Why would both a parent and child class implement the same interface?
Solution 1
As I understand interfaces (and my experimentation has reinforced), there is no purpose to having both the parent and the child implement the same interface.
No. Technically, it is completely redundant.
It does however document the fact that you intend SoapFacadeImpl
to be a SoapFacade
and it ensures that you get a compile error, if you (or someone else) decides to remove implements SoapFacade
from the base class.
You see this pattern everywhere in the standard Java Collections API. ArrayList
implements List
even though its base class (AbstractList
) already, does. Same holds for HashSet
/ AbstractSet
and the Set
interface.
Solution 2
If you use the interface also as a marker. Class.getInterfaces();
will only return directly instanced interfaces.
Riggy
Programmer for 18 years now, working primarily in Java and on the web.
Updated on June 03, 2022Comments
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Riggy about 2 years
I inherited some legacy Java (1.4) code and this design decision appears regularly. I can't understand if there's any purpose or reason to it.
public interface SoapFacade extends iConfigurable{ } public class SoapFacadeBase implements SoapFacade{ ... } public class SoapFacadeImpl extends SoapFacadeBase implements SoapFacade{ ... }
As I understand interfaces (and my experimentation has reinforced), there is no purpose to having both the parent and the child implement the same interface. In this scenario, everything from
SoapFacade
is implemented inSoapFacadeBase
, but the method iniConfigurable
is implemented inSoapFacadeImpl
. However, that doesn't create a need to haveSoapFacadeImpl
implementSoapFacade
.Is there something I don't know about interfaces that would give this pattern some purpose or benefit? Are there underlying costs beyond lack of clarity that should drive refactoring it? Or should it simply be refactored for clarity/simplicity?