Class comparator: compare and sort classes and their superclasses. : Class « Reflection « Java Tutorial

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/* 
 * JCommon : a free general purpose class library for the Java(tm) platform
 * 
 *
 * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
 * 
 * Project Info:  http://www.jfree.org/jcommon/index.html
 *
 * 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, 
 * USA.  
 *
 * [Java is a trademark or registered trademark of Sun Microsystems, Inc. 
 * in the United States and other countries.]
 *
 * --------------------
 * ClassComparator.java
 * --------------------
 * (C)opyright 2003-2005, by Thomas Morgner and Contributors.
 *
 * Original Author:  Thomas Morgner (taquera@sherito.org);
 * Contributor(s):   David Gilbert (for Object Refinery Limited);
 *
 * $Id: ClassComparator.java,v 1.3 2005/10/18 13:24:19 mungady Exp $
 *
 * Changes
 * -------
 * 02-May-2003 : Initial version
 * 
 */

import java.io.Serializable;
import java.util.Comparator;

/**
 * The class comparator can be used to compare and sort classes and their
 * superclasses. The comparator is not able to compare classes which have no
 * relation...
 * 
 * @author Thomas Morgner
 */
public class ClassComparator implements Comparator, Serializable {

  /** For serialization. */
  private static final long serialVersionUID = -5225335361837391120L;

  /**
   * Defaultconstructor.
   */
  public ClassComparator() {
    super();
  }

  /**
   * Compares its two arguments for order. Returns a negative integer, zero, or
   * a positive integer as the first argument is less than, equal to, or greater
   * than the second.
   * 
   * 
   * Note: throws ClassCastException if the arguments' types prevent them from
   * being compared by this Comparator. And IllegalArgumentException if the
   * classes share no relation.
   * 
   * The implementor must ensure that <tt>sgn(compare(x, y)) ==
   * -sgn(compare(y, x))</tt>
   * for all <tt>x</tt> and <tt>y</tt>. (This implies that
   * <tt>compare(x, y)</tt> must throw an exception if and only if
   * <tt>compare(y, x)</tt> throws an exception.)
   * 
   * 
   * The implementor must also ensure that the relation is transitive:
   * <tt>((compare(x, y)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
   * <tt>compare(x, z)&gt;0</tt>.
   * 
   * 
   * Finally, the implementer must ensure that <tt>compare(x, y)==0</tt>
   * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
   * <tt>z</tt>.
   * 
   * 
   * It is generally the case, but <i>not</i> strictly required that
   * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, any
   * comparator that violates this condition should clearly indicate this fact.
   * The recommended language is "Note: this comparator imposes orderings that
   * are inconsistent with equals."
   * 
   * @param o1
   *          the first object to be compared.
   * @param o2
   *          the second object to be compared.
   * @return a negative integer, zero, or a positive integer as the first
   *         argument is less than, equal to, or greater than the second.
   */
  public int compare(final Object o1, final Object o2) {
    final Class c1 = (Class) o1;
    final Class c2 = (Class) o2;
    if (c1.equals(o2)) {
      return 0;
    }
    if (c1.isAssignableFrom(c2)) {
      return -1;
    } else {
      if (!c2.isAssignableFrom(c2)) {
        throw new IllegalArgumentException("The classes share no relation");
      }
      return 1;
    }
  }

  /**
   * Checks, whether the given classes are comparable. This method will return
   * true, if one of the classes is assignable from the other class.
   * 
   * @param c1
   *          the first class to compare
   * @param c2
   *          the second class to compare
   * @return true, if the classes share a direct relation, false otherwise.
   */
  public boolean isComparable(final Class c1, final Class c2) {
    return (c1.isAssignableFrom(c2) || c2.isAssignableFrom(c1));
  }
}