On 9/21/2013 11:43 AM, Marcin Wiśnicki wrote:
First of all EnumSet/EnumMap are not general purpose collections, they
are already implementation specific.
Also, since you can't subclass enum, class of contained values will be
the same as declaring class.
No, not necessarily.
Specialized enum constants can be implemented by having anonymous
subclasses of the enum type.
The following two enums declarations are semantically equivalent:
// 1st
public enum MetaSyntVar {
FOO(21),
BAR(42);
private int answer;
MetaSyntVar(int answer) {
this.answer = answer;
}
public int answer() {
return answer;}
}
// 2nd
public enum MetaSyntVar {
FOO {
public int answer() {
return 21;}},
BAR {
public int answer() {
return 42;}};
public abstract int answer();
}
The former compiles down to one class file; the latter compiles down to
three.
-Joe
>From EnumSet documentation: "All of the elements in an enum set must
come from a single enum type".
I don't see how exposing getElementType() can hinder reification. I'm
going to say that it doesn't. Please provide counterexample if you
think I'm mistaken.
Here are your use cases:
1. Remove ugly hacks from serialization libraries that were known to
cause problems with different JVM implementations in the past (for
example see this code from jackson: http://goo.gl/yudBps)
2. A Visual editor of some sort may display EnumSet as a list of
checkboxes if it knows what type of enum values can be contained
within this set.
On Sat, Sep 21, 2013 at 8:15 PM, Remi Forax <fo...@univ-mlv.fr> wrote:
On 09/21/2013 05:27 PM, Marcin Wiśnicki wrote:
Third party serialization libraries and other tools need to know the
type of EnumSet elementType. Currently this field is package-private
and said libraries have to resort to non-portable reflection hacks.
Please add EnumSet#getElementType() and EnumMap#getKeyType() methods.
I've submitted this request to bugs.sun.com last month, where it
received Bug Id: 9006352. There was no further communication and that
bug is still not visible which doesn't surprise me as I've never had
any luck with bugs.sun.com even when submitting actual bug :(
PS. Please keep me CC'ed.
Hi Marcin,
I've read the bug file but failed to find a description of a compelling use
case for these methods.
Adding these two methods goes against an important rule:
try to avoid for sub-types of Collection, methods that are implementation
specific, the history has proven multiple times that implementations of
collections change a lot. This is a general rule, that we may want to ignore
but it has to worth it.
Also, I think it worth to understand what's elementType represent.
elementType is not the class of the values contained in the set but the
declaring class (Enum.getDeclaringClass) of the enum values. So despite
being a Class, elementType is a type information that only exists has a
field because generics are not reified. This has two consequences, the first
one is that if you want to create an EnumSet, you need a way to have an
elementType and the second one is that it may make the work of the people
that want to reify generics in a future version of Java harder.
and don't think I'm the dragon that keep the treasure, this is just my
humble opinion and I try to do my best to explain why adding these methods
is not obvious.
regards,
Rémi
