Should I create seperate interfaces as a way to centralize annotation?

Question

Assume I have these classes:

@Entity
@Data
class Student{
    @Size(min = 8)
    public String name;
    public int age;
    ...
}

@Data
class StudentDTO{
    @Size(min = 8)
    public String name;
    ....
}

To reduce duplicate annotations, I've figured out a simple trick like this:

interface HasName {
    @Size(min=8)
    String getName();
}
@Entity
@Data
class Student implements HasName {
    public String name;
    public int age;
}
@Data
class StudentDTO implements HasName{
    public String name;
}

There could be more fields.

Pro:

• No duplicate annotation
• Annotation will be unified

Cons:

• Increase complexity
• Increase dependency

But it look like a code smell to me, so is it good or bad?

Answer 1

Your strategy will work, but I can't see as a good solution. Your classes would implement so many interfaces that the objective to have a Interface in your class will be lost if you mix with more meaningful interfaces: the ones that really will acts like contracts in your code.

The classic solution for this kind of annotation problem is create a new annotation that aggregates other annotations, like:

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Size(min = 8)
public @interface Name {

}

And to use:

@Name
private String name;

The @Name annotation will continue to be replicated, but you can change the min size (per example) and add another annotations to the name (like @NotNull) on one single place.

Generally, I don't like to use annotations to do the validation for my classes. So, if you already trying to unify the validation without spreading the annotation , try to do the validation on the constructor without any annotation, something more OOP, like:

private Name name;

And Name:

class Name {
    Name(String name) {
        if (name.size() < 8) {
            //throw some exception
        }
    }
}