Is a HashMap a proper data structure

Question

I store in a HashMap 3 types of object.

HashMap>

[\'Lorry\', [list of lorries]]
[\'Sport\', [list of sport\'s cars]]
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Answer 1

A HashMap is the right data structure for the job, but in your case you might consider using two HashMaps: One holding the relation 'Car Type' -> 'Cars of that Type', and a second one for the relation 'ID' -> 'Car with that ID'.

Answer 2

The basic approach is sound, however since you only want to store each instance once, a Set is a better choice than a List for the map entry value:

Map<String, Set<Car>> typeCache = new HashMap<String, HashSet<Car>>();

The contains() method of HashSet is very fast indeed, so finding if your map contains a particular instance in it values is not going to cost much.

Using two maps would probably be better though - once for each type of lookup, so also use:

Map<String, Object> idCache = new HashMap<String, Object>();