Use ValueObjects Not Entity.
In the registration case, a UserName value object could be introduced. Create a Username object when receiving the registration. Implement validation in the constructor of the UserName.
See this question and this presentation for more detail.
Edit1:
1.How to handle cases where different validation rules applied for different context.
For example: The username must not have numbers for certain type of members, but it is required for other types of members?
Maybe different factory methods could do that. like UserName.forGoldenCardMember(...) or UserName.forPlainMember(...). Or make MemberType （a hierachy maybe） to validate UserName.
Another alternative solution is use AggregateFactory（AccountFactory in this case）.
2.Is constructor the only place to put the validation code? I did read online about two points of view: an object must always be valid vs. not always. Both present good arguments, but any other approach?
I prefer valid approach personally. Passing an maybe invalid value object harms encapsulabilty.
Edit2:
Require
a) validation business rule based on context(different username rules for member types)
b) keep validating all business rules even if one of them fail
Stick with Single responsibility principle by using Value Object(MemberType this case).
AggregateFactory could be introduced to ease the application layer(coarser granularity).
class AccoutFactory {
Account registerWith(Username username, MemberType type, ....) {
List<String> errors = new ArrayList<String>();
errors.addAll(type.listErrorsWith(username));
errors.add(//other error report...
if (CollectionUtils.isEmpty(errors)) {
return new Account(username,....);
} else {
throw new CannotRegisterAccountException(errors);
}
}
}
Edit3:
For questions in the comments
a) Shouldn't the Username object be the one that has a method that returns the error like
the listErrorsWith()? After all, it is the username that has different rules for different member type?
We could check this question from another perspective: MemberTypes have different rules for username. This may replace if/else block in the Username.listErrosWith(String, MemeberType) with polymorphism;
b) If we have the method in the MemberType, the knowledge will not be encapsulated in the Username.Also, we are talking about making sure Username is always valid.
We could define the validity of Username without MemberType rules. Let’s say "hippoom@stackoverflow.com" is a valid username, it is a good candidate for GoldenCard member but not good for SilverCard member.
c) I still can't see how performing validation that returns a list of errors without getting the list from exception thrown by the constructor or static method. Both does not look ideal IMHO.
Yes, the signature of listErrorsWith():List looks weired, I'd rather use validate(username) with no returning value(throw exception when fails). But this will force the cilent to catch every validation step to run validations all at once.
