There is no standard small implementation of Map in Java. HashMap is one of the best and most flexible Map implementations around, and is hard to beat. However, in the very small requirement area — where heap usage and speed of construction is paramount — it is possible to do better.
I have implemented SmallCollections on GitHub to demonstrate how this might be done. I would love some comments on whether I have succeeded. It is by no means certain that I have.
Although the answers offered here were sometimes helpful, they tended, in general, to misunderstand the point. In any case, answering my own question was, in the end, much more useful to me than being given one.
The question here has served its purpose, and that is why I have ‘answered it myself’.