Doug Lea wrote:
I don't think so. Relaying the subset methods to the *Set classes
was just an implementation expediency under the mis-thought that they
would take care of recursive subsets etc rather than needing
special implementations for KeySets. But the byproduct for remove()
is clearly wrong so they do need separate implementations.
This remembers me from an old pet peeve, the fact that java.util.HashSet
is implemented as a wrapper over java.util.HashMap. This sucks
performance-wise, due to the additional indirections everywhere, and
even more important, because every entry single object is larger than
necessary - it contains a key and a value fields, but a single field
would be necessary for HashSet. If you have a Set with millions of
entries, the overhead of one useless pointer per entry is significant.
Plus, the wrapping implementation uses a dummy, fixed Object instance as
the value for all entries - and this may create another subtle
performance problem: large numbers of "cross-space" references in the
heap. I know that in most GCs only old-to-young refs are evil, but it
seems that in some GCs any cross-space reference is bad (requires remset
tracking), like Detlef et al's new "Garbage-First" collector (if I
understood it).
Back in J2SE1.2 time, the runtime savings from the wrapping
implementation could be significant... but, today? The HashMap* classes
inside the latest rt.jar are 18,5Kb total (uncompressed), versus 3Kb
from HashSet. That's a 12Kb of space saving, plus some small saving in
JIT time too. I'd say that this is an irrelevant advantage, even in
JavaME. The performance bonus of a tuned HashSet, however modest, would
certainly be much more significant. I know it's ugly from a maintenance
POV to have two large classes that will be 90% identical, but to
paraphrase the movie Some Like It Hot, nothing is perfect...
A+
Osvaldo
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