Introduce @Contract annotation

[marcphilipp]

• Introduce @Contract annotation
• Add @Contract annotations and simplify code

Resolves #4722.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[sbrannen]

contact or contract? PR description and commit messages diverge from the code.

@Contract as in nullability contracts.

I updated the PR's description, and I'm sure @marcphilipp will update the code/commits. 😉

[marcphilipp]

Thanks for spotting my typo!