This enanchment is useful for applications that make heavy use of BitSet objects as sets of integers, and therefore they need to make a lot of calls to cardinality() method, which in BitSet class require linear time in the number of words in use by the bit set. This optimization reduces the cost of calling cardinality() to constant time, as it simply returns the value of the field, and it also try to make as little effort as possible to update the field, when needed.
Link to the Pull Request: https://github.com/openjdk/jdk/pull/12133
