in reply to Re^2: Open to debate on mixins and traits.
in thread Open to debate on mixins and traits.

I like to describe Java interfaces as "multiple inheritance from abstract base classes that live in a secondary type system." Here are four drawbacks:

The first problem is the most damning from the purist's perspective. Java's designers recognized a real problem (a single-rooted inheritance scheme is inadequate to express real solutions), but they created a secondary type system and didn't take it seriously.

From a practical perspective, the second problem is worse. The designers recognized that a single-rooted inheritance scheme is inadequate to express complex behaviors of real problems, but they completely failed to allow for any code reuse! "Objects that implement this interface all behave similarly, but multiple inheritance is bad so you'll have to copy and paste code to make it work."

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Re^4: Open to debate on mixins and traits.
by lachoy (Parson) on Jun 02, 2004 at 23:02 UTC

    I agree that most problems with interfaces derive from the multiple inheritance workarounds rather than the interfaces themselves. But from a non-purist perspective they're generally quite nice to work with, particularly when you're designing a large system. And using copy-and-paste instead of MI is IME never an actual problem because you can either define it in a common base class or move it to another class and use composition.

    I also agree that for most of the standard library interfaces are not used enough. There are exceptions (collections, io) but they prove the rule.

    As a workaround (to a workaround I guess) there's a good deal of activity around different AOP solutions to implement mixins, even applying them to compiled classes. The only problem with this is the typical problem with AOP -- you don't know by looking at the implementation what it can do or how it can be used, you've got to have knowledge from somewhere else (configuration files or other classes). Or you have to rely on an IDE to do it for you, and then you're tied to a particular tool, which may be even worse.

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