Question

Say I have a Hashtable<String, Object> with such keys and values:

apple => 1
orange => 2
mossberg => 3

I can use the standard get method to get 1 by “apple”, but what I want is getting the same value (or a list of values) by a part of the key, for example “ppl”. Of course it may yield several results, in this case I want to be able to process each key-value pair. So basically similar to the LIKE '%ppl%' SQL statement, but I don’t want to use a (in-memory) database just because I don’t want to add unnecessary complexity. What would you recommend?

Update:
Storing data in a Hashtable isn’t a requirement. I’m seeking for a kind of a general approach to solve this.

Answer 1

The obvious brute-force approach would be to iterate through the keys in the map and match them against the char sequence. That could be fine for a small map, but of course it does not scale.

This could be improved by using a second map to cache search results. Whenever you collect a list of keys matching a given char sequence, you can store these in the second map so that next time the lookup is fast. Of course, if the original map is changed often, it may get complicated to update the cache. As always with caches, it works best if the map is read much more often than changed.

Alternatively, if you know the possible char sequences in advance, you could pre-generate the lists of matching strings and pre-fill your cache map.

Update: Hashtable is not recommended anyway – it is synchronized, thus much slower than it should be. You are better off using HashMap if no concurrency is involved, or ConcurrentHashMap otherwise. Latter outperforms a Hashtable by far.

Apart from that, out of the top of my head I can’t think of a better collection to this task than maps. Of course, you may experiment with different map implementations, to find the one which suits best your specific circumstances and usage patterns. In general, it would thus be

Map<String, Object> fruits;
Map<String, List<String>> matchingKeys;