Re: Glitch in Java Collections (No descendingMap in LinkedHashMap)

From:

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

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 10 Oct 2012 18:20:13 -0400

Message-ID:

<k54sau$htf$1@dont-email.me>

On 10/10/2012 1:45 PM, Jim Janney wrote:

Jim Janney <jjanney@shell.xmission.com> writes:

Eric Sosman <esosman@comcast-dot-net.invalid> writes:

On 10/10/2012 11:00 AM, Jim Janney wrote:

Jim Janney <jjanney@shell.xmission.com> writes:

Jan Burse <janburse@fastmail.fm> writes:
[...]
If I really needed that functionality I'd probably try maintaining my
own access-order list in parallel to the map.

Oops, stupid me. The other way is to define a comparator based on
insertion order, and then use a SortedMap.

Defining the comparator might be something of a struggle,
especially if the same object instance could be referred to by
two different LinkedHashMaps.

The trick is finding a way to link the ordering information (probably a
counter assocated with the map) with the keys themselves. This is the
kind of problem where IdentityHashMap comes in handy.

Voila:

import java.util.Map;
import java.util.TreeMap;
import java.util.WeakHashMap;

public class InsertionOrderedMap<K,V> extends TreeMap<K,V> {
   private long nextInsertionRank = 0;
   private Map<K, Long> insertionRanks = new WeakHashMap<K, Long>();
   [... in which a sequence number is stored.]

Okay, fine, but how does this qualify as an "other" way?
To my eye it looks exactly like "my own access-order list in
parallel to the map," and not something "other" at all.

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