Question

For example if we have a class Person and a class Student extends Person.
The we may have:

public interface PersonQueryBuilder<T extends Person> {
    PersonQueryBuilder<T> withName(String name);

    PersonQueryBuilder<T> withAgeBetween(int from, int to);

    List<T> getResultList();
}

public interface StudentRepository<T extends Student> extends PersonQueryBuilder<T> {
    StudentRepository studying(Course course);
}

So why when I have an StudentRepository, both the withName() and withAgeBetween() methods return a PersonQueryBuilder and not and StudentRepository? This is very annoying. Is there any “elegant” way of solving this problem?

Answer 1

This is kind of funny. I propose the following (ugly?) solution that doesn’t require you to redefine the methods in the interface as Mark’s answer does:

interface PersonQueryBuilder<T extends Person, U extends PersonQueryBuilder<T,U>> {
    U withName(String name);

    U withAgeBetween(int from, int to);
}

interface StudentRepository extends PersonQueryBuilder<Student, StudentRepository> {
    StudentRepository studying(Course course);
}

Also you’re using raw types as results in your current implementations of your with methods.