The 6 UML Class Diagram Relationships

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The six relationships in UML class diagrams are crucial for understanding software design, but their similar terms can be confusing at first. Grouping similar concepts together and summarizing them in a table helps you grasp and memorize them at a glance.

UML Class Diagram Relationships Summary Table

There are 6 relationships in total, consisting of 2 strong/weak has-a relationships (Composition/Aggregation), 2 strong/weak uses-a relationships (Association/Dependency), and 2 strong/weak is-a/can-do relationships (Generalization/Realization).

| Relationship | Core Concept | Diagram | Keywords | Simple Example | | :----------------- | :---------------------- | :------------------ | :--------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------- | ----------- | | Composition | Strong has-a | Filled Diamond | Shared Lifecycle, Essential Part | House and Room (If the house is destroyed, the room is also destroyed.) | | Aggregation | Weak has-a | Empty Diamond | Independent Lifecycle, Optional Part | Department and Student (Even if the department is closed, the student still exists.) | | Association | Strong uses-a | Solid Arrow | Member Variable, Lasting Relationship | Soccer Team and Striker (The soccer team always uses the striker.) | | Dependency | Weak uses-a | Dashed Arrow | Parameter, Brief Use | Developer and Book (The developer uses the book briefly.) | | Generalization | is-a (Inheritance) | Solid Hollow Arrow | Inheritance, Parent-Child | Cat is an Animal. (Animal is more general than a cat. Divided into superclass (Animal) and subclass (Cat), Animal < | -- Cat) | | Realization | can-do (Implementation) | Dashed Hollow Arrow | Interface, Fulfilling a contract | Robot can clean. (Grouped by the 'capability' to clean. 'Cleanable' < | .. 'Robot') |

Class Diagrams for Each Relationship

Composition

• Keywords: Shared Lifecycle, Essential Part
• Example: House and Room (If the house is destroyed, the room is also destroyed.)

Aggregation

• Keywords: Independent Lifecycle, Optional Part
• Example: Department and Student (Even if the department is closed, the student still exists.)

Association

• Keywords: Member Variable, Lasting Relationship
• Example: Soccer Team and Striker (The soccer team always uses the striker.)

Dependency

• Keywords: Method Parameter, Brief Use
• Example: Developer and Book (The developer uses the book briefly.)

Generalization

• Keywords: Inheritance, Parent-Child
• Example: A Cat is an Animal. (Animal is more general than a cat. Divided into superclass (Animal) and subclass (Cat), Animal <|-- Cat)

Realization

• Keywords: Interface, Fulfilling a contract
• Example: A Robot can clean. (Grouped by the 'capability' to clean. 'Cleanable' <|.. 'Robot')

Exam Preparation Problems

Review Problems

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