Question

I have a Student class and a StudentPool class defined as follows:

public class Student {
    public Student copy () {
        return new Student();
    } 
}

and

public class StudentPool<T extends Student> {
    public T copyStudent(T t) {
        return t.copy();
    }
}

thus, the copyStudent method cannot be compiled and I have to use un-safe type casting. I cannot understand why Java does regard this as illegal?

Edit:
user845279, lcfseth, and Bohemian: I think the following revision to Student may cause the similar scenario, cast a parent class into its child class, but this version can pass compiling:

public class Student {
    public <T extends Student> T copy() {
       T rv = null; 
       return rv;
    } 
}

Edit:
forget the above code: rv can be either null or un-safely casted.

Answer 1

The problem is that although Student returns Student from copy(), subclasses would also return Student… not their own type.

Here’s an approach to fix your problem:

public static interface Copyable<T> {
    public T copy();
}

public static class Student implements Copyable<Student> {
    @Override
    public Student copy() {
        return new Student();
    }
}

public static class StudentPool<T extends Student> {
    public T copyStudent(Copyable<T> t) {
        return t.copy();
    }
}

As a side note, it seems you don’t need generics here.