Function with generic type as member variable

Question

I have a function which maps a class object to an instance of this class. Basically:

 Function<Class<T>, T> fun;

I can define this function within a method but when trying to put this in a member variable, the compiler complains because the type T is unknown.

So, T is not specific to the enclosing object. It might differ from call to call. What is the correct syntax to define such a function on class level?

EDIT

I would like to clarify. I have class A

public class A {
  public <T> T get(Class<T> clazz) {...}
}

Now, I want to hand a pointer to that function to class B like this:

public class B {
  <T> Function<Class<T>, T> fun;
  
  public <T> T get(Class<T> clazz) {
    return fun.apply(clazz);
  }
}

However, the Syntax for <T> Function<Class<T>, T> fun is incorrect. Is there a way to keep the information, that fun will always retain the type T?

My current solution is

public class B {
  Function<Class<?>, ?> fun;

  public <T> void setFun(Function<Class<T>, T> fun) {
    this.fun = fun;
  }

  public <T> T get(Class<T> clazz) {
    return (T) fun.apply(clazz);
  }
}

This works (and is obviously correct via invariant) but kinda ugly because of the required cast.

Answer 1

In Java (and JVM) values are not polymorphic, methods are.

So correct is to define it with a method

<T> Function<Class<T>, T> fun();

Polymorphic values in Java

---

Regarding EDIT.

Now, I want to hand a pointer to that function to class B like this:

public class B {
  <T> Function<Class<T>, T> fun;
 
  public <T> T get(Class<T> clazz) {
    return fun.apply(clazz);
  }
}

Once again, a value (including a field) can't be polymorphic, method can.

• So you can make fun a method

   public abstract class B {
      abstract <T> Function<Class<T>, T> fun();
  
      public <T> T get(Class<T> clazz) {
          return this.<T>fun().apply(clazz);
      }
   }
  

  You can't write setter like setFun. Its signature should be something like

   public void setFun(<T> Function<Class<T>, T> fun); // pseudocode
  

  rather than

   public <T> void setFun(Function<Class<T>, T> fun);
  

  <T> Function<Class<T>, T> is called rank-N type and it's absent in Java

  What is the purpose of Rank2Types?

  https://wiki.haskell.org/Rank-N_types

• Alternatively, you can wrap polymorphic fun with a class (interface)

  public interface PolyFunction {
      <T> T apply(Class<T> clazz);
  }
  
  public class B {
      PolyFunction fun;
  
      public void setFun(PolyFunction fun) {
          this.fun = fun;
      }
  
      public <T> T get(Class<T> clazz) {
          return fun.apply(clazz);
      }
  }
  

  PolyFunction looks like your A so maybe you want

  public class B {
      A a;
  
      public void setA(A a) {
          this.a = a;
      }
  
      public <T> T get(Class<T> clazz) {
          return a.get(clazz);
      }
  }