Is it a feature abuse to introduce default methods in new interfaces?

[Daniel Fuchs]

Hi Jaikiran,

I don't think this is an abuse, and I'd say that the decision on
whether to provide a default implementation for a new interface
probably depends on how well you estimate that the default
implementation will match what concrete subclass would have to
implement. Even if that's not an authoritative answer ;-)

I don't know if that's representative but Stream.Builder for
instance, came with a default add( ) method:

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.Builder.html

best regards,

-- daniel

On 21/02/2019 14:35, Jaikiran Pai wrote:
> I have been trying to find some authoritative answer on when _not_ to
> use default methods on interfaces (a feature introduced in Java 8). The
> official documentation in Java, about this feature, talks about the
> motivation[1] behind introducing this feature and explains how
> _existing_ interfaces can be changed to add default methods without
> breaking binary compatibility:
> 
> "Default methods enable you to add new functionality to the interfaces
> of your libraries and ensure binary compatibility with code written for
> older versions of those interfaces."
> 
> This and some other docs don't mention whether it's an accepted practice
> or abuse of default methods feature, when dealing with new interfaces.
> Is it fine to introduce new interfaces with default methods? I have
> tried searching the mailing list archives to see if it's discussed
> previously but my search terms haven't found anything relevant. Any
> previous authoritative answer, similar to Stuart Mark's reply on
> stackoverflow (on a different topic) about Optional usage[2], is also
> good enough.
> 
> [1] https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html
> 
> [2]
> https://stackoverflow.com/questions/23454952/uses-for-optional/23464794#23464794
> 
> -Jaikiran
> 
>