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Re: Generics ?

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On 12/17/11 10:20 AM, Knute Johnson wrote:
> [...]
> Example #4 Roedy's first suggestion compiles without warning but it is
> no longer generic. Which brings me back to my real question, can you
> extend a generic class and still be generic?

As "markspace" says: of course you can.

But you have to do it correctly.  (It also helps if you don't keep your 
code examples secret, such as having a statement that calls 
"model.addElement()" that is important to you, but which you don't 
actually share with the rest of us).

None of your examples actually followed the examples I provided 
precisely.  In each case, you made a mistake:

   • example #1: you tried to declare a class name as if it had a 
generic type parameter, but used "String" as if it were the name of the 
parameter.

   • example #2: the declaration of the class is fine, but then in your 
code you made the illegal assumption that the type parameter "E" is 
actually "String", by trying to pass an instance of String to the 
addElement() method.

   • example #3: you failed to specify "String" as the type parameter 
for the base class, meaning you're extending it without generic support, 
hence the "unchecked" warning.

   • example #4: you can't make a class generic, but at the same time 
fix the type parameter as something specific.

Frankly, I think example #4 is the most likely to be what you want.  You 
appear to want to explicitly pass a string literal to the supporting 
types within your own derived class.  If that's the case, then you have 
made the choice of the type parameter already.  It _has_ to be String.

If you actually want to declare a generic type yourself (e.g. so that 
you can reuse it generically), then you are not permitted to make any 
assumptions about the type parameter's value.  That includes not passing 
string literals to a generic member, such as your example of the call to 
addElement().

Pete

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Thread

Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-15 21:48 -0800
  Re: Generics ? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-15 22:03 -0800
    Re: Generics ? Tassilo Horn <tassilo@member.fsf.org> - 2011-12-16 08:30 +0100
      Re: Generics ? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-16 00:08 -0800
  Re: Generics ? Roedy Green <see_website@mindprod.com.invalid> - 2011-12-16 02:10 -0800
    Re: Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-16 22:01 -0800
      Re: Generics ? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-16 23:30 -0800
        Re: Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-17 10:20 -0800
          Re: Generics ? markspace <-@.> - 2011-12-17 10:54 -0800
            Re: Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-17 11:25 -0800
              Re: Generics ? Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-17 21:20 -0800
                Re: Generics ? "Qu0ll" <Qu0llSixFour@gmail.com> - 2011-12-18 21:08 +1100
                Re: Generics ? Lew <lewbloch@gmail.com> - 2011-12-18 08:17 -0800
                Re: Generics ? markspace <-@.> - 2011-12-18 08:43 -0800
          Re: Generics ? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-17 11:33 -0800
            Re: Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-17 11:40 -0800
              Re: Generics ? Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-17 11:47 -0800
              Re: Generics ? Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-17 15:13 -0500
      Re: Generics ? Roedy Green <see_website@mindprod.com.invalid> - 2011-12-17 02:46 -0800
  Re: Generics ? "John B. Matthews" <nospam@nospam.invalid> - 2011-12-17 11:18 -0500
    Re: Generics ? Knute Johnson <nospam@knutejohnson.com> - 2011-12-17 11:16 -0800

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