Re: what do you mean I can't (someObj instanceof MyGenericType) ?

Joshua Cranmer <>
Fri, 10 Aug 2007 22:10:11 GMT
Sideswipe wrote:

if(T instanceof arg) ... // compile error

arg is a variable not a typename.

if(T.class.isAssignableFrom(arg.getClass())) // compile error

What is 'T.class'? At runtime, the type argument is statically compiled
into the class file.

      var = (T)arg;

Is this even legal?

Is this better?

No. Not by a long shot.

When one needs to work with generics, one ends up working with Class
objects a lot. Your class again:

class C<T> implements I {
     private Class<T> tClass;
     public C(Class<T> tClass) {
         this.tClass = tClass;

     public void method1(Object arg) {
         T var = tClass.cast(arg);
         // [...]

Even better would be to make your interface generic as well, so that
less runtime checks would be needed.

Java Generics are NOT C++ Templates. Only one class file is ever used,
so the types of type arguments are erased to the tightest bound known,
which is generally Object. Because each invocation of T is more or less
are equivalent to Object (or the relevant type erasure), constructs like
  T[], T.class, (T), and instanceof T are prohibited.

For more information, read Sun's generics information (look at one of
the more recent generics threads for the link; I don't know it off the
top of my head).

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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