Support extending validator

[jolangloys]

It would be nice to be able to extend an existing validator to apply it on the root of the current validated object, in order to use a validator created for a super type (or just to create a hierarchy of validator)

For example, let's say I have two classes like these :

public class Person {
    private String name;

    public static final validator = ValidatorBuilder.of(Person.class)
                            .constraint(Person::getName, "name", Constraint::NotNull)
                            .build();

    // Etc...
}


public class Employee extends Person {
    private String serviceId;

    public static final validator = ValidatorBuilder.of(Employee.class)
                            // Apply the validator Person.validator here
                            .constraint(Employee::getServiceId, "service", Constraint::NotNull)
                            .build();

    // Etc ...
}

In this case, the work around I have come out with is .nest(Person.class::cast, "person", Person.validator) but it's not super readable. and I end up with a prefix "person.Xxx" which doesn't really mean anything.
Something like .extends(Person.validator) would be perfect

[making]

@jolangloys Thanks for your feedback. That's what I didn't come up with.
Introduction extends might cause complex situations like multiple inheritance.

I'm thinking to add delegate or copyOf to support your case.

[making]

@jolangloys
I've added ValidatorBuilder.cast method so that you can share Person 's validator with Employee. This should be simpler than extending validator.

Please take a look at a test case.
3c56a4c -> 82c6990 (updated)

[making]

Sorry I'll revert 3c56a4c
This implementation mutates ValidatorBuilder ...

[making]

also added clone method.

See 82c6990

[jolangloys]

Thank you for your answer and work.

But the problem here is that we usually don't retain instance of ValidatorBuilder but we build them directly into Validator so the clone() and cast() methods won't help in this case.

I don't really think multiple inheritance of validator should be a problem. However, I can see the problem extracting internal attributes from the Validator class.

I can suggest you a compromise : create a method in the Validator class ValidatorBuilder<S extends T> extend(Class<T> clazz) (or whatever the name) in order to obtain a ValidatorBuilder on a sub class of an existing validator.

If I take my example from the beginning, I will then have :

public class Employee extends Person {
    private String serviceId;

    public static final validator = Person.validator.extend(Employee.class)
                            .constraint(Employee::getServiceId, "service", Constraint::NotNull)
                            .build();

    // Etc ...

[making]

@jolangloys I also considered to add mutate method to Validator. The usage looks like

Person.validator.mutate() // returns ValidatorBuilder
               .cast(Employee.class)
               .constraint(Employee::getServiceId, "service", Constraint::NotNull)
               .build();

pretty close to your suggestion.

The problem here is the cross reference between Validator <-> ValidatorBuilder I've ever resolved in #7 .

If you have any more idea that doesn't have the cross reference, suggestion or PR is welcome.

[jolangloys]

Hello,

It's up to you, but I don't think that having a class and its own builder tight coupled with circular dependency is a problem. If you change one, you'll always need to change the other. To be honest, I have been surprised to find them in different package.

However, If you want to keep the pure unidirectional relationship, my first suggestion will be better then. For me, You'll then have two way of accomplishing this:

• Extract the attributes of the Validator. For example use package private methods to and an intermediate class
• Add an attributes to Validator & ValidatorBuilder like includedValidator which will be another Validator or a Set of Validator which will be called at validation time.