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Groups > comp.lang.java.programmer > #11712

Re: Converting Sets

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On 2/2/2012 7:06 AM, Mayeul wrote:
> On 02/02/2012 14:32, Roedy Green wrote:

>> Ah, but this this you created is NOT a Set<Y>. You cannot add
>> arbirary Y to it, just more X. I feel queasy. How could the compiler
>> keep track that setOfY could only contain X.

>
> It cannot.


I think this is covered in Effective Java.  Generics are a compile time 
thing.  If you want runtime, you have to roll your own (I don't know of 
any classes in the API that allow you to have a runtime type parameter, 
although I guess there may be some).

class MySet extends SomeSet {

   Class type;

   MySet( Class type ) { this.type = type; }

   void add( Object o ) {
     if( o instanceof type ) super.add( o );
   }
}

I think is the above is the gist of how EJ handles it.  Add a type token 
(the "type" variable) and test for types as they are added.  If you want 
generics too, add them in:

class MySet<T> extends Set<T> {

   Class<? extends T> type;

   MySet( Class<? extends T> type ) { this.type = type; }

   void add( T o ) {
     if( o instanceof type ) super.add( o );
   }
}

Not compiled, but I think that gives the right idea.

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Thread

Converting Sets Roedy Green <see_website@mindprod.com.invalid> - 2012-02-01 23:34 -0800
  Re: Converting Sets Mayeul <mayeul.marguet@free.fr> - 2012-02-02 12:47 +0100
    Re: Converting Sets Roedy Green <see_website@mindprod.com.invalid> - 2012-02-02 05:32 -0800
      Re: Converting Sets Mayeul <mayeul.marguet@free.fr> - 2012-02-02 16:06 +0100
        Re: Converting Sets markspace <-@.> - 2012-02-02 09:21 -0800
          Re: Converting Sets Jim Janney <jjanney@shell.xmission.com> - 2012-02-02 13:13 -0700
            Re: Converting Sets markspace <-@.> - 2012-02-02 13:46 -0800
          Re: Converting Sets Roedy Green <see_website@mindprod.com.invalid> - 2012-02-02 16:29 -0800
            Re: Converting Sets Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-02 17:34 -0800
              Re: Converting Sets Arne Vajhøj <arne@vajhoej.dk> - 2012-02-04 20:49 -0500
                Re: Converting Sets Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-06 12:55 -0800
                Re: Converting Sets Arne Vajhøj <arne@vajhoej.dk> - 2012-02-06 20:47 -0500
                Re: Converting Sets Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-06 17:51 -0800
                Re: Converting Sets Arne Vajhøj <arne@vajhoej.dk> - 2012-02-06 20:57 -0500
            Re: Converting Sets markspace <-@.> - 2012-02-03 14:12 -0800
              Re: Converting Sets Lew <lewbloch@gmail.com> - 2012-02-03 22:32 -0800
            Re: Converting Sets Lew <lewbloch@gmail.com> - 2012-02-03 22:30 -0800
          Re: Converting Sets Lew <lewbloch@gmail.com> - 2012-02-03 22:33 -0800
      Re: Converting Sets Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-02 11:02 -0800
    Re: Converting Sets Roedy Green <see_website@mindprod.com.invalid> - 2012-02-02 18:25 -0800
      Re: Converting Sets markspace <-@.> - 2012-02-03 14:25 -0800

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