Re: exporting a HashMap
Arne Vajh??j wrote:
Roedy Green wrote:
Lew wrote:
And anyway, generics and arrays do mix, just not well. To say merely that they
don't mix, without the qualifier "well", is inaccurate and misleading, and
certainly no help to newbies. Better would be to explain the limits, why they
exist, and what the alternatives are.
If you were charged with writing a paragraph or to for newbies to
explain the impedance mismatch of arrays and generics what would you
say?
"generics and arrays do mix, just not well" perhaps?
Along with some specifics. As stated in the cited post, you should explain
the limits and what to do about them. It isn't especially complicated
material. For example, and this is just off the top of my head to stimulate
discussion, not presented as a paragon of prose:
You should not mix arrays and generics.
Generics and arrays do not mix well. That's because arrays "remember" their
underlying type at runtime, but generics just become 'Object' at runtime
through the process of "type erasure". [1] The compiler will not let you
create an array of generic types, unless the generic parameter comprises
entirely unadorned wildcard ('?') characters. So
Foo<?> [] bunchaFoos = new Foo<?> [NUMFOOS];
is legal, but
Foo<Bar> [] bunchaFoos = new Foo<Bar> [NUMFOOS];
is not. Other things like casting and reflection get really difficult, too.
For almost everything you want to do mixing arrays and generics you can use
'ArrayList' instead of an array. The syntax is a little more verbose but the
type safety and expressiveness compensate.
[1] In technical terms, an array is a "reifiable" type - it can be made "real"
in the JVM. Consequently its base type must also be reifiable. A generic
type, except for the pure wildcard '?' generics, is not reifiable because of
erasure. So the compiler won't let you make an array of a generic type.
--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg