Java Compile error when return Generic Class type

Question

public interface Parent{
}

public class Child implements Parent{
}

public <T extends Parent> Class<T> getClass(){
  return Child.class;  // compile error, add cast to Class<T>
}

I expect there is no error about above code, however I get compile error when I return Child.class.

Answer 1

You can't tell Java to always return Child.class regardless of T. There might be other classes extending from parent that aren't a Child. Also, since you are not using T anywhere, your code doesn't make much sense. Two possible solutions:

public Class<? extends Parent> getAClass() {
    return Child.class;
}

or perhaps

public <T extends Parent> Class<T> getAClass(Class<T> clazz){
    return clazz; 
}

The second example compiles, but would probably make more sense if T was declared at class level.

Also, I renamed your method to avoid a name clash with the existing getClass() method.

Answer 2

Review the following

public class Main {
    public interface Parent{}
    public class Child implements Parent{}        
    public class BadChild implements Parent {}

    public static <T extends Parent> Class<T> getClazz() {
        return Child.class;  // compile error, add cast to Class<T>
    }

    public static void main(String[] args) {
        Class<BadChild> clazz = Main.<BadChild>getClazz();
    }
}

Calling method getClazz() with BadChild type is valid for the caller side. If you could compile your method that would lead to runtime errors. That's why this is forbidden.