Composition over Inheritance usage

Question

I was having an argument with a colleague of mine.

Which of the following 2 code snippets feels better to you considering that "composition should be used instead of inheritance"?

1)

class JsonToSpecificTransformer implements Transformer {

    private final JsonToObjectTransformer jsonTransformer = Transformers.fromJson(Specific.class);

    @Override
    public Message<?> transform(Message<?> message) {
        return jsonTransformer.transform(message);
    }
}

2)

class JsonToSpecificTransformer2 extends JsonToObjectTransformer {
    public JsonToSpecificTransformer2() {
        super(Specific.class);
    }
}

The specific class would show something like this:

@Data
class Specific {
    private int field1;
    ...
}

I will comment a little bit about these 2 code snippets:

1) uses composition and keeps and inner object so that it doesn't get reinitialized every time.

2) uses inheritance.

Thanks!

Edit:

I am using a DSL IntegrationFlow something like:

@Autowired
private JsonToSpecificTransformer jsonToSpecificTransformer;

@Bean
public IntegrationFlow flow() {
   return IntegrationFlows.from("myChannel")
      .transform(Transformers.fromJson(service2.Specific.class)
      or
      .transform(jsonToSpecificTransformer)
      ...
}

Hope this previous code clarifies a little bit about my intentions.

And trying to summarize:

@Bean
public JsonToObjectTransformer jsonToSpecificTransformer() {
   return Transformers.fromJson(Specific.class);
}

---
@Autowired
//@Qualif...
private JsonToObjectTransformer  jsonToSpecificTransformer;

@Bean
public IntegrationFlow flow() {
   return IntegrationFlows.from("myChannel")
      .transform(jsonToSpecificTransformer)
      ...
}

Answer 1

Indeed. Doesn't look like you should abuse framework classes for this reason. They may change in the future breaking your code.

IMHO no inheritance, no composition at all.

You simply can have a bean like this:

@Bean
JsonToObjectTransformer jsonToSpecificTransformer() {
    return Transformers.fromJson(Specific.class);
}

And use it as a plain Transformer reference whenever you need. What is good in this that you don't need to have any extra classes to support at all.