Preparing for the 0.2 draft

[Joshua Bloch]

Mark,

On Thu, Feb 4, 2010 at 12:27 PM, Mark Mahieu <markmahieu at googlemail.com>wrote:

>
> On 4 Feb 2010, at 05:56, Joshua Bloch wrote:
>
> > Neal found 48 locations in OpenJDK 6 in which he claimed that evaluating
> > "this" in the surrounding context would be helpful. I decided to take a
> > closer look at them in hopes of learning something.
>
> <snip>
>
> > 4 could not take advantage of the
> > "enclosing scope semantics for this," because the anonymous class has a
> > method that shadows the one in the enclosing class (see (1) below for an
> > example)
>
> If "this" were evaluated in the context of the enclosing class, how could
> shadowing come into play here?  Wouldn't the former preclude the latter?
>

This class of uses concern SAM types, exclusively. Technically you're right.
But evaluating "this" in the enclosing context of a "SAM lambda" is a bit
harder to fathom. If you're implementing the compare method of a Comparator
instance and you invoke the compare method, you probably expect a recursive
invocation.


>
> > (1) src/share/classes/com/sun/java/util/jar/pack/Histogram.java:187
> >
> > class Histogram {
> >    public double getBitLength(int value) {
> >        double prob = (double) getFrequency(value) / getTotalWeight();
> >        return - Math.log(prob) / log2;
> >    }
> >
> >    private final BitMetric bitMetric = new BitMetric() {
> >        public double getBitLength(int value) {
> >            return Histogram.this.getBitLength(value);
> >        }
> >    };
> > }
> >
> > The qualified this is necessary only because the anonymous class gives
> its
> > sole method the same name as the one that it wants to call in its
> enclosing
> > instance. But you can't name the method in a lambda expression, so having
> > "this" evaluated in the surrounding context would not be necessary (or
> > helpful) here.
>
> The method is named anyway, by the BitMetric interface.  Surely it's
> whether or not that is visible in the body of an equivalent lambda which
> determines how helpful a given approach is?  For this particular example,
> the "evaluated in surrounding context" approach works out the same as the
> approach in the v0.1 draft spec, I think.  Or is that your point?
>

I was purpose vague in discussing "class 4." I do believe the examples in
this class don't benefit from evaluating this in the surrounding context,
but it's hard to discuss them precisely until we firm up our story on SAM
types.

        Josh

P.S.  Also I reviewed 48 (!) uses, which limited the amount of time I could
spend thinking about each one.