UML - hierarchy of relationships

Dependence - general relationship

Every relationship is a dependency.

Definition

The source element depends on the target element and may be affected by changes to it

UML syntax source to target

- - - >

PlantUml

@startuml
  title Dependence 
  class Order
  class Account
  Order ..> Account
@enduml

Code example

class Order {
}
class Account {
}

Asosjacja

Definition

The description of a set of links between objects

UML syntax source to target

---> or --- (bidirectional)

PlantUml

@startuml plantuml
  title Association
  class Order
  class Account
  Order --> Account
@enduml

Code example

class Order {
  void checkAccount(Account account); //not necessarily as parameter
}
class Account {
}

The Order class uses the Account class. Order needs access to account.
Therefore, changes made in Account can lead to errors in the Order class.
If the Order class knows nothing about the Account class, then changes in the Account class don't matter to it.

Aggregation

Definition

The target element is a part of the source element

UML syntax source to target

<>---> <>---

(empty diamond)

PlantUml

@startuml 
  title Aggregation
  class Order
  class Account
  Order O-- Account
@enduml

Code example

class Order {
	List<Account> accounts;
    void checkAccounts();
}
class Account {
    public String accountName();
}

The Order class contains objects of the Account class, and the Account class can exist independently.

Composition

Definition

A strong (more constrained) form of aggregation

UML syntax source to target

<+>--->

(filled diamond)

PlantUml

@startuml
    title Composition
    class Order
    class Account
	Order *-- Account
@enduml

Code example

class Order {
	List<Account> accounts;
    createAccount(Strig name);
}
class Account {
}

The Order class contains objects of the Account class, and the Account class can NOT exist independently because it relies on being part of the Order class.

Realization (implementation)

Definition

The source element guarantees to carry out the contract specified by the target element

UML syntax source to target

- - - |>
Dotted Line with Diamond Arrowhead

PlantUml

@startuml
  title Realization 
  class Writable
  class CsvWritable
  CsvWritable --|> Writable
@enduml

Code example

class Writable implements CsvWritable {}

The CsvWritable class implements the Writable interface.

Generalization (inheritance)

Definition

The source element is a specialization of the more general target element and may be substituted for it

UML syntax source to target

---|> arrow with full diamond

PlantUml

@startuml
    title Generalization 
    class Child
    class Parent
	Child --|> Parent
@enduml

Code example

class Child extends Parent {}

The Child class inherits from the Parent class.

Dependency stereotype

Use

<<use>>

The most common dependency stereotype is «use», which simply states that the client makes use of the supplier in some way. If you see just a dashed dependency arrow with no stereotype, then you can be pretty sure that «use» is intended.

PlantUML

@startuml
'https://plantuml.com/sequence-diagram

title use stereotype
class Client {
check(s:Supplier)
get():Supplier
doSomething()
}
class Supplier 

Client ..> Supplier :<<use>>
@enduml

Source

• UML 2 and the Unified Process Jim Arlow
• Core Java Volume 1 - Fundamentals (11th Edition)