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Groups > comp.lang.java.programmer > #19996

Re: optimsed HashMap

From Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups comp.lang.java.programmer
Subject Re: optimsed HashMap
Date 2012-11-27 08:56 -0500
Organization A noiseless patient Spider
Message-ID <k92gpu$2ah$1@dont-email.me> (permalink)
References <8i70b8d0pm6ibk03ti4t2pv60jd0bctlcs@4ax.com> <b9Tss.19809$W21.10542@newsfe27.iad> <k914ve$kl4$1@dont-email.me> <jc0ts.12172$v83.1077@newsfe09.iad>

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On 11/27/2012 5:01 AM, Arved Sandstrom wrote:
> On 11/26/2012 09:28 PM, Eric Sosman wrote:
>> On 11/26/2012 6:44 PM, Daniel Pitts wrote:
>>>
>>> I don't think your use-case needs this kind of (read "micro")
>>> optimization.  However, I have often wished for a way to get a Map.Entry
>>> for a key, which could then be used to insert a new value efficiently.
>>>
>>> Map.Entry<K,V> getOrAddEntry(K key);
>>>
>>> Map.Entry<K,V> e = map.getOrAddEntry(myKey);
>>> if (e.getValue() == null) {
>>>     e.setValue(newValue(myKey));
>>> }
>>> useValue(e.getValue());
>>>
>>> Alas, not exactly possible to add it now.
>>
>>      java.util.concurrent.ConcurrentMap has putIfAbsent().
>>
> It's almost certainly a reflection (no pun intended) of what I do with
> Maps, but a common variant use case for me is
>
> putEmptyListAndAddValueToListIfAbsent
>
> I don't normally use ConcurrentHashMap so the usual idiom for me is a
> contains() check that may put() the new empty list as a value,
> afterwards the list (empty or otherwise) is always got, and the actual
> value added to it.

     I usually write it as

	List<V> list = map.get(key);
	if (list == null) {
	    list = new SomeKindOfList();
	    map.put(key, list);
	}
	list.add(newValue);

... which involves one extra lookup per mapped key, not one
extra lookup per query.

     The imaginary getOrAddEntry() would be nice, although it
might complicate the Map.Entry interface itself (you'd want a
way to distinguish "This key maps to null" from "This key is
newly entered").

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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Thread

optimsed HashMap Roedy Green <see_website@mindprod.com.invalid> - 2012-11-23 17:12 -0800
  Re: optimsed HashMap Arne Vajhøj <arne@vajhoej.dk> - 2012-11-23 20:19 -0500
  Re: optimsed HashMap markspace <-@.> - 2012-11-23 17:33 -0800
    Re: optimsed HashMap Roedy Green <see_website@mindprod.com.invalid> - 2012-11-23 22:42 -0800
      Re: optimsed HashMap Roedy Green <see_website@mindprod.com.invalid> - 2012-11-24 03:34 -0800
        Re: optimsed HashMap Knute Johnson <nospam@knutejohnson.com> - 2012-11-24 08:39 -0800
          Re: optimsed HashMap Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2012-11-24 15:14 -0800
      Re: optimsed HashMap Arne Vajhøj <arne@vajhoej.dk> - 2012-11-24 13:24 -0500
      Re: optimsed HashMap markspace <-@.> - 2012-11-24 10:44 -0800
        Re: optimsed HashMap "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-11-25 13:40 +0000
      Re: optimsed HashMap Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-11-26 22:03 +0100
        Re: optimsed HashMap Robert Klemme <shortcutter@googlemail.com> - 2012-11-26 23:32 +0100
          Re: optimsed HashMap Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-11-27 03:24 +0100
            Re: optimsed HashMap Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-11-27 03:35 +0100
              Re: optimsed HashMap Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-27 08:44 -0500
                Re: optimsed HashMap Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-11-27 14:20 -0800
                Re: optimsed HashMap Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2012-11-30 03:35 +0100
  Re: optimsed HashMap Patricia Shanahan <pats@acm.org> - 2012-11-23 19:51 -0800
    Re: optimsed HashMap "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-11-24 10:21 +0000
      Re: optimsed HashMap Roedy Green <see_website@mindprod.com.invalid> - 2012-11-24 03:39 -0800
        Re: optimsed HashMap Robert Klemme <shortcutter@googlemail.com> - 2012-11-24 16:24 +0100
          Re: optimsed HashMap "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-11-25 13:50 +0000
            Re: optimsed HashMap Robert Klemme <shortcutter@googlemail.com> - 2012-11-25 15:30 +0100
      Re: optimsed HashMap "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-11-26 21:13 +0000
    Re: optimsed HashMap Arne Vajhøj <arne@vajhoej.dk> - 2012-11-24 13:16 -0500
  Re: optimsed HashMap v_borchert@despammed.com (Volker Borchert) - 2012-11-24 08:05 +0000
  Re: optimsed HashMap Silvio <silvio@internet.com> - 2012-11-26 11:57 +0100
  Re: optimsed HashMap Jim Janney <jjanney@shell.xmission.com> - 2012-11-26 11:13 -0700
  Re: optimsed HashMap Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-11-26 15:44 -0800
    Re: optimsed HashMap Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-26 20:28 -0500
      Re: optimsed HashMap Arved Sandstrom <asandstrom2@eastlink.ca> - 2012-11-27 06:01 -0400
        Re: optimsed HashMap Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-27 08:56 -0500
      Re: optimsed HashMap Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-11-27 14:16 -0800

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