Overriding return type in extended interface - Bad idea?

Question

In Java, you can do the following :

public interface IEngine{}
public interface ICoolEngine extends IEngine{}

public interface Car
{
   IEngine getEngine();         


        

Answer 1

No, that's fine. Since ICoolEngine extends IEngine, any object implementing ICoolEngine can be treated as if it's an IEngine (without all the ICoolEngine-specific methods of course). You'll just have to be aware of the type difference depending on which interface you are working with in each situation, and make sure not to use ICoolEngine methods that aren't defined in IEngine (assuming that, in your actual code, there are additional methods listed in the equivalent of ICoolEngine).

It's not a bad practice to do this; you're simply using the power of polymorphism.