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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. <br> 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 bayard@generationjava.com
* @author JIDE Software
*/
public class ComparableComparator implements Comparator<Object>, Serializable {
private static final long serialVersionUID = -291439688585137865L;
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;
}
/**
* Constructs a ComparableComparator.
*/
public ComparableComparator() {
}
@SuppressWarnings("unchecked")
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<Object>) o1).compareTo(o2);
result2 = ((Comparable<Object>) 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());
}
}
}