Java - Initialize a HashMap of HashMaps

Question

I am new to java and practicing by creating a simplistic NaiveBayes classifier. I am still new to object instantiation, and wonder what to do to initialize a HashMap of Has

Answer 1

Recursive generic data structures, like maps of maps, while not an outright bad idea, are often indicative of something you could refactor - the inner map often could be a first order object (with named fields or an internal map), rather than simply a map. You'll still have to initialize these inner objects, but it often is a much cleaner, clearer way to develop.

For instance, if you have a Map> you're often really storing a map of A to Thing, but the way Thing is being stored is coincidentally a map. You'll often find it cleaner and easier to hide the fact that Thing is a map, and instead store a mapping of Map where thing is defined as:

public class Thing {
    // Map is guaranteed to be initialized if a Thing exists
    private Map data = new Map();

    // operations on data, like get and put
    // now can have sanity checks you couldn't enforce when the map was public
}

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Also, look into Guava's Mulitmap/Multiset utilities, they're very useful for cases like this, in particular they do the inner-object initializations automatically. Of note for your case, just about any time you implement Map you really want a Guava Multiset. Cleaner and clearer.