Class Diagram

Class Name

• Example: Bike
• If abstract class use italics: Vehicle
• If interface, mentioned at the top: <<interface>>

Fields

• public (+)
• private (-)
• protected (#)
• Package Local (~)

Relationships

• Association A ━━━━━━ B
  • A stores B as a field and vice versa
• Aggregation: Whole ◇━━━━━━ Part
  • Logical grouping, Parts have independent lifecycle
  • It is confused with Association
• Composition: Owner ◆━━━━━━ Part
  • Part cannot exist independently
  • Lifecycle is controlled by owner
• Inheritance: Child ━━━extends━━━▷ Parent
• Realization: Cat -------implements------▷ Animal
• Dependency: A --------⇒ B
  • A uses B temporarily
  • Generally not shown in the diagrams

Relationship Strength

• Dependency < Association < Aggregation < Composition

Practical Uses

• Interface (Realization): -------implements------▷
• Inheritance: ━━━extends━━━▷
• Aggregation (Logical grouping, no ownership) ?????
• Composition (Strong ownership of lifecycle): ◆━━━━━━
• Association (if none of the above matches): ━━━━━━

Association

• solid single line: A ━━━━━━ B
  • Bidirectional: (without arrow A ━━━━━━ B)
    • A can call B and B can call A
    • Bus and BusStop
  • Unidirectional: (with arrow A ━━━━━━> B)
    • A can call B but B cannot call A
    • User and Desktop
• generally an attribute of the dependent class is an instance of the independent class.
• Objects know each other, but lifecycles are independent
• Bidirectional: Bus and BusStop

class Bus {
    private List<BusStop> busStops;
    
    void addStop(BusStop busStop) {
        busStops.add(busStop)
    }
}
 
class BusStop {
    private List<Bus> buses;
    
    void addBus(Bus bus) {
        buses.add(bus);
    }
}

• Unidirectional: User and Desktop

class User {
    private Desktop desktop;
}

HAS-A relationship

• Examples:
  • Vehicle has a Tyre
• Types:
  • Aggregation
  • Composition

Aggregation

• empty diamond: A ◇━━━━━━ B (unfilled diamond + solid line)
• weak ownership/grouping (empty/weak diamond)
  • B can exist without A
• It is special type of association and confusing
• It does not have defined semantics, hence need to understand the context
• https://stackoverflow.com/questions/9640885/uml-aggregation-vs-association
• Example:
  • Module ◇━━━━━━ Components
  • If Module does not exist Component can still exist
  • Components can be reused in other Modules

Composition

• aka “Part of” relationship
• filled diamond: A ◆━━━━━━ B (filled diamond + solid line)
• strong ownership (full/strong diamond)
  • B cannot exist without A
• A owns B and controls its lifecycle
• It is special type of Aggregation
• Example:
  • HttpRequest ◆━━━━━━ Header
  • If HttpRequest instance is destroyed, Headers are also destroyed

Inheritance

• aka “IS-A” relationship
• denotes generalization/specialization relationship
• “extends” in java
• A ━━━━━━▷ B (sold line + triangle)
• A is subclass of B and B is superclass of A

Realization

• aka interface
• “implements” in java
• A -------------▷ B (dashed line + triangle)
• mention <<interface>> at the top of the table name
• A implements B, B is interface or abstract class

Dependency

• A --------⇒ B (dashed line with arrow)
• A depends on B, if there are changes in B, A will be affected
• Dependency is weaker than association
• generally the independent class is used in the dependent class as a
  • parameter variable of method
  • local variable inside the method
• It is not stored as a field, hence the class is temporarily used but any change in contract may cause problems
• https://stackoverflow.com/questions/1152335/does-an-association-imply-a-dependency-in-uml
  • I feel the confusing thing is that the term “dependency” has different meaning in general software engineering context compared to UML context

//@assoc  The Player(A) has? some Dice(B)
class Player {
    Dice myDice;
}
 
//@dep    The Player(C) uses some Dice(D) when playing a game
class Player {
    rollYahtzee(Dice someDice);
}

Multiplicity

• How many instances of each class
• Exact: a
• Range: a…b
• Anything: Asterisk (*) (0 to anything)
• Examples:
  • one-to-one: A ━(1)━━━━━━(1)━ B
  • one-to-ranged: A ━(1…3)━━━━━━━(1)━ B
  • one-to-many: A ━(1)━━━━━━━(*)━ B
  • one-to-many with minimum 1: A ━(1)━━━━━━━(1…*)━ B

Role

• What is the role/relationship in association
• Example:
  • Guest ━━eats━━ Food
  • Cook ━━cooks━━ Food
• Generally uses is used:
  • A ━━uses━━> B

Examples

Bank Account

• Notice: generally (but not necessarily) we have List<> to show aggregation or composition
• If it is not “HAS-A” then probably it has association (“USES” ??) relationship

Java Collections

• Notice: inheritance (extends) vs implementation (implements)