Question

I have:

interface I<T> {}

and:

interface I1 extends I<Clazz1> {}

I have also I2, I3 and so on which extend I<Clazz2>, I<Clazz3> etc.

I don’t know about Clazz1 in runtime. How could I get this information?

I tried:

subInterface1.getInterfaces()[0].getTypeParameters()[0]

but in only knows about T, not about Clazz1.

Answer 1

When you do

subInterface1.getInterfaces()[0].getTypeParameters()[0]

you are getting the first formal type parameter of the Class object representing the interface I, which is indeed the variable T. Instead you need to use

subInterface1.getGenericInterfaces()[0]

to get the Type object representing the particular instantiation of I that has been extended by I1. This will be an instance of ParameterizedType, so you can cast to that and then use getRawType() and getActualTypeArguments() to extract the raw type I and the actual type parameter value Clazz1 respectively. Note that the return type of getActualTypeArguments() is Type[] rather than Class[] because the actual arguments might not be concrete classes, they could be variables or wildcards (or indeed other parameterized types, in a case such as List<List<String>>).