如何轻松访问Java（或Google番石榴）中的嵌套地图

Question

I have a nested Hashmap (using JDK 7) as defined below

private static HashMap<SourceSystemIdEnum, HashMap<String, HashMap<StatsEnum, Double>>> statsCache = new HashMap<SourceSystemIdEnum, HashMap<String, HashMap<StatsEnum, Double>>>();

All the keys for all the maps (nested as well) are created at runtime and the value of outer map is another map; the values of next level of map is another map; and the value of inner most map is just a Double (it is not going to be collection).

I use above data structure to maintain cache (tree like object hierarchy) where the values of inner most map is updated every second (that is Double type).

I came across http://tomjefferys.blogspot.com/2011/09/multimaps-google-guava.html , while looking for a better/easier nested maps. But the value of inner most map is never going to be collection, so google Guava didnt seem relevant at first look(?)

I also came across this How to iterate through Nested Map and Multiset? - Java/Guava and even iteration doesnt be any better in guava (it seems).

I am trying to reduce routine (broiler plate) code when iterating or updating the values of my scenario (map within map within map) and how can I rewrite my existing code to switch to Google Guava's multimap (or is that going to be any better than regular hashmap of jdk7 ).

****EDIT**** I agree it is unusual to have such deep nesting. I can have a list of maps, but then lookup is going to be costly. Here is the breakdown of maps

HashMap : the outer map represents various data feeds as the keys(ex:NDAQ,CBOE,NYSE,AMEX)

HashMap the mid level map represents various tickers as the keys(ex:CSCO,INTC,MSFT..)

HashMap: the outer map represents values of various statistical parameters (EX: mean, median, skew,kurtosis) as the keys and it is these Double values that get updated every minute

NOTE: All the keys in above 3 maps are pretty much known in advance (so maps dont grow or resize during runtime-it is only Double value in inner most map that gets updated every minute)

Answer 1

Could a Guava Table help with your problem?

Ultimately, it's unusual to have a problem where you need to have so many indirections. What kind of data are you trying to represent in this structure? What kinds of operations are you trying to perform?

Answer 2

It strikes me that you are using collection types as a substitute for a proper complex key.

If I can boil down what you're trying to accomplish, it is to look up a stock statistic based on three things: the feed, ticker, and statistic type. It seems to me that this HashMap structure is being used simply for caching and lookups, and that the hierarchy is not otherwise relevant.

Given that, let's define a complex key:

public final class StockStatisticKey {
    private final SourceSystemIdEnum systemId;
    private final String tickerName;
    private final StatsEnum statType;

    public StockStatisticKey(SourceSystemIdEnum systemId, String tickerName, StatsEnum statType) {
            this.systemId = systemId;
            this.tickerName = tickerName;
            this.statType = statType;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof StockStatisticKey) {
            StockStatisticKey other = (StockStatisticKey) obj;
            boolean equal = true;
            equal &= Objects.equal(systemId, other.systemId);
            equal &= Objects.equal(tickerName, other.tickerName);
            equal &= Objects.equal(statType, other.statType);

            return equal;
        }

        return false;
    }

    public int hashCode() {
        return Objects.hashCode(systemId, tickerName, statType);
    }
}

Now you can have a Cache<StockStatisticKey, Double> where you can very quickly look up your value based on this complex key. This should perform every bit as well as your nested HashMaps, and adds to it semantic value.

You may also consider some variant of this, where the SourceSystemIdEnum and the ticker name together form one key, the StatusEnum as second key, and these values would go into a Guava Table .

Answer 3

It seems Trees is good data structure (as @Ata suggested). Because there is no standard JDK implementation of tree(??) and not convinced with adding open source tree implementations (from maven), I am going to stick with my existing nested maps. I will update this answer, if I find better solution