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POSimple database-like collection class in Java
 

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    <p>The problem: Maintain a bidirectional many-to-one relationship among java objects.</p> <p>Something like the Google/Commons Collections bidi maps, but I want to <em>allow</em> duplicate values on the forward side, and have <em>sets</em> of the forward keys as the reverse side values. Used something like this:</p> <pre><code>// maintaining disjoint areas on a gameboard. Location is a space on the // gameboard; Regions refer to disjoint collections of Locations. MagicalManyToOneMap&lt;Location, Region&gt; forward = // the game universe Map&lt;Region, &lt;Set&lt;Location&gt;&gt;&gt; inverse = forward.getInverse(); // live, not a copy Location parkplace = Game.chooseSomeLocation(...); Region mine = forward.get(parkplace); // assume !null; should be O(log n) Region other = Game.getSomeOtherRegion(...); // moving a Location from one Region to another: forward.put(parkplace, other); // or equivalently: inverse.get(other).add(parkplace); // should also be O(log n) or so // expected consistency: assert ! inverse.get(mine).contains(parkplace); assert forward.get(parkplace) == other; // and this should be fast, not iterate every possible location just to filter for mine: for (Location l : mine) { /* do something clever */ } </code></pre> <p>The simple java approaches are: 1. To maintain only one side of the relationship, either as a <code>Map&lt;Location, Region&gt;</code> or a <code>Map&lt;Region, Set&lt;Location&gt;&gt;</code>, and collect the inverse relationship by iteration when needed; Or, 2. To make a wrapper that maintains both sides' Maps, and intercept all mutating calls to keep both sides in sync.</p> <p>1 is O(n) instead of O(log n), which is becoming a problem. I started in on 2 and was in the weeds straightaway. (Know how many different ways there are to alter a Map entry?)</p> <p>This is almost trivial in the sql world (Location table gets an indexed RegionID column). Is there something obvious I'm missing that makes it trivial for normal objects?</p>
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    1. COI'm reluctant to put this as an answer because I don't know what you're actually doing, but for what its worth, I think you're just using the wrong data structure. What you describe isn't a map or a set, its a graph where all of the vertices are adjacent to any number of other vertices. You'll get a better programming model + runtime by using a proper graph data structure rather than glue together sets and maps haphazardly.
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    2. COProbably. All I'm after from Maps and Sets is the very basic semantics and the log-n performance. If a better structure provides a vaguely map-ish "view" of things, perfect. Am exploring http://jung.sourceforge.net/ thanks to a now-deleted answer and it looks promising.
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    3. COIs the cost of insertions important to you? (Also, do you mean otherset in that last line?)
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