Modernize codebase with Java improvements

[gnodet]

This PR includes several improvements to modernize the codebase:

1. Replace IllegalArgumentException with ClassCastException for type cast failures
2. Replace IllegalArgumentException with NullPointerException for null checks
3. Use modern Java collections API (toList() instead of collect(Collectors.toList()))
4. Replace custom null checks with Objects.requireNonNull
5. Add @nonnull and @nullable annotations throughout the codebase

These changes make the code more maintainable, safer, and aligned with modern Java practices.