[adrianN]

But that's what profiling is for.

[arjie]

That argument favours hashmaps by default, though. After all, they're all right at small sizes and much better at big sizes.

[usefulcat]

Profiling won't tell you what the chances are that the collection will someday need to contain more than N elements.

[adrianN]

You don't replace every map in your code with a distributed Key-Value store, in case it might need to store terabytes in the future. Most of the time you have a pretty good idea whether a map has to store a hundred, or a hundred thousand elements.