Re: Enumset.contains

Joshua Cranmer <Pidgeot18@verizon.invalid>
Sat, 10 Nov 2012 20:38:21 -0600
On 11/10/2012 6:56 PM, Roedy Green wrote:

On Sat, 10 Nov 2012 00:30:28 +0100, Robert Klemme
<> wrote, quoted or indirectly quoted
someone who said :



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

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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