Programmatically assess relationship between type variables in class hierarchy

Question

Suppose I have a Java class hierarchy defined as follow:

interface Bar<T> {}

class Foo<A,B> implements Bar<B> {}

How can I programmatically assess (using reflection) that the type parameter of Bar in Foo is the second of foo's parameters and not the first (B instead of A)?

I've tried using TypeVariable#getName() in order to compare the names, but when I apply getGenericInterfaces() to Foo<A,B> I get Bar<T> and not Bar<B>

Solution (thanks to @LouisWasserman): use Foo.class.getGeenricInterfaces()[0].getActualTypeParameters() returns the correct TypeVariable (B instead of T, in the previous example)

How are you going about calling getGenericInterfaces to Foo? — Louis Wasserman, Jan 20 '15 at 17:44
I'm developing a static analyzer and I need to properly map type relationships in order to perform type unification; i.e. I would like to know programmatically asses that Bar is a supertype of Foo. I would like to reason about types without instantiating concrete types, and for this I need to get the mapping before — Francesco Bellomi, Jan 20 '15 at 17:45
@LouisWasserman using getGenericIntergaces I get access to Bar and I don't know how to test if T~B or T~A — Francesco Bellomi, Jan 20 '15 at 17:52
@FrancescoBellomi: That doesn't answer my question; I wanted to see the code you were using to call `getGenericInterfaces`. In particular I want to know how you're calling `getGenericInterfaces` on `Foo` instead of a raw `Foo.class`. — Louis Wasserman, Jan 20 '15 at 17:53
@LouisWasserman you're right. I was using `Foo.class.getGenericInterfaces()[0].getRawType().getGenericTypes()`. Using `Foo.class.getGenericInterfaces()[0].getActualTypeArguments()` brings `Bar` as desired. — Francesco Bellomi, Jan 20 '15 at 23:03

score 0 · Answer 1 · answered Jan 20 '15 at 18:16

well using TypeVariable#getName() return the type as it appears in the source code in your case it's normal to get Bar<T>. TypeVariable Doc

Using reflection in generic Classes can't help, because of Type Erasure. Erasure of Generic Types

I've the same issue in some personal projects, I tried to change the design of my class, have a look at the example below:

Instead of this:

public class Mapper<T> {

   public Mapper(){
   }
}

I used this:

public class Mapper {
   private Class<?> entityClazz;
   public Mapper(Class<?> entity){
       this.entityClazz = entity
       //Here I've donne all reflection issues i want ! 
   }
}

You can use Class#isAssignableFrom() Doc to test assignability between Class Objects.
I hope this helps, good luck !

Programmatically assess relationship between type variables in class hierarchy

1 Answers1