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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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package org.jbox2d.testbed.tests;

import org.jbox2d.collision.Manifold;
import org.jbox2d.collision.shapes.CircleShape;
import org.jbox2d.collision.shapes.PolygonShape;
import org.jbox2d.common.Settings;
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.TestbedTest;

public class OneSidedTest extends TestbedTest{

	enum State
	{
		e_unknown,
		e_above,
		e_below,
	};
	
	float m_radius, m_top, m_bottom;
	State m_state;
	Fixture m_platform;
	Fixture m_character;
	
	@Override
	public String getTestName() {
		return "One Sided";
	}

	@Override
	public void initTest(boolean argDeserialized) {
		// Ground
		{
			BodyDef bd = new BodyDef();
			Body ground = getWorld().createBody(bd);

			PolygonShape shape = new PolygonShape();
			shape.setAsEdge(new Vec2(-20.0f, 0.0f), new Vec2(20.0f, 0.0f));
			ground.createFixture(shape, 0.0f);
		}

		// Platform
		{
			BodyDef bd = new BodyDef();
			bd.position.set(0.0f, 10.0f);
			Body body = getWorld().createBody(bd);

			PolygonShape shape = new PolygonShape();
			shape.setAsBox(3.0f, 0.5f);
			m_platform = body.createFixture(shape, 0.0f);

			m_bottom = 10.0f - 0.5f;
			m_top = 10.0f + 0.5f;
		}

		// Actor
		{
			BodyDef bd = new BodyDef();
			bd.type = BodyType.DYNAMIC;
			bd.position.set(0.0f, 12.0f);
			Body body = getWorld().createBody(bd);

			m_radius = 0.5f;
			CircleShape shape = new CircleShape();
			shape.m_radius = m_radius;
			m_character = body.createFixture(shape, 20.0f);

			body.setLinearVelocity(new Vec2(0.0f, -50.0f));

			m_state = State.e_unknown;
		}
	}
	
	@Override
	public void preSolve(Contact contact, Manifold oldManifold) {
		super.preSolve(contact, oldManifold);
		
		Fixture fixtureA = contact.getFixtureA();
		Fixture fixtureB = contact.getFixtureB();

		if (fixtureA != m_platform && fixtureA != m_character){
			return;
		}

		if (fixtureB != m_character && fixtureB != m_character){
			return;
		}

		Vec2 position = m_character.getBody().getPosition();

		if (position.y < m_top + m_radius - 3.0f * Settings.linearSlop){
			contact.setEnabled(false);
		}
	}

}