I have never been happy with validation code I wrote, I found this blog post and wanted to explore the idea.
We want to guarantee our data is valid so downstream systems don't have to duplicate the validation step. To do this we need to guarantee type safety and avoid null values in our fields.
The implementation has 3 classes MetadataParser, UpdateField, and Metadata.
The magic comes from creating the class UpdateField that describes our "update" action. This allows us to only change values in the Metadata object when they are valid, otherwise we flag the update with an error, and it never occurs.