;
import com.unclain.udevelop.prueba1.ingame.EntityPlayer;

public class Test implements ContactListener{

	private Fixture fA;
	private Fixture fB;
	private ScreenTestOne mainInstance;

	public Test(ScreenTestOne mainInstance, Fixture fA, Fixture fB){
		this.mainInstance = mainInstance;
		this.fA = fA;
		this.fB = fB;
		
		mainInstance = new ScreenTestOne(new GameScreen(new UDevelop1()));
	}
	
	@Override
	public void beginContact(Contact contact) {
		// TODO Auto-generated method stub
		fA = contact.getFixtureA();
		fB = contact.getFixtureB();
		
		if(fA.getBody() != null && fB.getBody() != null){
			if(fA.equals(mainInstance.player.fplayer) && fB.equals(mainInstance.ftest)){
				System.out.println("¡QUESO!");
			}
		}
	}

	@Override
	public void endContact(Contact contact) {
		// TODO Auto-generated method stub
		fA = contact.getFixtureA();
		fB = contact.getFixtureB();
		
		if(fA.getBody() != null && fB.getBody() != null){
			if(fA.equals(mainInstance.player.fplayer) && fB.equals(mainInstance.ftest)){
				System.out.println("¡QUESO!");
			}
		}
	}

	@Override
	public void preSolve(Contact contact, Manifold oldManifold) {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void postSolve(Contact contact, ContactImpulse impulse) {
		// TODO Auto-generated method stub
		
	}

}

I was searching but I don’t find nothing useful, so, How work ContactListener? How can I detect a contact between two Fixtures?

Thanks.

P.D: I’m spanish, so, thanks for try understand me. ;D