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From: Jim Janney <jjanney@shell.xmission.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Converting Sets
Date: Thu, 02 Feb 2012 13:13:10 -0700
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markspace <-@.> writes:

> 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.

See checkedSet in java.util.Collections.

-- 
Jim Janney