What are container classes?

So I have this container class called ShipStorage which store objects in an array. I have another class called FileManager which uses the object of ShipStorage. My professor said container classes should not be inputting or outputting(i think that means cannot use System.out.println). Will FileManager be considered a container class as well when it only uses the ShipStorage obj?

> My professor said container classes should not be inputting or outputting.

There is a general principle in OO design called Separation of Concerns. A class should do things that are its "concern" and not do things that are not its "concern".

(A dishwasher washes dishes. Heating microwave dinners is not its concern.)

In the design that you are implementing, the purpose of a container class is to contain objects. Inputting or outputting the objects is not its concern. That is (presumably) the concern of the FileManager class, or some other class.

This is a reasonable design decision for your teacher to make, and probably how I would design it too. Either way, this is what your teacher has said to do; i.e. the "requirements". Therefore, it is how your code should work.

> (i think that means cannot use System.out.println)

That is an example of the things that your container should not do ... in this project.

The Java programming language doesn't insist that applications are designed properly, but as you get more experience you will find that good design has many benefits, especially when you are dealing with complicated applications.