Minor improvement to anonymous classes

Brian Goetz brian.goetz at oracle.com
Fri Jul 30 14:52:23 UTC 2021

I have been working on a library where I've found myself repeatedly 
refactoring what should be anonymous classes into named (often local) 
classes, for the sole reason that I want to combine interfaces with an 
abstract base class:

     interface Foo { ... lots of stuff .. }
     abstract class AbstractFoo { ... lots of base implementation ... }

     interface RedFoo extends Foo { void red(); }

and I want a factory that yields a RedFoo that is based on AbstractFoo 
and implements red().  Trivial with a named class, but there's no reason 
I should not be able to do that with an anonymous class, since I have no 
need of the name.

We already address this problem elsewhere; there are several places in 
the grammar where you can append additional _interfaces_ with &, such as:

     class X<T extends Foo & Red> { ... }

and casts (which can be target types for lambdas.)

These are not full-blown intersection types, but accomodate for the fact 
that classes have one superclass and potentially multiple interfaces.  
It appears simple to extend this to inner class creation expressions:

     new AbstractFoo(args) & RedFoo { ... }

This would also smooth out a rough edge refactoring between lambdas and 
anonymous classes.

I suspect there are one or two other places in the spec that could use 
this treatment.

(Note that this is explicitly *not* a call for "let's do full-blown 
intersection types"; this is solely about class declaration.)

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