Re: Converting Sets

markspace <-@.>
Thu, 02 Feb 2012 09:21:13 -0800
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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