Explain java map of maps.

Regarding the Java map of maps - Both nested and combined keys have their places. bowmore gives a pro argument for composite keys, and a con argument for nested maps. Let me provide the loyal opposition:

Composite map keys work great when you're looking up a specific known item.

Nested maps work well when you need to rapidly find all the variations, kinds, and subtypes of type A. For example, choosing A (vs. B, C, ...) might be the first step in a decision tree. Once the user or algorithm or whatever picks A, then you need to know only about A's subtypes, and B..Z or B..ZZZZZ no longer matter.

Now you're dealing with a very tight and efficient lookup structure for the subsearch. If you try to do that with composite keys, you end up doing a full table scan a la [ (key, value) for (key, value) in prices.items() if key.startswith('A') ]. That's not an efficient operation, and will be slow if the map is at all large.

Nested maps also work well when the number of nesting levels may grow. The problem structure already extended from type to (type, subtype). Is there any chance the next rev will need (type, subtype, variation) or (type, subtype, version)? If so, a nested mapping approach can be cleanly extended. This, however, is a stylistic, second-order advantage, especially compared to the "easy subsearch" advantage above.