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The testbed for JBox2D, a 2d java physics engine, ported from the C++ Box2d engine.
There is a newer version: 2.2.1.1
/*******************************************************************************
 * Copyright (c) 2013, Daniel Murphy
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 * 	* Redistributions of source code must retain the above copyright notice,
 * 	  this list of conditions and the following disclaimer.
 * 	* Redistributions in binary form must reproduce the above copyright notice,
 * 	  this list of conditions and the following disclaimer in the documentation
 * 	  and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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 * POSSIBILITY OF SUCH DAMAGE.
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/**
 * Created at 5:18:10 AM Jan 14, 2011
 */
package org.jbox2d.testbed.tests;

import org.jbox2d.callbacks.ContactImpulse;
import org.jbox2d.collision.shapes.PolygonShape;
import org.jbox2d.common.MathUtils;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.BodyType;
import org.jbox2d.dynamics.Fixture;
import org.jbox2d.dynamics.contacts.Contact;
import org.jbox2d.testbed.framework.TestbedSettings;
import org.jbox2d.testbed.framework.TestbedTest;

/**
 * @author Daniel Murphy
 */
public class Breakable extends TestbedTest {
		
	Body m_body1;
	Vec2 m_velocity;
	float m_angularVelocity;
	PolygonShape m_shape1;
	PolygonShape m_shape2;
	Fixture m_piece1;
	Fixture m_piece2;
	
	boolean m_broke;
	boolean m_break;
	
	/**
	 * @see org.jbox2d.testbed.framework.TestbedTest#initTest(boolean)
	 */
	@Override
	public void initTest(boolean argDeserialized) {
		// Ground body
		{
			BodyDef bd = new BodyDef();
			Body ground = getWorld().createBody(bd);
			
			PolygonShape shape = new PolygonShape();
			shape.setAsEdge(new Vec2(-40.0f, 0.0f), new Vec2(40.0f, 0.0f));
			ground.createFixture(shape, 0.0f);
		}
		
		// Breakable dynamic body
		{
			BodyDef bd = new BodyDef();
			bd.type = BodyType.DYNAMIC;
			bd.position.set(0.0f, 40.0f);
			bd.angle = 0.25f * MathUtils.PI;
			m_body1 = getWorld().createBody(bd);
			
			m_shape1 = new PolygonShape();
			m_shape1.setAsBox(0.5f, 0.5f, new Vec2(-0.5f, 0.0f), 0.0f);
			m_piece1 = m_body1.createFixture(m_shape1, 1.0f);
			
			m_shape2 = new PolygonShape();
			m_shape2.setAsBox(0.5f, 0.5f, new Vec2(0.5f, 0.0f), 0.0f);
			m_piece2 = m_body1.createFixture(m_shape2, 1.0f);
		}
		
		m_break = false;
		m_broke = false;
	}
	
	/**
	 * @see org.jbox2d.testbed.framework.TestbedTest#postSolve(org.jbox2d.dynamics.contacts.Contact,
	 *      org.jbox2d.callbacks.ContactImpulse)
	 */
	@Override
	public void postSolve(Contact contact, ContactImpulse impulse) {
		if (m_broke) {
			// The body already broke.
			return;
		}
		
		// Should the body break?
		int count = contact.getManifold().pointCount;
		
		float maxImpulse = 0.0f;
		for (int i = 0; i < count; ++i) {
			maxImpulse = MathUtils.max(maxImpulse, impulse.normalImpulses[i]);
		}
		
		if (maxImpulse > 40.0f) {
			// Flag the body for breaking.
			m_break = true;
		}
	}
	
	void Break() {
		// Create two bodies from one.
		Body body1 = m_piece1.getBody();
		Vec2 center = body1.getWorldCenter();
		
		body1.destroyFixture(m_piece2);
		m_piece2 = null;
		
		BodyDef bd = new BodyDef();
		bd.type = BodyType.DYNAMIC;
		bd.position = body1.getPosition();
		bd.angle = body1.getAngle();
		
		Body body2 = getWorld().createBody(bd);
		m_piece2 = body2.createFixture(m_shape2, 1.0f);
		
		// Compute consistent velocities for new bodies based on
		// cached velocity.
		Vec2 center1 = body1.getWorldCenter();
		Vec2 center2 = body2.getWorldCenter();
		
		Vec2 velocity1 = m_velocity.add(Vec2.cross(m_angularVelocity, center1.sub(center)));
		Vec2 velocity2 = m_velocity.add(Vec2.cross(m_angularVelocity, center2.sub(center)));
		
		body1.setAngularVelocity(m_angularVelocity);
		body1.setLinearVelocity(velocity1);
		
		body2.setAngularVelocity(m_angularVelocity);
		body2.setLinearVelocity(velocity2);
	}
	
	/**
	 * @see org.jbox2d.testbed.framework.TestbedTest#step(org.jbox2d.testbed.framework.TestbedSettings)
	 */
	@Override
	public void step(TestbedSettings settings) {
		
		if (m_break) {
			Break();
			m_broke = true;
			m_break = false;
		}
		
		// Cache velocities to improve movement on breakage.
		if (m_broke == false) {
			m_velocity = m_body1.getLinearVelocity();
			m_angularVelocity = m_body1.getAngularVelocity();
		}
		
		super.step(settings);
	}
	
	/**
	 * @see org.jbox2d.testbed.framework.TestbedTest#getTestName()
	 */
	@Override
	public String getTestName() {
		return "Breakable";
	}
	
}