0.3.18 Contrasting UML Object, Activity, and Class Diagrams

Three UML Diagrams Answer Three Different Questions

You have now worked with three UML diagram types:

• object diagrams;
• activity diagrams;
• class diagrams.

All three can describe the same soccer system.

They do different jobs.

A useful shortcut is:

Object diagram: What specific things exist right now?

Activity diagram: What happens next?

Class diagram: What kinds of things can the system represent?

Object Diagrams Show Specific Instances

An object diagram focuses on a snapshot.

For example:

player1 : Player
-------------------------
name = "Jordan"
jerseyNumber = 7
isAvailable = true

This is one specific Player object.

The diagram may also show relationships to specific objects such as:

team1 : Team
match1 : Match

Object diagrams are useful when the important information is:

• object identity;
• current field values;
• relationships among specific instances.

Activity Diagrams Show Flow

An activity diagram focuses on actions and paths.

For example:

Start
  ↓
Read availability
  ↓
Is player available?
  ├─ [yes] → Add player to list
  └─ [no]  → Continue
  ↓
End

This diagram is not primarily about the internal field list for Player.

It is about the process.

Activity diagrams are useful when the important information is:

• sequence;
• decisions;
• alternate paths;
• where a process begins and ends;
• how information moves through actions.

Class Diagrams Show Types and Structure

A class diagram describes kinds of objects.

For example:

Player
--------------------------------
name : string
jerseyNumber : int
isAvailable : bool

This does not describe one specific player.

It says what information Player objects can contain.

Class diagrams are useful when the important information is:

• classes;
• attributes;
• attribute types;
• relationships among classes.

Compare the Same Soccer Idea

Suppose the system needs to represent player availability.

Object-diagram view

player1 : Player
-------------------------
isAvailable = true

Question answered:

What is true about this specific object right now?

Class-diagram view

Player
-------------------------
isAvailable : bool

Question answered:

What kind of information can Player objects contain?

Activity-diagram view

Is player available?
  ├─ [yes] → Add to lineup
  └─ [no]  → Continue

Question answered:

How does availability affect the process?

The concept is related across all three diagrams.

The representation changes because the question changes.

Do Not Force One Diagram to Do Another Diagram's Job

An object diagram should not become a substitute for process flow.

A class diagram should not be filled with one particular player's current values.

An activity diagram should not become a list of every attribute in the system.

Using the wrong representation makes the model harder to interpret.

Choose the diagram based on the problem you are trying to understand.

The Diagrams Can Support One Another

A class diagram can define:

Player
-------------------------
isAvailable : bool

An object diagram can show:

player1 : Player
-------------------------
isAvailable = true

An activity diagram can use that state in a decision:

Is player available?
  ├─ [yes] → Include player
  └─ [no]  → Exclude player

Those models are not duplicates.

They are complementary views.

UML and C# Will Also Connect

The same class attribute:

isAvailable : bool

can correspond to a Boolean value in C#.

The object diagram shows the current value.

The activity diagram shows how the value affects flow.

C# provides executable instructions that create the object, store the value, and make the decision.

That creates a useful chain:

Class structure → Object state → Process flow → Program code

Choose by the Question

Use this table as a mental guide:

The strongest diagram is not the one with the most information.

It is the one that answers the current question clearly.