Association

Association is a relationship where one class knows about another class and can use its services. It’s a “uses-a” or “knows-a” relationship where classes can exist independently.

What is Association?

Association represents a relationship where:

• Classes know about each other
• Classes can communicate with each other
• Classes have independent lifecycles
• Usually implemented with references or pointers

Key Characteristics

• Independent existence - Both classes can exist without each other
• Communication - Classes can call each other’s methods
• References - One class holds a reference to another
• Weaker than composition - No ownership implied

Unidirectional Association

One class knows about another, but not vice versa.

Bidirectional Association

Both classes know about each other.

Visual Representation

Real-World Example: Library System

Key Takeaways

Remember

• Association = “uses-a” or “knows-a” relationship
• Classes can exist independently
• Usually implemented with references
• Can be unidirectional or bidirectional
• Weaker than composition - no ownership implied
• Use when classes need to communicate but don’t own each other

When to Use Association

Use Association when:

• Classes need to communicate with each other
• Classes have independent lifecycles
• One class uses another’s services
• You need flexibility to change relationships
• Classes are loosely coupled

Examples:

• Teacher ↔ Course
• Student ↔ Course
• Order ↔ Product
• Loan ↔ Book
• Customer ↔ Order