Java - 2 Way 'Has A' Relationship

Question

I have just started a project to make my employer a management software. I have a niggling, but potentially simple, query that I can't seem to find any information on.

Is it prudent/good practice to have a 2 way 'has a' relationship between objects. So can, for example, a Client object 'have a' Site, and then the Site 'has a' Client, where the Client object is the Client that 'has' the Site?

public class Client {
    Site site;
}

public class Site {
    Client client;
}

Is there anything objectionable (no pun intended) to this, or is it fine? I am currently creating a mock-up UML for the project, and this has been bothering me.

Answer 1

Is there anything objectionable to this, or is it fine?

There's no definitive answer to this. The best answer is: it depends to the design of your application.

When to use it

If your Client object should navigate to the Site object and your Site object should navigate to the Client object, then the current example in your code is fine. Still, probably you will need some way to associate these elements, probably by an additional id field on one of the classes or in both.

If it happens that you work with a framework that helps you to bind the classes automatically like Hibernate, then maintaining the circular reference won't be a problem for you.

When not to use it

Basically, for text serialization, since it will generate an infinite loop. As already mentioned in Raibaz's answer, a library like Jackson will fall into infinite loop while serializing Client or Site class into a JSON string¹. Note that this is also valid when serializing to other String data like passing the objects through a JAX-WS web service in XML (more info: What happens to generic class in jax-ws webservice?).

¹ This can be solved using annotations (@Something) that belong to a specific library e.g. @JsonManagedReference and @JsonBackReference from Jackson library, as noted by @SimonAndréForsberg, but the downside of this solution is that your classes will have tight coupling with the library.

Answer 2

It is common, but I would consider how loosely coupled you want the child object to be from its parent. If you have the reference to the parent from the child object then you will not be able to reuse the object with another parent / no parent.