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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.
 ******************************************************************************/
package org.jbox2d.pooling.normal;

import java.lang.reflect.Array;
import java.lang.reflect.InvocationTargetException;

import org.jbox2d.pooling.IDynamicStack;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MutableStack implements IDynamicStack {

  private static final Logger log = LoggerFactory.getLogger(MutableStack.class);

  private T[] stack;
  private int index;
  private int size;
  private final Class sClass;

  private final Class[] params;
  private final Object[] args;

  public MutableStack(Class argClass, int argInitSize) {
    this(argClass, argInitSize, null, null);
  }

  public MutableStack(Class argClass, int argInitSize, Class[] argParam, Object[] argArgs) {
    index = 0;
    sClass = argClass;
    params = argParam;
    args = argArgs;

    stack = null;
    index = 0;
    extendStack(argInitSize);
  }

  @SuppressWarnings("unchecked")
  private void extendStack(int argSize) {
    T[] newStack = (T[]) Array.newInstance(sClass, argSize);
    if (stack != null) {
      System.arraycopy(stack, 0, newStack, 0, size);
    }
    for (int i = 0; i < newStack.length; i++) {
      try {
        if (params != null) {
          newStack[i] = sClass.getConstructor(params).newInstance(args);
        } else {
          newStack[i] = sClass.newInstance();
        }
      } catch (InstantiationException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      } catch (IllegalAccessException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      } catch (IllegalArgumentException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      } catch (SecurityException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      } catch (InvocationTargetException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      } catch (NoSuchMethodException e) {
        log.error("Error creating pooled object " + sClass.getSimpleName(), e);
        assert (false) : "Error creating pooled object " + sClass.getCanonicalName();
      }
    }
    stack = newStack;
    size = newStack.length;
  }

  /* (non-Javadoc)
   * 
   * @see org.jbox2d.pooling.IDynamicStack#pop() */
  public final E pop() {
    if (index >= size) {
      extendStack(size * 2);
    }
    return stack[index++];
  }

  /* (non-Javadoc)
   * 
   * @see org.jbox2d.pooling.IDynamicStack#push(E) */
  @SuppressWarnings("unchecked")
  public final void push(E argObject) {
    assert (index > 0);
    stack[--index] = (T) argObject;
  }
}