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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.
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 * 	  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.
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 * 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,
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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;

//Updated to rev 56->130->142 of b2DistanceJoint.cpp/.h

/**
 * Distance joint definition. This requires defining an
 * anchor point on both bodies and the non-zero length of the
 * distance joint. The definition uses local anchor points
 * so that the initial configuration can violate the constraint
 * slightly. This helps when saving and loading a game.
 * @warning Do not use a zero or short length.
 */
public class DistanceJointDef extends JointDef {
	/** The local anchor point relative to body1's origin. */
	public final Vec2 localAnchorA;

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

	/** The equilibrium length between the anchor points. */
	public float length;

	/**
	 * The mass-spring-damper frequency in Hertz.
	 */
	public float frequencyHz;

	/**
	 * The damping ratio. 0 = no damping, 1 = critical damping.
	 */
	public float dampingRatio;

	public DistanceJointDef() {
		type = JointType.DISTANCE;
		localAnchorA = new Vec2(0.0f, 0.0f);
		localAnchorB = new Vec2(0.0f, 0.0f);
		length = 1.0f;
		frequencyHz = 0.0f;
		dampingRatio = 0.0f;
	}

	/**
	 * Initialize the bodies, anchors, and length using the world
	 * anchors.
	 * @param b1 First body
	 * @param b2 Second body
	 * @param anchor1 World anchor on first body
	 * @param anchor2 World anchor on second body
	 */
	public void initialize(final Body b1, final Body b2, final Vec2 anchor1, final Vec2 anchor2) {
		bodyA = b1;
		bodyB = b2;
		localAnchorA.set(bodyA.getLocalPoint(anchor1));
		localAnchorB.set(bodyB.getLocalPoint(anchor2));
		Vec2 d = anchor2.sub(anchor1);
		length = d.length();
	}
}