Generics - Class of T

[AndrewM16921]

If I have a generically typed class with a type T, how do I determine the type of (that is, get a Class instance of) T at runtime?

Something like...

public class example<T>
{
  public example()
  {
    Class c = T.getClass();
  }
}

would I have to create a new instance of T...?
new T().getClass() ...?
Even then I foresee problems.

[Fubarable]

You can't. Generics are compile-time constructs only, not run-time. Read up on type erasure for more on this.

[AndrewM16921]

Ah... alright

[pbrockway2]

Notice that at compile time you know the type T. That is, you would declare a variable or return type as Example<String>, or Example<MyClass> or whatever. It is quite allowable to pass a Class<Whatever> to the constructor.

public class Eg<T> {

	T data;

	Eg(Class<? extends T> claz) throws Exception {
		T data = claz.newInstance();
	}
	
	public static void main(String[] args) throws Exception {
		Eg<Super> test = new Eg<Super>(Sub.class); // or Super.class
	}
}

class Super {}
class Sub extends Super {}

I'm not sure if that helps. Perhaps if you said what you are trying to do (as distinct from how you are trying to do it.)

[DarrylBurke]

I had the same question once. There's a sneaky way to get at the class by using a (possibly anonymous) subclass, discussed here: https://forums.oracle.com/forums/thr...readID=2244865

db

[AndrewM16921]

Hmm, I'll read that thread a bit. I was just experimenting with reflection, wasn't really trying anything specific per se. Just nice to know little tricks should the need to use them come up in the future.

[jim829]

Not certain how to cleanly do what you want (or uncleanly for that matter) because of type erasure. And even if you could get a type for T (e.g. String, Integer), Collections are not covariant so the type of any List<T> is simply List. But I did find an interesting article which directly addresses your issue. It can be viewed at Neal Gafter's blog: Reified Generics for Java.

v/r,
Jim