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Enumset.contains

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  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

#19679 — Enumset.contains

You might think

EnumSet.contains( subset ) would mean containsAnyOf or containsAllOf
but it is meaningless.

There is an EnumSet.containsAll but no EnumSet.containsAnyOf

It seems odd Set and EnumSet don't directly support the usual things
mathematicians do with sets,
union
intersection
isSubsetOf
isSuperSetOf

The operations would be so fast internally if Oracle used the binary
logic ops to handle bit strings, rather than flat-footed processing an
element at a time.

What think you?


-- 
Roedy Green Canadian Mind Products http://mindprod.com
Ironically, even though the Internet was created by the US military 
[DARPA (Defense Advanced Research Projects Agency)]
to withstand a nuclear attack, it is almost defenceless against malice
from any of its users

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#19681

On Friday, November 9, 2012 1:40:48 PM UTC-8, Roedy Green wrote:
> You might think
> EnumSet.contains( subset ) would mean containsAnyOf or containsAllOf

Only if you don't read the Javadocs for '[Abstract]Collection#contains()", which make it 
pretty obvious that 'contains()' applies to a single instance of the base type.

'EnumSet' is constrained to follow the semantics of the type it overrides with that method.

> but it is meaningless.

Nonsense.

It's meaning is clear from the Javadocs. Since an 'EnumSet' has as its base type the enum 'E', 
then any call to 'contains(someSubsetNotABaseInstance)' must return 'false'.

> There is an EnumSet.containsAll but no EnumSet.containsAnyOf

In this it is just like its parent type.

> It seems odd Set and EnumSet don't directly support the usual things
> 
> mathematicians do with sets,
> 
> union

'addAll()'.
http://docs.oracle.com/javase/7/docs/api/java/util/Set.html#addAll(java.util.Collection)

> intersection

'retainAll()'
http://docs.oracle.com/javase/7/docs/api/java/util/Set.html#retainAll(java.util.Collection)

> isSubsetOf

uberSet.retainAll(underSet).containsAll(underSet)

> isSuperSetOf

'containsAll()'
http://docs.oracle.com/javase/7/docs/api/java/util/Set.html#containsAll(java.util.Collection)

> The operations would be so fast internally if Oracle used the binary
> logic ops to handle bit strings, rather than flat-footed processing an
> element at a time.
> 
> What think you?

Three out of four of the operations you wish were directly supported are.

The fourth takes two whole calls to support.

-- 
Lew

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#19683

Lew <lewbloch@gmail.com> wrote:
> On Friday, November 9, 2012 1:40:48 PM UTC-8, Roedy Green wrote:
>> isSubsetOf
> uberSet.retainAll(underSet).containsAll(underSet)

why not just: uberSet.containsAll(underSet) ?

containsAnyOf (for EnumSets es1 and es2):
   es1.clone().retainAll(es2).size()>0
it requires actually constructing the intersection,
just to determine if that will be empty or not.

Maybe we're even just lucky, that there is containsAll() in
the Collection framework, or we would have to do it as:
  es1.clone().retainAll(es2).size() == es2.size()

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#19682

On 11/09/2012 10:40 PM, Roedy Green wrote:
> You might think

No.

> EnumSet.contains( subset ) would mean containsAnyOf or containsAllOf
> but it is meaningless.
>
> There is an EnumSet.containsAll but no EnumSet.containsAnyOf
>
> It seems odd Set and EnumSet don't directly support the usual things
> mathematicians do with sets,
> union
> intersection
retainAll()

> isSubsetOf
containsAll()

> isSuperSetOf
containsAll()

> The operations would be so fast internally if Oracle used the binary
> logic ops to handle bit strings, rather than flat-footed processing an
> element at a time.
>
> What think you?

You can use clone() and retainAll() and check return type, or use 
addAll() and check whether size < oldSize + other.size(), or write a 
loop in a few lines...

Cheers

	robert

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#19691

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.

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

-- 
Roedy Green Canadian Mind Products http://mindprod.com
Ironically, even though the Internet was created by the US military 
[DARPA (Defense Advanced Research Projects Agency)]
to withstand a nuclear attack, it is almost defenceless against malice
from any of its users

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#19692

On 11/10/2012 4:56 PM, Roedy Green wrote:
>   A proper intersection method for EnumSets would be implemented
> with a single AND machine instruction.
>


Have you checked the source code, Roedy?


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


Maybe.  Maybe Sun was just checking who could read the docs.

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#19693

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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