UML help C# Design Principles

Question

I have a problem understanding an UML below:

Specifically, what is the relationship between PersistentSet and ThirdPartyPersistentSet? What is the relationship between PersistentObject and ThirdPartyPersistentSet?

Please note that the UML is from Agile Principles, Patterns, and Practices in C# By Martin C. Robert, Martin Micah 2006. Chapter 10

Thanks in advance!

Answer 1

The relationship between PersistentSet and ThirdPartyPersistentSet is an Aggregation, which means the PersistentSet contains one or more ThridPartyPersistenSet instances. This is a "weak" relationship, meaning the instances of ThirdPartyPersistentSet can exist outside of the PersistentSet.

The relationship between PersistentObject and ThirdPartyPersistentSet is a Dependency, meaning basically ThirdPartyPersistentSet needs a PersistentObject to do it's work.

So, to translate this to code, your PersistentSet would contain something like this:

public class PersistentSet
{
    public List<ThirdPartyPersistentSet> Items { get; }
    ...
}

And your ThirdPartyPersistentSet would look something like this:

public class ThirdPartyPersistentSet
{
    private PersistentObject _object;
    public ThirdPartyPersistentSet(PersistentObject obj)
    {
        _object = obj;
    }
    ...
}