cleanup the code

[xzel23]

I will collect several PRs here since there are many places where the code can be simplified and putting all in a single PR will be hard to review.

1. resource handling: use try-with resources to reduce complexity and handle corner cases correctly; don't re-implement JDK functionality
2. remove unnecessary explicit boxing and unboxing
3. simplify operations on collections, for example use Map.putIfAbsent()/computeIfAbsent() and others
4. collapse Exceptions with the same catch clauses
5. replace StringBuffer with StringBuilder

Please keep this one open for the time being, I will provide iterative PRs.

[mistmist]

thanks Axel for your many efforts!
the PRs are appreciated, but i think a github issue for generic code cleanups isn't required... just put a short description of the cleanup in the commit message, file as many PRs as you like... it's always "open season" for self-contained code cleanups, and issues should be filed for large-scale refactorings that need discussion, and bugs.

[svanteschubert]

@xzel23
Thanks for the patches, it reminded me that this project started before JDK 1.5 / 5! :-)
If this issue can be closed, please do so or mention it in a commit, so it will be closed automatically by GitHub.

If you have a PR which is unfinished/ongoing, just set the draft status, we do the same.. ;-)

[xzel23]

@svanteschubert, @mistmist If it's ok, I'll keep this open as a todo-list for the time being.

[xzel23]

There are still some places left in the code, but I will close this now.