Designing a class implementing a method which signature types depends on the instance contained

Question

My goal is to define a Service class which process method accepts a RequestType argument that is defined by the contained ServiceType and returns an object of a type defined by ServiceType. By doing this I'm able to derive Services that can process a given Request that is compatible with the type of the Service , same for the Response.

I really don't know if it is possible and the following diagram is obviously wrong but depicts the result I'm struggling to achieve.

Answer 1

Many programming languages support generic types (C#), generics (Java) or class templates (C++). All three terms mean the same thing, but I honestly like the C++ terminology best. The term "class template" is good, because it implies that the class is not the actual class, but a template used by the compiler to create real classes. The intention is to solve the exact problem you are running into. I'll use C#, since that is the language I am most fluent with, but many OOP languages support this:

public abstract class Service<TRequest, TResponse>
{
    public virtual TResponse process(TRequest)
    {
        // common logic goes here
    }
}

The identifiers inside the angle brackets TRequest and TResponse are just placeholders for real classes that will be specified when initializing a new service object:

Service<GetBlogRequest, GetBlogReponse> service = new Service<GetBlogRequest, GetBlogReponse>();
GetBlogRequestrequest request = new GetBlogRequest(1200);
GetBlogReponseresponse response = service.process(request);

UML diagrams usually depict class templates or generics using the angle bracket notation as well:

+------------------------------------------+
|               <<abstract>>               |
| Service<TRequest, TResponse>             |
+------------------------------------------+
| +process(request : TRequest) : TResponse |
+------------------------------------------+