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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.
 * 
 * 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)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 ******************************************************************************/
/**
 * Created at 12:52:04 AM Jan 20, 2011
 */
package org.jbox2d.pooling.normal;

import java.lang.reflect.Array;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * @author Daniel Murphy
 */
public class OrderedStack {
  private static final Logger log = LoggerFactory.getLogger(OrderedStack.class);

  private final E[] pool;
  private int index;
  private final int size;
  private final E[] container;

  @SuppressWarnings("unchecked")
  public OrderedStack(Class argClass, int argStackSize, int argContainerSize) {
    size = argStackSize;
    pool = (E[]) Array.newInstance(argClass, argStackSize);
    for (int i = 0; i < argStackSize; i++) {
      try {
        pool[i] = argClass.newInstance();
      } catch (InstantiationException e) {
        log.error("Error creating pooled object " + argClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + argClass.getCanonicalName();
      } catch (IllegalAccessException e) {
        log.error("Error creating pooled object " + argClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + argClass.getCanonicalName();
      }
    }
    index = 0;
    container = (E[]) Array.newInstance(argClass, argContainerSize);
  }

  public final E pop() {
    assert (index < size) : "End of stack reached, there is probably a leak somewhere";
    return pool[index++];
  }

  public final E[] pop(int argNum) {
    assert (index + argNum < size) : "End of stack reached, there is probably a leak somewhere";
    assert (argNum <= container.length) : "Container array is too small";
    System.arraycopy(pool, index, container, 0, argNum);
    index += argNum;
    return container;
  }

  public final void push(int argNum) {
    index -= argNum;
    assert (index >= 0) : "Beginning of stack reached, push/pops are unmatched";
  }
}