Improve code formatting, compiler warnings and JavaDoc

[sclassen]

I would like to contribute a series of PullRequests to 2.x branch fixing up some code formatting, compiler warning and JavaDoc improvements.

This issue should serve as a container to bundle the PullRequests and maybe to discuss issues which arise.

As the plan is to NOT change any logic/behaviour I would not put this issue on the 2.x roadmap.

[Andres Almiray]

thank you for having a look at this @sclassen 😁

[Ray Tsang]

I think everything is merged, closing for now :D thnx @sclassen!