Question

I have to implement an RMI server which will be a front end for two other RMI services. So I decided a logical thing to do would be to have the interface for this implement the interfaces for the other two services.

public interface FrontEndServer extends Remote, BookServer, StudentServer {     // Block empty so far } 

However there is a method on the StudentServer

/**  * Allows a student to borrow a book  *   * @param studentID of the student who wishes to borrow a book  * @param bookID of the book the student wishes to borrow  * @throws RemoteException  * @throws StudentNotFoundException when a student is not found in the system  */ void addBookToStudent(int studentID, int bookID) throws RemoteException, StudentNotFoundException; 

I would like the FrontEndServer to also throw a BookNotFoundException as this service will also validate if the book actually exists before attempting to add the details in.

Is this possible or is my design idea completely off and this is actually a bad design idea as if the other interfaces change and all? And will I be better of writing the method signatures for all the methods inside the FrontEndServer?

Answer 1

If you extend an interface (the same applies if you implement an interface), you can’t override a method and have it throw more checked exceptions than the original. You can it throw the same or less but not more.

Think about it:

interface A {   void foo(); }  interface B extends A {   void foo() throws IOException; }  A a = new B() { ... } a.foo(); 

would potentially throw an IOException but you’d have no way of knowing. That’s why you can’t do it.

This of course is perfectly acceptable:

interface A {   void foo() throws IOException; }  interface B extends A {   void foo(); }  A a = new B() { ... } try {     a.foo(); } catch (IOException e) {     // must catch even though B.foo() won't throw one } 

Your BookNotFoundException could however extend RuntimeException or RemoteException. Not sure that’s a good approach however.