example of a reasonable Singleton

NaturalComparator.java

package v6ak.util;

import java.util.Comparator;

public final class NaturalComparator {

	private static final Comparator<? extends Comparable<?>> instance = new Comparator<Comparable<Object>>(){
		public int compare(Comparable<Object> o1, Comparable<Object> o2) {
			if( o1==null || o2==null ){
				throw new NullPointerException("Comparing null values is not supported!");
			}
			return o1.compareTo(o2);
		}
	};
	
	@SuppressWarnings("unchecked")
	public static <T extends Comparable<?>> Comparator<T> getInstance(){
		return (Comparator<T>) instance;
	}
	
	private NaturalComparator(){}
	
}

NaturalComparator.scala

package v6ak.util

import java.util.Comparator

object NaturalComparator extends Comparator[Comparable[Any]]{

	def apply[T]() = asInstanceOf[Comparator[T]]

	override def compare(o1:Comparable[Any], o2:Comparable[Any]) = {
		if( o1 == null || o2 == null ){
			throw new NullPointerException("Comparing null values is not supported!")
		}
		o1 compareTo o2
	}

}

scala-NaturalComparator-example.scala

import v6ak.util.NaturalComparator
import java.util.Comparator
import java.lang.Integer

def compareChars(a:Char, b:Char, cmp: Comparator[Char]){
	println(cmp.compare(a, b))
}

val intCmp = NaturalComparator[Integer]
val strCmp = NaturalComparator[String]
val universalCmp = NaturalComparator
val universalCmp2 = NaturalComparator()

println(intCmp.compare(Integer valueOf 1, Integer valueOf 2))
println(strCmp.compare("b", "a"))

println(intCmp eq strCmp) // intCmp and strCmp are the same, but they are casted to a different type

// Following two commented commands can't be compiled, of course. It does not make sense.
// println(universalCmp.compare("b", Integer valueOf 5)) // String would have to be able to compare itself with anything.
// println(universalCmp2.compare("b", Integer valueOf 5)) // There is not enough type info provided, so it can't be used.

// Following two commented commands can't be compiled. They make sense, but type information is too restrictive.
// println(universalCmp2.compare("a", "b"))
// println(universalCmp.compare("a", "b"))

compareChars('r', 'r', NaturalComparator()) // The type of comparator is inferred. However, it is neccessary to add braaces.