Re: Alternatives languages on the JVM: which one or is there no alternative?

From: (Volker Borchert)
29 Nov 2013 23:22:37 GMT
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>
          | |
          | |
                                          \ /

(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 <>

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