Constructor calls like new AtomicInteger(0) cause a volatile write that
can be saved in cases where the constructor parameter is the default
value.
See gh-23575
Apply checkstyle rule to ensure that private and package private
classes do not have unnecessary public methods. Test classes have
also been unified as much as possible to use default scoped
inner-classes.
Closes gh-7316
Andy Wilkinson
dreis2211
Phillip Webb
Madhura Bhave