Question

I have this interface

public interface MyInterface{
    Collection<T> Find(T t);
    Collection<T> FindAll();
    T FindById(int id);
    void Add(T t);
    void Remove(T t);
}

What must I do for the T FindById(int id) method if I need to be guaranteed that T will have an Id of type int? Should I create another interface that my object must extend to use FindById?

Answer 1

To really get that type-safety, as well as flexibility for non-int identifiers, try something like:

public interface IdentifiedThing<I extends Serializable> {
    I getId();
}

You can then lock down the typing:

public interface MyInterface<I extends Serializable, IT extends IdentifiedThing<I>> {
    Collection<IT> find(IT t);
    Collection<IT> findAll();
    IT findById(I id);
    void add(IT t);
    void remove(IT t);
}

(I’ve fixed your method capitalization to Java-style too 😉