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Re: Enumset.contains

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On 11/10/2012 6:56 PM, Roedy Green wrote:
> On Sat, 10 Nov 2012 00:30:28 +0100, Robert Klemme
> <shortcutter@googlemail.com> wrote, quoted or indirectly quoted
> someone who said :
>
>>> intersection
>> retainAll()
>
> These are methods of AbstractSet inherited and in my opinion
> preposterously pedestrian. You might as well have used a TreeSet as an
> Enum.  A proper intersection method for EnumSets would be implemented
> with a single AND machine instruction.

The implementations aren't actually inherited from AbstractSet. If you 
pay careful attention to the documentation, you'll notice that EnumSet 
is an abstract class. The actual implementation is one of two classes, 
RegularEnumSet or JumboEnumSet, which is selected based on the number of 
elements in the enum. In these subclasses, the methods are indeed 
overridden for increased performance.

 From the source code of java.util.RegularEnumSet (the actual 
implementation of EnumSets for enums with <= 64 characters):
     public boolean retainAll(Collection<?> c) {
         if (!(c instanceof RegularEnumSet))
             return super.retainAll(c);

         RegularEnumSet<?> es = (RegularEnumSet<?>)c;
         if (es.elementType != elementType) {
             boolean changed = (elements != 0);
             elements = 0;
             return changed;
         }

         long oldElements = elements;
         elements &= es.elements;
         return elements != oldElements;
     }

Up to the necessary boilerplate, it is exactly an AND machine instruction.

> Mathematicians have been using the term "intersection" for at least
> 100 years. I think it was improper of Sun/Oracle to rename the
> functions.

Except that java.util.Set isn't a set in the mathematical sense, but a 
specification of an abstract datatype called a "set" (a collection of 
items such that no item is contained more than once). There is a 
distinction between the two; in particular, a mathematical set is an 
intrinsically immutable object while the set datatype is definitely mutable.

Case in point: what would a.intersection(b) do?
1. Return a set c which consists of all elements that are both in a and 
b at the point of the function call.
2. Return a set c which gives you a view of the elements that are both 
in a and b at the time of any given function call in c (so if you remove 
an element from a, it may be removed from c as well).
3. Update a to consist only of the elements that it shares in common with b.

You can make arguments for doing any of these three things (and perhaps 
choose a better name than a noun for a method call). On the other hand, 
retainAll tells you exactly which of the three it will do.

To paraphrase one of my programming books, relationships in 
object-oriented programming don't necessarily follow the same 
relationships in mathematics, even if they have the same name.

-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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Thread

Enumset.contains Roedy Green <see_website@mindprod.com.invalid> - 2012-11-09 13:40 -0800
  Re: Enumset.contains Lew <lewbloch@gmail.com> - 2012-11-09 15:07 -0800
    Re: Enumset.contains Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-11-09 23:50 +0000
  Re: Enumset.contains Robert Klemme <shortcutter@googlemail.com> - 2012-11-10 00:30 +0100
    Re: Enumset.contains Roedy Green <see_website@mindprod.com.invalid> - 2012-11-10 16:56 -0800
      Re: Enumset.contains markspace <-@.> - 2012-11-10 17:10 -0800
      Re: Enumset.contains Joshua Cranmer <Pidgeot18@verizon.invalid> - 2012-11-10 20:38 -0600

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