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    <p>Here's a short example for libgdx. It shows how to create a <em>ContactListener</em> to show which fixtures are involved when contacts are made and broken. It also shows the use of <em>world.getContactList()</em> which will return a list of contacts that still exist after the physics step. This may miss contacts that were made and broken during the course of the physics step. If you are interested in these then you will want to implement a <em>ContactListener</em>, using <em>beginContact()</em> to detect when contacts are made and <em>endContact()</em> to detect when they are broken.</p> <pre><code>package hacks; import com.badlogic.gdx.Gdx; import com.badlogic.gdx.backends.lwjgl.LwjglApplication; import com.badlogic.gdx.backends.lwjgl.LwjglApplicationConfiguration; import com.badlogic.gdx.graphics.GL10; import com.badlogic.gdx.graphics.OrthographicCamera; import com.badlogic.gdx.math.MathUtils; import com.badlogic.gdx.math.Vector2; import com.badlogic.gdx.physics.box2d.Body; import com.badlogic.gdx.physics.box2d.BodyDef; import com.badlogic.gdx.physics.box2d.BodyDef.BodyType; import com.badlogic.gdx.physics.box2d.Box2DDebugRenderer; 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.FixtureDef; import com.badlogic.gdx.physics.box2d.Manifold; import com.badlogic.gdx.physics.box2d.PolygonShape; import com.badlogic.gdx.physics.box2d.World; public class Box2DDemoMain extends com.badlogic.gdx.Game { private static final float SCALING = 0.1f; private Box2DDebugRenderer debugRenderer; private OrthographicCamera camera; private World world; @Override public void create() { debugRenderer = new Box2DDebugRenderer(); camera = new OrthographicCamera(); createWorld(); createCollisionListener(); createGround(); createBox(); } private void createWorld() { Vector2 gravity = new Vector2(0, -10); world = new World(gravity, true); } private void createCollisionListener() { world.setContactListener(new ContactListener() { @Override public void beginContact(Contact contact) { Fixture fixtureA = contact.getFixtureA(); Fixture fixtureB = contact.getFixtureB(); Gdx.app.log("beginContact", "between " + fixtureA.toString() + " and " + fixtureB.toString()); } @Override public void endContact(Contact contact) { Fixture fixtureA = contact.getFixtureA(); Fixture fixtureB = contact.getFixtureB(); Gdx.app.log("endContact", "between " + fixtureA.toString() + " and " + fixtureB.toString()); } @Override public void preSolve(Contact contact, Manifold oldManifold) { } @Override public void postSolve(Contact contact, ContactImpulse impulse) { } }); } private void createGround() { PolygonShape groundShape = new PolygonShape(); groundShape.setAsBox(50, 1); BodyDef groundBodyDef = new BodyDef(); groundBodyDef.type = BodyType.StaticBody; groundBodyDef.position.set(0, -20); Body groundBody = world.createBody(groundBodyDef); FixtureDef fixtureDef = new FixtureDef(); fixtureDef.shape = groundShape; groundBody.createFixture(fixtureDef); groundShape.dispose(); } private void createBox() { PolygonShape boxShape = new PolygonShape(); boxShape.setAsBox(1, 1); BodyDef boxBodyDef = new BodyDef(); boxBodyDef.position.set(0, 20); boxBodyDef.angle = MathUtils.PI / 32; boxBodyDef.type = BodyType.DynamicBody; boxBodyDef.fixedRotation = false; Body boxBody = world.createBody(boxBodyDef); FixtureDef boxFixtureDef = new FixtureDef(); boxFixtureDef.shape = boxShape; boxFixtureDef.restitution = 0.75f; boxFixtureDef.density = 2.0f; boxBody.createFixture(boxFixtureDef); boxShape.dispose(); } @Override public void resize(int width, int height) { super.resize(width, height); float cameraWidth = Gdx.graphics.getWidth() * SCALING; float cameraHeight = Gdx.graphics.getHeight() * SCALING; camera.setToOrtho(false, cameraWidth, cameraHeight); camera.position.set(0, 0, 0); } @Override public void render() { super.render(); world.step(Gdx.graphics.getDeltaTime(), 8, 3); int numContacts = world.getContactCount(); if (numContacts &gt; 0) { Gdx.app.log("contact", "start of contact list"); for (Contact contact : world.getContactList()) { Fixture fixtureA = contact.getFixtureA(); Fixture fixtureB = contact.getFixtureB(); Gdx.app.log("contact", "between " + fixtureA.toString() + " and " + fixtureB.toString()); } Gdx.app.log("contact", "end of contact list"); } Gdx.gl.glClearColor(0.0f, 0.0f, 0.0f, 1.0f); Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT); camera.update(); debugRenderer.render(world, camera.combined); } public static void main(String[] args) { LwjglApplicationConfiguration config = new LwjglApplicationConfiguration(); config.title = Box2DDemoMain.class.getName(); config.width = 800; config.height = 480; config.fullscreen = false; config.useGL20 = true; config.useCPUSynch = true; config.forceExit = true; config.vSyncEnabled = true; new LwjglApplication(new Box2DDemoMain(), config); } } </code></pre>
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    1. COThank you so much :)
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    2. COwhat do the 8 and 3 mean in this Line? world.step(Gdx.graphics.getDeltaTime(), 8, 3);
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    3. COThey are the velocity iterations and position iterations respectively. These values are recommended by the Box2D manual. See section 2.4 - the manual is here: http://www.box2d.org/manual.html
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