A very simple unsynchronized stack. This one is faster than the java.util-Version. : Stack « Collections « Java Tutorial

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/**
 * 
 * JCommon : a free Java report library
 * 
 *
 * Project Info:  http://www.jfree.org/jcommon/
 *
 * (C) Copyright 2000-2006, by Object Refinery Limited and Contributors.
 *
 * This library is free software; you can redistribute it and/or modify it under the terms
 * of the GNU Lesser General Public License as published by the Free Software Foundation;
 * either version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
 * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License along with this
 * library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330,
 * Boston, MA 02111-1307, USA.
 *
 * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
 * in the United States and other countries.]
 *
 * ------------
 * $Id: FastStack.java,v 1.3 2008/09/10 09:22:05 mungady Exp $
 * ------------
 * (C) Copyright 2002-2006, by Object Refinery Limited.
 */

import java.io.Serializable;
import java.util.Arrays;
import java.util.EmptyStackException;

/**
 * A very simple unsynchronized stack. This one is faster than the
 * java.util-Version.
 * 
 * @author Thomas Morgner
 */
public final class FastStack implements Serializable, Cloneable {
  private Object[] contents;

  private int size;

  private int initialSize;

  /**
   * Creates a new empty stack.
   */
  public FastStack() {
    this.initialSize = 10;
  }

  /**
   * Creates a new empty stack with the specified initial storage size.
   * 
   * @param size
   *          the initial storage elements.
   */
  public FastStack(int size) {
    this.initialSize = Math.max(1, size);
  }

  /**
   * Returns <code>true</code> if the stack is empty, and <code>false</code>
   * otherwise.
   * 
   * @return A boolean.
   */
  public boolean isEmpty() {
    return this.size == 0;
  }

  /**
   * Returns the number of elements in the stack.
   * 
   * @return The element count.
   */
  public int size() {
    return this.size;
  }

  /**
   * Pushes an object onto the stack.
   * 
   * @param o
   *          the object.
   */
  public void push(Object o) {
    if (this.contents == null) {
      this.contents = new Object[this.initialSize];
      this.contents[0] = o;
      this.size = 1;
      return;
    }

    final int oldSize = this.size;
    this.size += 1;
    if (this.contents.length == this.size) {
      // grow ..
      final Object[] newContents = new Object[this.size + this.initialSize];
      System.arraycopy(this.contents, 0, newContents, 0, this.size);
      this.contents = newContents;
    }
    this.contents[oldSize] = o;
  }

  /**
   * Returns the object at the top of the stack without removing it.
   * 
   * @return The object at the top of the stack.
   */
  public Object peek() {
    if (this.size == 0) {
      throw new EmptyStackException();
    }
    return this.contents[this.size - 1];
  }

  /**
   * Removes and returns the object from the top of the stack.
   * 
   * @return The object.
   */
  public Object pop() {
    if (this.size == 0) {
      throw new EmptyStackException();
    }
    this.size -= 1;
    final Object retval = this.contents[this.size];
    this.contents[this.size] = null;
    return retval;
  }

  /**
   * Returns a clone of the stack.
   * 
   * @return A clone.
   */
  public Object clone() {
    try {
      FastStack stack = (FastStack) super.clone();
      if (this.contents != null) {
        stack.contents = (Object[]) this.contents.clone();
      }
      return stack;
    } catch (CloneNotSupportedException cne) {
      throw new IllegalStateException("Clone not supported? Why?");
    }
  }

  /**
   * Clears the stack.
   */
  public void clear() {
    this.size = 0;
    if (this.contents != null) {
      Arrays.fill(this.contents, null);
    }
  }

  /**
   * Returns the item at the specified slot in the stack.
   * 
   * @param index
   *          the index.
   * 
   * @return The item.
   */
  public Object get(final int index) {
    if (index >= this.size) {
      throw new IndexOutOfBoundsException();
    }
    return this.contents[index];
  }
}