在 Java 中向下转型？ 这是一个令人沮丧的问题吗？

Question

So I have a POJO class let´s call it: InnerDomainObject. Then I have an object representing this object, with a few more fields, for communication towards different clients (it s an API DTO): OuterDomainObject

Because the DTO has in fact all of the POJOs fields, I made OuterDomainObject inherit from InnerDomainObject.

Now I need to somehow cast InnerDomainObject to OuterDomainObject --> not possible.

I want to avoid writing a constructor iterating through all the fields. I want to avoid writing useless code.

I just want OuterDomainObject to be created out of InnerDomainOBject´s values and then add some to it before sending it to the client.

What´s the best way of doing this ?

Answer 1

You sound like you are using the Adapter Pattern . You shouldn't need to cast an InnerDomainObject to an OuterDomainObject. You should use composition: the OuterDomainObject should hold a reference to an InnerDomainObject, which will likely be passed into a constructor. When a client invokes a method on an OuterDomainObject, if that method exists in InnerDomainObject, the OuterDomainObject should call that method on its instance of InnerDomainObject. Instead of casting an InnerDomainObject foo to an OuterDomainObject, just create a new OuterDomainObject and pass in foo : new OuterDomainObject(foo) . You will need to write some simple glue code, but the result is very clean.