Covariant overrides on the Buffer Hierachy

Peter Levart peter.levart at
Tue Apr 22 07:05:21 UTC 2014

On 04/22/2014 04:30 AM, Zhong Yu wrote:
> On Mon, Apr 21, 2014 at 7:07 AM, Ulf Zibis <Ulf.Zibis at> wrote:
>> So to me, this is clearly a language matter, not to solve by tricky
>> footprint and/or performance lasting code on APIs:
>> People want a _smart syntax_ for consecutive method invocations on the same
>> object, that's all!
> Amen to that!
> However I'm a little concerned with the mental effort a reader needs
> to figure out the implicit method receiver. I'd like to see more
> explicitness in establishing the implicit method receiver, something
> like Pascal's with statement.
>      expr "."
>      {
>          // expr is the implicit receiver for this block
>          foo(); // interpreted as
>      }
> so that we could write
>      CharBuffer.allocate(26).{position(2);put("C");position(25);put("Z");}
>      string.{ mySub=subString(indexOf("C"), indexOf("Q")); }
>      new HashMap<>().{ put(n1,v1); put(n2,v2); }
>      new Button().{ color(red); text("hello"); onClick(event->color(blue)); }

Some questions about this syntax:

What is the type of :

     new HashMap<>().{ put(n1,v1); put(n2,v2); }

is it an expression or a statement? It should be an expression, 
otherwise it's not of much use. It should be possible to do:

     Map<Integer, String> map = new HashMap<>().{ put(n1,v1); put(n2,v2); };

...but that looks very similar to an anonymous class syntax, doesn't it? 
It actually behaves similarly to, for example:

     Map<Integer, String> map = new HashMap<>() {{ put(n1,v1); 
put(n2,v2); }};

I don't know if this is good or bad.

It certainly spawns all kinds of questions that were on the agenda for 
lambda expressions, like:
- what's the meaning of "this" inside the .{ ... } block?
- what's the meaning of "break", "continue", "next", "return", .... (I 
guess it should be the same as with the ordinary lexical block: { ... }, 
although it's rather unusual to be able to for example "break" out of a 
loop inside the evaluation of an expression)

Regards, Peter

> Zhong Yu

More information about the core-libs-dev mailing list