I thought of that ... it started in
LUCENE-5248 where I want to keep a growable bitset alongside the docs/values arrays to mark whether a document has an updated value or not (following Rob's idea). When I implemented that using OpenBitSet, I discovered the bug and opened LUCENE-5272. As I worked on fixing the bug, I realized OBS has other issues as well and thought that perhaps I can use FixedBitSet, only grow it by copying its array. This is doable even without the ctor, since I can call getBits() and do it like that:
FixedBitSet newBits = new FixedBitSet(17); System.arraycopy(oldBits.getBits(), 0, newBits.getBits(), 0, oldBits.getBits().length);
I then noticed there is a ctor already in FixedBitSet which copies another FBS so I thought just to improve it. It seems more intuitive to do t than let users figure out they can grow a FixedBitSet like above?