I'm confused with designing a client software with database integration to what should be a member variable of the class or just a query to the database. Let me be specific with a trivial example:

If I have, lets say, a Student class, which has a list of "friends" that are Student objects. Should my software design have an ArrayList<Student> as a member variable of the Student class or should the Database deal with the relationship itself and the Student class doesn't account for those "friends"? How should a proper UML be in this case?

CodePudding user response：

You need a one-to-many relationship between Student and friends in both the relational database and the object model.

CodePudding user response：

This question is broader than you may think, as there are many ways to deal with it. Here some first ideas:

In principle, you could very well load from the database a Student object. You'd create it based on data in a row in a STUDENTS table. Since the friendship between students is a many-to-many association, you'd have to read an association table, say STUDENT_FRIENDS, relevant pairs of ids of befriended students.

But how to use these tables in the application?

• A first possibility would be to load each Student friend and insert it in the ArrayList<Student>. But each loaded student is like the first student and could have oneself friends that you'd have to load as well! You'd end up loading a lots of students, if not all, just for getting the single one you're interested in.
• A second possibility would be use an ArrayList<StudentId> instead of an ArrayList<Student> and populate it. You'd then load the friends just in time, only when needed. But this would require some more important changes in your application.
• A third possibility is not to expose an ArrayList. Not leaking the internals is always a good idea. Instead use a getter. So you'd load the friends only if student.getFriends() is called. This is a convenient approach, as you'd have a collection of friends at your disposal, but avoid being caught in a recursive loading of friends of friends.

In all the cases, you may be interested in using a repository object to get individual or collections of students, and encapsulate the database handling.

Advice: as said, there are many more options, the repository is one approach but there are also active records, table gateways and other approaches. To get a full overview, you may be interested in Martin Fowler's book Patterns of Enterprise Application Architecture.