Concept · Core

UML Relationships

The arrows ARE the design. Dependency, association, aggregation, composition and inheritance say exactly how tightly two classes are bound — and who owns whose lifecycle.

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1/2The strength spectrum
usesdriveshas*owns1..*
Order
+ total()
TaxService
Driver
Car
Team
Player
House
Room
The strength spectrum
 1// ⇢ depend  — uses a parameter, stores nothing
 2class Order  { total(TaxService t){ ... } }
 3 
 4// →  assoc  — holds a long-lived reference
 5class Driver { Car car; }
 6 
 7// ◇ aggregate — has-a, shared, part outlives whole
 8class Team   { List<Player> roster; }
 9 
10// ◆ compose — owns, parts die with the whole
11class House  { House(){ rooms = new Room[n]; } }
State
⇢ dependuses transiently (a param)
✦

DEPENDENCY (⇢, the weakest). Order just USES TaxService — it’s a method parameter, nothing is stored on Order. When the call ends, the link is gone. Like taking a taxi: you use it, you don’t own it.