Clarify model class fields interface regarding mandatory / optional / default values

[laurentg]

The model classes is our internal main reference regarding the specification of optional/mandatory fields. Currently we uses Optional<T> when a field is marked as optional in the specs, with another getter supplying by design a non-null value with a default. However Optional is better understood in language context, and since we are inside a validator, even mandatory value can be null.

We should clarify how to model the following:

• Mandatory values:
  • that cannot be null by design, such as a mandatory ID in DAO scope. Example: stop time trip ID.
  • that can be null for invalid input data:
    • without a sensible default to use by a validator. Example: stop name
    • with a sensible default to use by a validator. Example: route type, in speed checks we can fallback on a default type to get the max speed
• Optional values:
  • that have a default value provided by the specs. Example: timepoint, pickup/dropoff type, route colors...
  • that do not have a default value provided by the specs:
    • with a sensible default value that can be used by validators. Example: ?
    • without a sensible default value that can be used by validators. Here a null is valid. Example: route URL.
• Conditionally required, for example stop name (where boarding areas or generic nodes make it optional), parent_station...

Maybe creating dedicated holders such as Optional<T>, for example Required<T> etc...