Add size method for immutable collection builders

It’s often necessary to know the size of the collection being built. For example, to special case if this is the first element, or if the collection is empty. The workarounds are ugly.

This should be free for most builders since they already track the size. I only care about this for List, Set, and Map, so if it’s fine if it can’t be added for more exotic collections like ListMultimap.

[electrum]

@jbduncan yes, exactly. Apologies for not explaining more clearly.

@lowasser that’s a great point about duplicates for ImmutableSet.

We could add a size() method for ImmutableList, ImmutableMap, and any others where it makes sense and is free. Additionally, we could add an isEmpty() method for most/all builders. Knowing if the collection is empty will cover a good number of use cases, though sometimes you do need the size (most often for ImmutableList).

[cpovirk]

In the cases where people have asked for this internally, we’ve found that their code reads better if it separates the “add things to builder” and “read from resulting collection” steps, rather than intermixing the two, even in the small way you suggest here. I would imagine that there are exceptions, of course, but overall, I fear that people will use this to make code worse rather than better.

We’ve also found that people try to avoid calling build() on an empty builder. That turns out not to be necessary because build() is free in that case.