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 *       "This product includes software developed by the
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 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
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package com.jidesoft.comparator;

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

/**
 * A Comparator that compares Comparable objects. Throws ClassCastExceptions if the objects are not
 * Comparable, or if they are null. Throws ClassCastException if the compareTo of both objects do
 * not provide an inverse result of each other as per the Comparable javadoc.  This Comparator is
 * useful, for example, for enforcing the natural order in custom implementations of SortedSet and
 * SortedMap. 
 If both objects are null, they will be treated as equal. If one is null and the
 * other is not, the null value will be treated as smaller then non-null value.
 *
 * @author [email protected]
 * @author JIDE Software
 */
public class ComparableComparator implements Comparator, Serializable {

    private static final ComparableComparator instance =
            new ComparableComparator();

    /**
     * Return a shared instance of a ComparableComparator.  Developers are encouraged to use the
     * comparator returned from this method instead of constructing a new instance to reduce
     * allocation and GC overhead when multiple comparable comparators may be used in the same VM.
     *
     * @return an instance of ComparableComparator.
     */
    public static ComparableComparator getInstance() {
        return instance;
    }

    private static final long serialVersionUID = -291439688585137865L;

    /**
     * Constructs a ComparableComparator.
     */
    public ComparableComparator() {
    }

    public int compare(Object o1, Object o2) {
        if (o1 == null && o2 == null) {
            return 0;
        }
        else if (o1 == null) {
            return -1;
        }
        else if (o2 == null) {
            return 1;
        }

        if (o1 instanceof Comparable) {
            if (o2 instanceof Comparable) {
                int result1;
                int result2;
                try {
                    result1 = ((Comparable) o1).compareTo(o2);
                    result2 = ((Comparable) o2).compareTo(o1);
                }
                catch (ClassCastException e) {
                    return o1.getClass().getName().compareTo(o2.getClass().getName()); // fall back to compare the string
                }

                // enforce comparable contract
                if (result1 == 0 && result2 == 0) {
                    return 0;
                }
                else
                if (result1 < 0 && result2 > 0) { // to make sure the two results are consistent
                    return result1;
                }
                else
                if (result1 > 0 && result2 < 0) { // to make sure the two results are consistent
                    return result1;
                }
                else {
                    // results inconsistent
                    throw new ClassCastException("The two compareTo methods of o1 and o2 returned two inconsistent results. Please make sure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y.");
                }
            }
            else {
                // o2 wasn't comparable
                throw new ClassCastException("The second argument of this method was not a Comparable: " + o2.getClass().getName());
            }
        }
        else if (o2 instanceof Comparable) {
            // o1 wasn't comparable
            throw new ClassCastException("The first argument of this method was not a Comparable: " + o1.getClass().getName());
        }
        else {
            // neither were comparable
            throw new ClassCastException("Both arguments of this method were not Comparables: " + o1.getClass().getName() + " and " + o2.getClass().getName());
        }
    }
}