如何设计此方法的签名

Question

I need to define a method, which takes a request and returns a response:

  public MyResponse submit(MyRequest request){                      A
      ....
  }

This looks simple. However:

MyRequest can has a number of child classes

 public class HolidayRequest extends MyRequest {}
 public class FundingRequest extends MyRequest {}
 public class BudgetRequest extends MyRequest {}

The same applies to the response class:

 public class HolidayResponse extends MyResponse {}
 public class FundingResponse extends MyResponse {}
 public class BudgetResponse extends MyResponse {}

A user of the method submit can pass in a request of any type and get a corresponding response object, eg: Pass in HolidayRequest will return HolidayResponse.

Although the interface defined above still works, the user has to cast the return value of submit from MyResponse to a specific response type.

I then think to change the method to:

  public <T extends MyResponse, S extends MyRequest>                B
  T submit(S request){
      ....
  }

This allows the return type to be a specific type rather than the parent type. However, a user can still write code like:

  HolidayResponse response = submit(new FundingRequest());

and the code will still compile but fail at runtime.

Then I realise the submit method does not need to know which type of request it is, so there is no need to use type parameter for MyRequest, I think I can maybe further simplify it to:

  public <T extends MyResponse>                                    C
  T submit(MyRequest request){
      ....
  }

But again, this one has the same problem as mentioned above, user can pass in a request type and expect another response type.

My question is that how I shall properly define the signature of this method? Which one of the A, B and C definition above will you use? Or there might be more appropriate way to define it?

Thank you very much.

Answer 1

You have to link your Request to your Response .

For example, change the MyRequest class to

public abstract class MyRequest<T extends MyResponse> {

Now your HolidayRequest would look like:

public class HolidayRequest extends MyRequest<HolidayResponse> {

Now your method signature can force the MyReponse to correspont:

public <R extends MyResponse, S extends MyRequest<R>> R submit(S request){

}