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A 2D java physics engine, a port of the C++ Box2d engine. This is the core physics engine.
There is a newer version: 2.2.1.1
/*******************************************************************************
 * Copyright (c) 2011, 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.
 *     * Neither the name of the  nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 * 
 * 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 DANIEL MURPHY 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) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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/*
 * JBox2D - A Java Port of Erin Catto's Box2D
 * 
 * JBox2D homepage: http://jbox2d.sourceforge.net/
 * Box2D homepage: http://www.box2d.org
 * 
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 * 
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 * 
 * 1. The origin of this software must not be misrepresented; you must not
 * claim that you wrote the original software. If you use this software
 * in a product, an acknowledgment in the product documentation would be
 * appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 * misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 */

package org.jbox2d.dynamics.joints;

import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;

/**
 *  Revolute joint definition. This requires defining an
 *  anchor point where the bodies are joined. The definition
 *  uses local anchor points so that the initial configuration
 *  can violate the constraint slightly. You also need to
 *  specify the initial relative angle for joint limits. This
 *  helps when saving and loading a game.
 *  The local anchor points are measured from the body's origin
 *  rather than the center of mass because:

 *  
 *  you might not know where the center of mass will be.
 *  if you add/remove shapes from a body and recompute the mass,
 *     the joints will be broken.
 */
public class RevoluteJointDef extends JointDef {
	

	/**
	 *  The local anchor point relative to body1's origin.
	 */
	public Vec2 localAnchorA;

	/**
	 *  The local anchor point relative to body2's origin.
	 */
	public Vec2 localAnchorB;

	/**
	 *  The body2 angle minus body1 angle in the reference state (radians).
	 */
	public float referenceAngle;

	/**
	 *  A flag to enable joint limits.
	 */
	public boolean enableLimit;

	/**
	 *  The lower angle for the joint limit (radians).
	 */
	public float lowerAngle;

	/**
	 *  The upper angle for the joint limit (radians).
	 */
	public float upperAngle;

	/**
	 *  A flag to enable the joint motor.
	 */
	public boolean enableMotor;

	/**
	 *  The desired motor speed. Usually in radians per second.
	 */
	public float motorSpeed;

	/**
	 *  The maximum motor torque used to achieve the desired motor speed.
	 *  Usually in N-m.
	 */
	public float maxMotorTorque;
	
	public RevoluteJointDef() {
		type = JointType.REVOLUTE;
		localAnchorA = new Vec2(0.0f, 0.0f);
		localAnchorB = new Vec2(0.0f, 0.0f);
		referenceAngle = 0.0f;
		lowerAngle = 0.0f;
		upperAngle = 0.0f;
		maxMotorTorque = 0.0f;
		motorSpeed = 0.0f;
		enableLimit = false;
		enableMotor = false;
	}

	/**
	 * Initialize the bodies, anchors, and reference angle using the world
	 * anchor.
	 * @param b1
	 * @param b2
	 * @param anchor
	 */
	public void initialize(final Body b1, final Body b2, final Vec2 anchor) {
		bodyA = b1;
		bodyB = b2;
		bodyA.getLocalPointToOut(anchor, localAnchorA);
		bodyB.getLocalPointToOut(anchor, localAnchorB);
		referenceAngle = bodyA.getAngle() - bodyB.getAngle();
	}

}