Enthuware

In the exam, you may get a question that says "100 characters", in that case, ensureCapacity() may not be a valid option.

And then what would be valid way?
I see there is even no such simple field in StringBuilder nor AbstractStringBuilder as capacity, it's somehow not that clearly for me calculated...

There is no way to ensure the capacity at exactly 100 (or any specific number). The point is to read the problem statement carefully.