Re: Indexing by multiple keys

From:

Lew <noone@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 31 Jul 2009 23:21:27 -0400

Message-ID:

<h50cbo$952$1@news.albasani.net>

markspace wrote:

  class Person {
    int ssn;
    String name = "";
  }

  HashMap<Object,Person> map = new HashMap<Object,Person>();

  Person person = new Person();
  map.put( person.ssn, person );
  map.put( person.name, person );

Now "person" is in the map twice, once under SSN and once by their name.
As Arne said, this is done by reference so there's no wasted space or
extra copies or anything bad like that.

Lew wrote:

There could be wasted space. Using your example:

map.put( person.name, person );
map.put( new String( person.name ), person );

will create two instances of name strings with the same value,
neither of which can be GCed while the person lives and is in the map.

Peter Duniho wrote:
> It's true that, with the first string coming from the Person instance,
> having that entry no longer in the HashMap won't allow the string
> instance to be GC'ed (it's still referenced by the Person instance).
> But if you reverse the order, so that the newly constructed string is
> added first, then when the second call to put() happens, that first
> newly constructed string will be collectable (assuming no other code,
> where it's retained elsewhere, of course).

Sure, but my point was a response to the notion that the Map as construed in
the example unequivocally would have "no wasted space or extra copies or
anything bad like that". All it took was one example of how that isn't true.

--
Lew