Re-specification and re-abstraction

[Greg Dennis]

>>> The tension here is between respecting the existing language behavior,
>>> intended or not -- where exact redeclarations of interface methods is a
>>> no-op -- and how we want this "new" feature to work, and who loses when
>>> existing usage patterns conflict with the desired usage patterns for the
>>> new feature.
>>
>> The no-op-ness of "exact redeclaration of interface method" is respected
>> either way if the definition of "exact" is adequate.
>
> Yes, if you exactly redeclare the default too, it is also a no-op.  So,
> more precise statement: "exact redeclaration of the return type, name,
> and signature, but no default clause."

Long-time lurker here chiming in. It seems to me a problem with this
calling this an "exact redeclaration" is that all of these things may
stay the same (return type, name, signature), and yet the *contract*
of the method may still have changed. That new contract may be given
in the Javadoc accompanying the method, but does not require any
change in method signature.

For example, consider class C:

  class C {
    /**
     * Precondition: a is not null
     */
    R method m(A a);
  }

When a precondition is false, the method can do anything --- return a
value, throw an exception, go into an infinite loop, anything. Now
class D may extend it and drop the precondition:

  class D {
    /**
     * If a is null, returns null.
     */
    R method(A a);
  }

Since the precondition is dropped, D is still a true subtype of C. But
if the author of C were to later provide a default implementation to m
that throws an exception when given a null argument, it would break
the contract of D. That seems very undesirable.

The argument is that the current semantics preserves the "no-op"
behavior of a redeclaration. Another way to look at it is that a
redeclaration has always meant "reabstract" and that the "no-op"
treatment has merely been an optimization since concrete
implementations in interfaces were never before possible :)

I'll now scurry back into lurker mode ...

Greg