Get the the type of the class of a generic type?

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> Class<? extends T> -- but T is unbounded, so it is basically bounded by Object , which means it returns Class<? extends Object> 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 .

Answer 2

Class<T> clazz  = (Class<T>) obj.getClass();
return a.test(clazz);

Answer 3

Try

@SuppressWarnings("unchecked")
T test() {
    return (T) a.test(obj.getClass());
}

Answer 4

In complement of the other answers, when defining your function as

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

if you are using the classOfT parameter just to determine the type of T , you might be better off defining your method with no parameter at all. For instance, do this instead of the above declaration:

public class A{
    public <T> T test() { ... }
}

That way you can just call it using this qualified method-call syntax. This is how I would rewrite your code:

public class A{
    public <T> T test() {
          // ...
    }        

    public static void main(String[] args) {
          A a = new A();
          String result = a.<String>test(); 
    }
}

public class B <T> {
    private A a = new A();

    T test() {
        return this.a.<T>test();
    }
}

This way you do not need to pass the generic A#test() function a class parameter, which would possibly have been unused at runtime. Of course if you do use the classOfT at runtime, this solution may not be that much interesting for you...