default methods in interfaces

[Paul Sandoz]

Hi Mohan,

The primary use-case for default methods is to evolve interfaces without breaking compatibility [*]. Without such a mechanism we would not have been able to evolve the Collections API and integrate it with the Streams API.

See section 10 of:

  http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-final.html

What you are asking may have been discussed before, search here:

  http://openjdk.markmail.org/search/?q=lambda-dev

Default methods can be used as a limited form of traits to mix-in behaviour but not state, so they ain't like Scala's traits. Any state (or context) needs to be passed as parameters to those methods. Also note that because there is no state then there are no state-realted issues associated multiple inheritance (Java already has multiple inheritance of types; default methods add some additional inheritance rules for resolving conflicts, but fundamentally they are inherited like other methods). 

In terms of patterns for this kind of usage, i don't have a clear answer. I expect some may evolve over time as developers evolve and implement new interfaces that provide certain mix-in behaviour. Note that we have been careful not to add the @FunctionalInteface annotation to all interfaces that could be used as the target type of lambda expressions and method references. In certain cases such non-annotated interfaces are more intended to provide mix-in behaviour, so there is clue where to look and watch.

Hth,
Paul.

[*] Note that there is still a small risk that the methods added will clash with methods on implementing classes or other interfaces.

On Oct 29, 2013, at 8:15 AM, mohan.radhakrishnan at polarisft.com wrote:

> Hi,
>         There is no user list and this is a basic question. The default 
> methods in interfaces are being used to implement 'MixIn' classes(AspectJ 
> does it by weaving classes ) and some references even mention multiple 
> inheritance. What is the general design pattern in application code that 
> this is intended for ? Is this  related to functional programming styles 
> in some way ?
> 
> Should I just search older threads for this type of discussion ?
> 
> Thanks,
> Mohan
> 
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