Classes should not be declared inside `package-info.java` files.

> Typically package-info.java contains only a package declaration, preceded
> immediately by the annotations on the package. While the file could
> technically contain the source code for one or more classes with package
> access, it would be very bad form.

-- [JLS 7.4]

[JLS 7.4]: https://docs.oracle.com/javase/specs/jls/se8/html/jls-7.html#jls-7.4
