Check FunctionalInterfaces and implementations

[mkalb]

• Visibility of class variables (protected, package, private)
• Functional method implementations (final or not final)
• Getter methods for class variables (protected final)
• final classes
• default methods (more or less)
• JavaDoc (Tags like see, author, since, ...)
• Existing examples (stexfires-examples)
• JUnit Tests for all methods and constructors
• Generics: <? super T> and <? extends T>
• Class names consistent
• Parameter names consistent
• Method names consistent
• Many constructors or many static methods
• Null handling (Record, Category, RecordId, ...)
• Find usages and extensions (review, compare, ...)
• Synchronize or thread-safe necessary (Supplier, long, double, Collections, Arrays, ...)
• Method overload (generics, lambda friendly)