The implementation of default methods

[Brian Goetz]

There are several layers of this issue.

1.  How to document implementation properties of the default in a way 
that does *not* commit to doing things this way forever.  This is 
analogous to "This implementation currently..." notes that are sometimes 
seen in class method documentation, and I don't think requires much more 
than that.  For example, if the default implementation uses a bad 
algorithm, it could document this as "this implementation currently uses 
an O(n^2) algorithm; subclasses that care about performance should 
probably provide a better implementation."  This does not commit us to 
maintaining a bad algorithm forever, but does provide useful detail to 
users and subclass maintainers.

2.  How to document what a subclass can count on, so they can make a 
decision as to whether they need to override the method or not.  Here, 
Doug's examples with putIfAbsent and friends are good; we document "this 
implementation behaves as if...", which is a kind of specification for 
the default, and the default had better continue to behave that way.

3.  Retrofitted defaults in already-specified classes.  This is the 
Iterator.remove issue being discussed on corelibs, where the issue is 
whether adding a default for remove that throws UOE now becomes part of 
the specification for Iterator.remove, and whether all compliant 
implementations much have a similarly-behavior Iterator.remove.  This 
one is trickier than (2) because it may be changing the specification of 
existing code.  So the question here is, what does it mean to add a 
default on Iterator.remove?  Does that make undue work for JDK vendors 
who don't use our java.util classes?  How do we write the spec to make 
it clear that having a default that throws UOE *is* part of the spec now?

Here's my proposed answers:

1.  Document "Implementation note: The default implementation 
currently..."

2.  Document "The default implementation behaves as if..."  (Or whatever 
Doug's proposed wording is.)

3.  Document "The default implementation MUST"




On 12/14/2012 8:06 AM, David Holmes wrote:
> Okay we now have several threads across the public lists debating the
> issue of whether the implementations of default methods that we are
> adding to the core libraries are the required implementation for those
> default methods or simply an implementation.
>
> Can we get a definitive position on this?
>
> Thanks,
> David