Java - Handling generics with inheritance

Question

I have a class that represents a vendor service and all their services have an authentication and an execute method.

I started thiking of an abstract class that represents this as below. The thing is each of their services require a different request object, so I thought of using generics. The problem is that if use it, I can't handle the specifics of each request object. Each children must use some methods from the type.

1) Should I try to make this way I'm trying, or remove this executeRequest method from the abstract class and each subclass implement it with the correct type?

2) I always hear "prefer composition over inheritance". Should I move the executeRequest to an interface?

Thanks in advance!

public abstract class VendorService  {

    private final VendorInitialization VendorInitialization;

    //a bean with some auth params
    public VendorService(VendorInitialization VendorInitialization) {
        this.VendorInitialization = VendorInitialization;
    }

    protected abstract <T> boolean validateRequest(T requestObject) throws VendorServiceBadRequest;

    protected abstract <T, P> P executeRequest(T requestObject);
}


public class VendorServiceAllocation extends VendorService {

    public VendorServiceAllocation(VendorInitialization VendorInitialization) {
        super(VendorInitialization);
    }

    @Override
    protected <T> boolean validateRequest(T requestObject) throws VendorServiceBadRequest {

        //List<BeanAllocation> requestObject = new Arraylist<>(); //I was using like this before

        //TODO: how to handle it as list of on this specific case?
        if (requestObject == null || requestObject.size() == 0) {
            throw new VendorServiceBadRequest(String.format("The list must have at least one element"));
        }

        //TODO: requestObject.get(0).getMySpecificFieldFromBeanAllocation will not work

        //some checks
        return true;
    }

    @Override
    protected <T, P> P executeRequest(T requestObject) {
        //executes and return a list of objects specific to this class
        return new List<BeanAllocationResponse>();
    }
}

Edit, for clarification: In the child class VendorServiceAllocation, I need to use some methods that are specific of that type.

E.g.: Inside executeRequest, I need to call requestObject.customFunctionFromChild()

I think your abstract implementation is just fine; you just need to make sure that the functionality you abstract to the generic parent is useful for all of the children. — Mapsy
– Mapsy, Commented Jul 18, 2016 at 17:15
Thinking as a executeRequest, all children will use it. But if I think of the objects, each children will receive a different object. I should remove it from the abstract then? Or do you think I could use generics, something? — Felipe S.
– Felipe S., Commented Jul 18, 2016 at 17:20
you're making the abstract methods generic while the class is not, instead make the class generic , then you could have sub-class Service1 that inherits Parent<Integer> and another that inherits Parent<String>(Integer and String are just examples for illustration ) — niceman
– niceman, Commented Jul 18, 2016 at 17:26
oh nice!! this is what I was looking! the way you suggested, each children will know its type and I can work it.. please comment it as answer so I can mark it correct.. :) — Felipe S.
– Felipe S., Commented Jul 18, 2016 at 17:33
It's not immediately clear if you need type parameters. I think just working with types as interfaces (e.g. make a Request class that validateRequest accepts) — Kedar Mhaswade
– Kedar Mhaswade, Commented Jul 18, 2016 at 17:38

matt · Accepted Answer · 2016-07-18 17:59:16Z

3

I think niceman hit the nail on the head, though I am not quite sure what you are asking. eg.

abstract class Service<T,P>{
    abstract public P processRequest(T t);
}

Then you can implement it in one of two ways.

class StringService extends Service<String, String>{
    public String processRequest(String t){
         return t;
    }
}

Or you could leave it to still be Generic and the actual instances would have the different types.

class OtherService<T> extends Service<T, String>{
    public String processRequest(T t){
        return t.toString();
    }
}

Where you could use it as,

OtherService<Integer> is = new OtherService<>();

answered Jul 18, 2016 at 17:59

matt

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1 Comment

Felipe S. Over a year ago

this is exactly what I wanted.. I upvoted your answer, but I asked for @niceman to post his reply as an answer as he replied first.. hope you understand! :)

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