Suppose you're receiving bytes representing a User at the edge of your system. If you put json bytes into your parser and get back a User, then put your User through validation, that means you know there are both 'valid' Users and 'invalid' Users.

Instead, there should simply be no way to construct an invalid User. But this article pushes a little harder than that:

Does your business logic require a User to have exactly one last name, and one-or-more first names? Some people might go as far as having a private-constructor + static-factory-method create(..), which does the validation, e.g.

  class User {
    private List<String> names;
    private User(List<String> names) {..}
    public static User create(List<String> names) throws ValidationException {
       // Check for name rules here
    }
  }

Even though the create(..) method above validates the name rules, you're still left holding a plain old List-of-Strings deeper in the program when it comes time to use them. The name rules were validated and then thrown away! Now do you check them when you go to use them? Maybe?

If you encode your rules into your data-structure, it might look more like:

  class User {
      String lastName;
      NeList<String> firstNames;
      private User(List<String> names) throws ValidationException {..}
  }

If I were doing this for real, I'd probably have some Name rules too (as opposed to a raw String). E.g. only some non-empty collection of utf8 characters which were successfully case-folded or something.

Is this overkill? Do I wind up with too much code by being so pedantic? Well no! If I'm building valid types out of valid types, perhaps the overall validation logic just shrinks. The above class could be demoted to some kind of struct/record, e.g.

  record User(Name lastName, NeList<Name> firstNames);

Before I was validating Names inside User, but now I can validate Names inside Name, which seems like a win:

  class Name {
      private String value;
      private Name (String name) throws ValidationException {..}
  }