UML association understanding problem

Question

I've been using UML for a while and I've read few articles, books, forums about it but I still don't REALLY understand when two classes should be connected with association line (a simple line or arrow (or are these not the same?)). I'll provide three examples - can you tell me which one will cause the two classes to be in this relationship?

1.

//a field of OtherClass
    public class MainClass
    {
        private OtherClass other;
    }

2.

//method argument
    public class MainClass
    {
        public void Action(OtherClass other)
        { }
    }

3.

//method return value
    public class MainClass
    {
        public OtherClass Action()
        { }
    }

4.

//used inside a method
    public class MainClass
    {
        private Something something;

        public void Action()
        {
            OtherClass other = something.GetOtherClass();
        }
    }

Answer 1

First of all an arrow represents navigability of the association. Single arrow means unidirectional relation, in this case only the source class knows about the target class. Arrow on both ends means bidirectional relation, where both classes know about each other. If no arrows are present the association can be either bidirectional by default or suppressed for the sake of readability. In practice you should draw arrows only when you want to emphasize the direction of the association.

When it comes to your second question, only the first case describes an (unidirectional) association between MainClass and OtherClass. Neither arguments nor return values imply association in the UML sense (although both imply dependency). In the last example there is an association between MainClass and Something class through the something attribute. As a rule of thumb you should look for associations in attributes.

Please note that there is a concept of dependency in UML and it is represented by a dashed line.

Pozdrowienia!