Generic method input generic Objects of type

Question

I am refactoring code... Considering this chain of inheritance...

public class Base

public class Simple extends Base

public class Secure extends Simple

public class Framework extends Secure

public class Application extends Framework

and the class

public class ApplicationCallback

Currently, ApplicationCallback has this method public abstract ApplicationCallback#doSomething(Application app), but I want it to be accessible by Base.... without changing the code everywhere from ApplicationCallback#doSomething(Application app) to ApplicationCallback#doSomething(Base base).

Is there a way I can define a method that I can define to prevent the code change everywhere?

Kind of like ApplicationCallback#doSomething(T<? extends Base> app), so that when overriding, I can just put ApplicationCallback#doSomething(Base base) and ApplicationCallback#doSomething(Simple simple) etc etc.

The question is about @Override ApplicationCallback#doSomething()

That looks like an awefully deep inheritance hierarchy, I doubt that's a good idea. Also, it's not clear to me what you mean by "accessible by Base". — daniu
– daniu, Commented Apr 20, 2018 at 14:22
"without changing the code everywhere from ApplicationCallback#doSomething(Application app) to ApplicationCallback#doSomething(Base base)" - You wouldn't have to change the calling sides since you widen the parameter type from Application to Base. — Turing85
– Turing85, Commented Apr 20, 2018 at 14:23
@Turing85 When using @Override, it would have to be changed to ApplicationCallback#doSomething(Base base). Is there a way to make it generic so that anything that extends Base is acceptable in the method input? — theAnonymous
– theAnonymous, Commented Apr 20, 2018 at 14:25
as @Turing85 said if you change the definition to doSomething(Base base) it will work for anything that extends Base. However if you want to strongly type doSomething you need to add a generic to the class definition. ApplicationCallback<T extends Base> { doSomething(T base)} — JEY
– JEY, Commented Apr 20, 2018 at 14:28

user8681 · Accepted Answer · 2018-04-20 14:31:54Z

1

You should introduce a generic parameter on the ApplicationCallback class. That allows you to define callbacks that take only certain subtypes:

class Callback<T extends Base> { abstract void doSomething(T t); }

That would allow you to use methods from the subtype you defined, for example if the Secure class adds a getSecureThing() feature, you could:

Callback<Base> base = new Callback<Base>() { 
  void doSomething(Base base) {
    // getSecureThing not available
  }
}
Callback<Application> app = new Callback<Application>() { 
  void doSomething(Application app) {
    // app.getSecureThing is available
  }
}

answered Apr 20, 2018 at 14:31

user8681

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8 Comments

user8681 Over a year ago

This does require you to change all Callback implementations. If you're using IntelliJ, it has a feature called 'introduce type parameter' that will help you do that automatically.

Turing85 Over a year ago

If you need the generic parameter only on the doSomething(...)-method, only set this method generic: public <T extends Base> abstract void doSomething(T t);

user8681 Over a year ago

That won't work. Try to implement it and actually call the callbacks - you'll see why.

Turing85 Over a year ago

Then my testcase seems to be missing something. It compiles and runs for me. Could you give me a hint?

user8681 Over a year ago

@Turing85 Try to call the getSecureThing method from my example in a callback. It won't be available, because you can't force the callback implementation to only take Secure instances.

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