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From: v_borchert@despammed.com (Volker Borchert)
Newsgroups: comp.lang.java.programmer
Subject: Re: Alternatives languages on the JVM: which one or is there no alternative?
Date: 29 Nov 2013 23:22:37 GMT
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Robert Klemme wrote:

> What does dynamic_cast in C++ do differently?  As far as I am informed 
> it involves a type check as well.  I mean, you can have the same in Java 
> by catching the exception (which is what you need to do in C++ IIRC).

No you don't, that is exactly the raison d'etre of dynamic_cast.

  if (o instanceof C) {
    final C c = (C) o;
    // do something with c
  }

does _two_ typechecks

  final C c = dynamic_cast<C>(o);
  if (c != null) {
    // do something with c
  }

would do _one_ typecheck and a cheap IFNONNULL

> > class Foo<T> {
> >      // nonstatic inner class uzes 4 bytes for the
> >      // implicit this$0 and 4 more bytes padding
> >      class TPtr<T> extends WeakReference<T> {
> >          Foo foo() {
> >              return Foo.this;
> >          }
> >      }
> >
> >      void gced() {
> >      }
> > }
> 
> Since Foo inherits Object you could as well make Foo<T> inherit 
> WeakReference<T> avoiding the additional bytes per instance.

Sorry, I forgot the "extends Bar<T>".

> I find it difficult to discuss this without knowing more about your 
> requirements and situation.  But then again, maybe you don't want or 
> cannot disclose that.

One example, for a Cache<Key,Value> that supports mixed flavor Value
references, and a subclass which additionally has entries doubly
linked, somewhat similar to LinkedHashMap, I want

                     AbstractEntry<K,V>                 WeakReference<V>
                            |                                    |
                            |                                    |
                           / \                                   |
          +---------------+---+----------+-------------+         |
          |                              |             |         |
          |                              |             |         |
  AbstractLinkedEntry<K,V>         StrongEntry<K,V>    |         |
          |                              |             |         |
          |                              |             |         |
          +---------------+---+----------+             +--+---+--+
          |                \ /                             \ /
          |                 |                               |
          |                 |                               |
          |          StrongLinkedEntry<K,V>             WeakEntry<K,V>
          |                                                 |
          |                                                 |
          +------------------------------+---+--------------+
                                          \ /
                                           |
                                           |
                                     WeakLinkedEntry<K,V>


(soft flavor omitted for safe of readability)


-- 

"I'm a doctor, not a mechanic." Dr Leonard McCoy <mccoy@ncc1701.starfleet.fed>
"I'm a mechanic, not a doctor." Volker Borchert  <v_borchert@despammed.com>