Re: Converting Sets

From:
Jim Janney <jjanney@shell.xmission.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 02 Feb 2012 13:13:10 -0700
Message-ID:
<2pliol804p.fsf@shell.xmission.com>
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

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