On 08.06.2013, at 23:51, Aleksey Shipilev <aleksey.shipi...@oracle.com> wrote:
> Hm, sounds interesting. > > What's about yet another "factory"-like default in Comparator: > > Comparator<C> Comparator.make(Class<C> klass) > - returns synthetic comparator using C.compareTo iff C implements > Comparable<C> > - returns null otherwise Err... "make" actually suggests we better do the zero-return Comparator if there is no Comparable<C> on the class. Which fits some of the things you want below. > > That will naively instantiate the Comparator, but there is the possibility to > reuse e.g. ClassValue to lazily associate the default comparator with the > class. > > -Aleksey. > > On 08.06.2013, at 19:50, Doug Lea <d...@cs.oswego.edu> wrote: > >> >> Suppose you have two objects, x and y, both of the same >> type (x.getClass() == y.getClass()) and both known >> to be instanceof Comparable. >> >> Can you compare them? That is, can you safely call >> (Comparable)x).compareTo(y); >> >> Almost always, the answer is yes, because almost all >> Comparable classes are of form "class C implements Comparable<C>" >> >> But there's nothing about interface Comparable that forces >> this to be true. It would be weird but legal if both >> are of form, say, "class C implements Comparable<Byte>". >> Which means that you cannot compare them to each other. >> >> There's currently no direct way to check if you have a normal >> commutative (bidirectionally applicable) pair of comparables. >> >> It can be checked using reflection though, using mechanics >> that are both messy and slower than they might be if >> implemented as a JDK utility method. >> >> There a few possible API choices for how to do this. >> >> The most basic is >> >> boolean isCommutativelyComparable(Class<C?> c); >> >> returning true if c represents a class form C implements Comparable<C>. >> (It would be OK although a little annoying to use if it restricted >> argument type to Class<Comparable<?>>.) >> >> A more general one is: >> >> Class<?> commutativelyComparableClassFor(Object x) >> >> that returns a class C such that >> (x instanceof C) and (C implements Comparable<C>) >> or null if there is no such class. The return type could >> be strengthened to Class<Comparable<?>> >> >> This allows use of the result to match up objects of >> subclasses of commutatively comparable classes. >> >> The most handy form is: >> >> int compareIfCommutativelyComparable(Comparable x, Comparable y); >> >> that returns compareTo result if both of same commutatively >> comparable type, else 0. >> >> Maybe this one is *too* handy though. It's not hard to >> get the same effect manually given the above methods. >> >> Does anyone have any thoughts about introducing at >> least the first, and possibly the others into the >> java.util.Comparables utility class? If so, there would >> be a few immediate usages within JDK. >> >> I'm surprised that this hasn't come up for consideration >> before. Or maybe I haven't noticed previous discussions? >> >> -Doug >> >> >>
