Question

I have to devise a function that will take as input a keyword and will output a category id.
Ex:

f('dog') returns _ANIMAL
f('chair') returns _FURNITURE

I already have the mapping and I could just iterate over the tag array each time, but I have a feeling this is not the best solution.

Is there a special data structure (I’m thinking of ternary search trees) in the Java libraries for this specific task? Should I just use HashMap (or maybe Set (since there are few categories))?

P.S. This mapping is fixed, I do not need to add or remove elements from it once it is built.

Answer 1

If I understand you correctly, then HashMap sounds like exactly what you want. You wouldn’t want to iterate through an entire array each time, because with many function calls and/or a large array your program would wind up running slowly. With a HashMap, pulling a value (your category) from a key (your keyword) happens more or less immediately, in constant time.

You can build the map like this:

HashMap map = new HashMap();
map.put("dog", "animal");
map.put("chair", "furniture");
map.put("cat", "animal");

And then map.get("dog") returns “animal”, map.get("chair") returns “furniture”.

As others have indicated, enums would work well (and a tiny bit faster) for this too—with the caveat that they’re fixed at compile time and thus cannot be changed during execution.