Get the Type of a generic field at runtime, How to?

Question

Hello,
As the topic already says, i need to get the Type of a particular field of a class. This field is declared private and generic. In C# there is a method
Type Object.getTypeIs there any specific way to do this in Java 1.5?
Please excuse my poor english.
Thanks in advance.
Markus

Answer 1

McNepp wrote:
endasil wrote:
McNepp wrote:
If you want to know the parametrized type (String in the example), I think there is no way of knowing this in Java 1.5 or Java 1.6, since the parametrized type is erased and not available at run time.The type of a parameterized field is not erased.For most intents and purposes, it is. Type erasure refers to the fact that at runtime, there are not actually multiple class binaries depending on the generic arguments to a class. Therefore, an ArrayList<T> is actually just an ArrayList with no generics.
Frankly, I don't understand why you insist that the information on generic fields that the OP was asking about is lost at runtime.I wasn't trying to insist that. At the time, I was replying more to Saish and trying to reaffirm that most information about generics is lost at run-time. I mistakenly ignored how you qualified it with "field."
What you write about instances of generic classes losing their type information is of course correct, albeit not to the point of the original question.Nope, you're right. I was just trying to reconcile the fact that many people get confused that there's any information available at run-time, and so start down the path of thinking that type erasure doesn't exist. But it very much does.
The original question was about how to obtain the type of a generic field.And I did show in my example that even that is fairly limited, given that if the type is provided by the parameter of the class, it doesn't give you anything useful (I'm not trying to say you said it would!).
The compiler preservers this information in the class file, so it can be obtained at runtime. Frameworks like JPA put this to use extensively, proving that it is of real value.Definitely. However I don't see this having as much to do with generics as basic reflection functionality. If you can get the type of a field at run-time, you should be able to get the parameters as well! That should in no way belittle its value, though. But I would have guessed (knowing little about) that JPA wouldn't put that to use so much as the type parameters of an accessor return type or mutator argument type. Especially since I thought we'd shown that you would need your fields to be non-private for JPA to be able to gain information about their type.
Edit: getDeclaredField works fine with private members, and returns the expected "java.lang.String" from jschell's example above
Edited by: endasil on 28-Apr-2009 10:39 AM