Intro to Class Relationships

Class relationships define how classes interact with each other in an object-oriented system. Understanding these relationships is crucial for designing maintainable, flexible, and well-structured software.

Why Class Relationships Matter

Class relationships help you:

• Model real-world relationships accurately
• Design flexible systems that can evolve
• Communicate design clearly with diagrams
• Avoid common pitfalls in system design
• Write maintainable code with proper coupling

Types of Class Relationships

There are four main types of class relationships, ordered from weakest to strongest:

1. Dependency - “Uses temporarily” (weakest)
2. Association - “Uses-A” or “Knows-A”
3. Aggregation - “Has-A” (weak ownership)
4. Composition - “Has-A” (strong ownership)

Relationship Strength Comparison

Quick Decision Guide

Ask These Questions:

1. “Does Class A use Class B temporarily?”

   • Yes → Dependency
   • No → Continue

2. “Does Class A know about Class B?”

   • Yes → Association
   • No → Continue

3. “Does Class A contain Class B?”

   • Yes → Continue
   • No → Association

4. “Can Class B exist without Class A?”

   • Yes → Aggregation
   • No → Composition

Visual Summary

Key Concepts

Lifecycle Ownership

• Dependency: No ownership - temporary use
• Association: No ownership - independent lifecycles
• Aggregation: Weak ownership - parts can exist independently
• Composition: Strong ownership - parts cannot exist without whole

Multiplicity

Relationships can have different multiplicities:

• 1 - Exactly one
• 0..1 - Zero or one (optional)
• 1..* - One or more
• * - Many (zero or more)

Direction

Relationships can be:

• Unidirectional - One class knows about another
• Bidirectional - Both classes know about each other

Real-World Examples

Dependency Example

• Order uses Calculator temporarily to calculate total
• Calculator is passed as parameter, not stored

Association Example

• Teacher teaches Course
• Teacher knows about Course, Course can exist without Teacher

Aggregation Example

• University has Students
• Students can exist without University (can transfer)

Composition Example

• Car has Engine
• Engine cannot exist without Car (destroyed when Car is destroyed)

Next Steps

Now that you understand the basics, explore each relationship type in detail:

• Association - Classes that know about each other
• Aggregation - Weak “has-a” relationships
• Composition - Strong “has-a” relationships
• Dependency - Temporary use relationships
• All Together - Complete example showing all relationships

Remember

Understanding class relationships helps you:

• Choose the right relationship type for your design
• Create flexible, maintainable systems
• Communicate your design effectively
• Avoid over-coupling or under-coupling