Java: getting inner type in nested parameterized types (reflection)

Question

Most of the documentation regarding type erasure handling in Java assumes that the use case is handling a type like SomeType<ParamType>. I am trying to process method parameter for the following method:

public void setOtherReferenceRanges(List<ReferenceRange<T>> referenceRanges)

When the container class is instantiated with a type DvQuantity, this signature should become public void setOtherReferenceRanges(List<ReferenceRange<DvQuanitity>> referenceRanges) in runtime.

Using reflection one can see that the List has an actualTypeArgument which is ReferenceRange<T>. Since reflection uses class information, I would not expect it to give me ReferenceRange<DvQuantity>. However, when I created the class containing this method, I passed the DvQuantity type as T. So the type filling in T should be available to Java runtime, but I could not find a way of getting it. I end up with a TypeVariableImpl object accessed via reflection, which does not seem to contain any useful data.

Can you think of any ways to discover this information in runtime?

Answer 1

When you say

when I created the class containing this method

I guess you mean when you create an object of that type, for example:

foo = new ContainerClass<DvQuantity>();

In that case, because of erasure, there is no way to recover the type DvQuantity.

However, if you create a class passing a type parameter to the superclass, like this

class DvQuantityContainerClass extends ContainerClass<DvQuantity> {...}
...
foo = new DvQuantityContainerClass();

Or, shorter, an inline anonymous subclass (which looks almost like the first example but with a subtle but important difference):

foo = new ContainerClass<DvQuantity>(){};

Then you can recover the type parameter, because you recover the type parameter used to extend a superclass at runtime. Unfortunately, Java itself doesn't provide an easy way to now get the type of the DvQuantityContainerClass.setOtherReferenceRanges method with the T filled in. For that, I've written gentyref, to do advanced reflection on generic types:

Method m = DvQuantityContainerClass.class.getMethod("setOtherReferenceRanges", List.class);
// this will return List<ReferenceRange<DvQuanity>>, like you are lookingn for
return GenericTypeReflector.getExactParameterTypes(m, DvQuantityContainerClass.class)