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

Specifically, what is the relationship between PersistentSet and ThirdPartyPersistentSet?

PersistentSet has-many ThirdPartyPersistentSets

What is the relationship between PersistentObject and ThirdPartyPersistentSet?

ThirdPartyPersistentSet is dependent on (uses-a) PersistentObject

all of the lines in a uml class diagram indicate dependencies of one sort or another except for the dashed like to a comment (dog-eared box). a solid line with no arrows indicates a two (2) way (bi-directional) dependency.

Answer 2

Specifically, what is the relationship between PersistentSet and ThirdPartyPersistentSet?

The solid diamond <|>-----> is Composition ("has a") where the "parts" are destroyed when the "whole" is. In the image below, if you destroy a Car, you destroy the Carburetor.

The empty diamond < >-----> is Aggregation ("has a") where the "parts" might are not destroyed when the "whole" is. In the image below, if you destroy a pond, you don't necessarily destroy the ducks (they move to a different pond if they are smart).

_{(source: wikimedia.org)}

What is the relationship between PersistentObject and ThirdPartyPersistentSet?

This is a dependency relationship. See my answer here for further information.

So when does a dependency relationship change to an association relationship when using parameter passing?

If you store the parameter locally, then it changes from a dependency relationship, to an association relationship. If you only use the parameter locally, then it stays a dependency.

C# Code example:

// Association 
public class ThirdPartyPersistentSet
{
    private PersistentObject _object;
    public ThirdPartyPersistentSet(PersistentObject obj)
    {
        _object = obj; // Store it to a local variable.
        // Now ThirdPartyPersistentSet 'knows' about
        // the PersistentObject.
    }
}

// Dependency
public class ThirdPartyPersistentSet
{
    public ThirdPartyPersistentSet(PersistentObject obj)
    {
        obj.GetSomething(); // Do something with obj,
        // but do not store it to a local variable.
        // You only 'use' it and ThirdPartyPersistentSet
        // does not 'know' about it.
    }
}

Answer 3

The black diamond represents composition, containment (as in having a field of the type pointed by the arrow): PersistentSet entities contain ThirdPartyPersistentSet entities When PersistenSet is destroyed, all ThirdPartyPersistenSet objects contained will also be destroyed.

The dashed line represents dependency, as in makes a function call that has a parameter of the type pointed by the arrow): ThirdPartyPersistentSet depends on PersistentObject

Look at the Wikipedia entry for more details

Answer 4

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;
    }
    ...
}

Answer 5

Specifically, what is the relationship between PersistentSet and ThirdPartyPersistentSet?

http://en.wikipedia.org/wiki/Class_diagram#Aggregation

What is the relationship between PersistentObject and ThirdPartyPersistentSet?

http://en.wikipedia.org/wiki/Class_diagram#Dependency