Question

A class which has a method declared as this:

public class A{
    public <T> T test(java.lang.Class<T> classOfT)
}

Normally it can be called against an object like this:

A a = new A()
String s = "test";
a.test(String.class); //I wrote this as s.class at first,which is wrong.Thanks Nick.

Now I would like to generalize the object passed to it, so I declare a class like this:

public class B <T>{
    private A a   = new A();
    private T obj = null;
    T test(){return a.test(obj.getClass()); } 
}

But the code wont’ compile.I am wondering is it possible to achieve my goal?

Thank you in advance Java gurus 😉

Answer 1

T obj = ...;
obj.getClass()

The last line returns Class<? extends T> — but T is unbounded, so it is basically bounded by Object, which means it returns Class<? extends Object>, which is the same as Class<?>.

So doing this:

T test () { return a.test(obj.getClass()); }

Will actually invoke a.test with a parameter of type Class<?>, which returns an Object, and not a T.

Just cast the parameter to Class<T> or the return type to T and it should work — although I am yet to understand why you need something like this. Also, there is this strange error in the original post:

 String s = "test";
 a.test(s.class);

Doing "test".class is wrong — it should be String.class.