Clean-up and refactoring for 2.0

[Michael1993]

Closes nothing. A very self-indulgent PR/branch.

EDIT: Closes #673. Maybe?

Proposed commit message:

Refactor and document code (#672)

This commit simplifies some code with Java 11, adds some missing
documentation and fixes some code smells as marked by IntelliJ.

PR: #672

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[Bukama]

Some small notes, most personal preferences. Most important ones most probably those about documentation update.

Can't say if changing the ! xx.isPresent() into isEmpty() results in the same output - so I trust the build here :D

[Bukama]

Please provide a commit message, so we can merge this