Author: Tianle Yuan

UML & Object relationships⚓︎

Let's see how to express OOP by using UML class diagram. With UML we then import the relationship between classes.

UML
Object relationships

UML⚓︎

Unified Modeling Language (UML) is a type of static structure diagram used to show the structure of a system, including: - Classes - Attributes (Properties) - Methods (Operations) - Relationships (between objects)

1.1 Classes⚓︎

How to express class?

A class in UML should encapsulate the class name, state (attributes), and behavior (methods).

C++

class BankAccount{
    public:
        int m_ownerID;
        float m_balance;
    public:
        int Deposit(int amount){...};
        int Withdrawal(int ID){...};
}

Let's say we have a class as above; then we can show the diagram below (w/o signature):

Class with signatureClass without signature

classDiagram
    class BankAccount{
        +m_ownerID : int
        +m_balance : float
        +Deposit(amount : int) int
        +Withdrawal(amount : int) int
    }

classDiagram
    class BankAccount
    BankAccount : +m_ownerID
    BankAccount : +m_balance
    BankAccount : +Deposit()
    BankAccount : +Withdrawal()

1.2 Visibility⚓︎

How to set visibility of attribute and method?

We can change the symbol before the attributes or methods as below:

+ denotes public attributes or method
- denotes private attributes or method
# denotes protected attributes or method

C++

class BankAccount{
    private:
        int m_ownerID;
    protect:
        float m_balance;
    public:
        int m_age;

}

Let's say we have a class as above; then we can show the diagram below (w/o signature):

classDiagram
    class BankAccount{
        -m_ownerID : int
        #m_balance : float
        +m_age : int
    }

1.3 Multiplicity on relations⚓︎

How to express multiplicity between classes?

Multiplicity or cardinality in class diagrams indicates the number of instances of one class that can be linked to an instance of the other class.

The different cardinality options are :

1 Only 1
0..1 Zero or One
1..* One or more
* Many
n n
0..n zero to n
1..n one to n

classDiagram
    direction TB
    Customer "1" --> "*" Ticket
    Student "1" --> "1..*" Course
    Galaxy --> "many" Star : Contains

1.4 Annotations on classes⚓︎

How to add annotations on classes?

Some common annotations include:

<<Interface>> To represent an Interface class
<<Abstract>> To represent an abstract class
<<Service>> To represent a service class
<<Enumeration>> To represent an enum

classDiagram
    direction TB
    class A{
        <<interface>>
    }
    class B{
        <<Abstract>>
    }

    class C{
        <<Service>>
    }

    class D{
        <<Enumeration>>
    }

Object relationships⚓︎

Now we have the UML diagram as a tool for visualizing relationships. Let's look at the relationships between objects.

How to express relationships between classes?

Below are all the relationships used among classes.

DependencyAssociationAggregationCompositionRealizationInheritance

Intuition: The behavior of one object depends on another object.
Iff: class A depends on class B (The edition of class B will affect class A).
E.g: A car (A) needs to have tires, an engine, and oil (B) to work correctly.

classDiagram
    direction LR
    classA ..> classB : Dependency

Intuition: The relationship among multiple objects.
Iff: class A depends on class B
- object A can visit info in object B
E.g: An order form (A) associated with one customer object and multiple product objects (B).

classDiagram
    direction LR
    classA --> classB: Association

Intuition: The relationship between the whole and the parts. The parts can survive without the whole.
Iff: class A depends on class B
- object A can visit info in object B
- object A is constructed by object B
- object A do not controls the life-circle of object B(object B can survive without object A)
E.g: An order form (A) associated with one customer object and multiple product objects (B).

classDiagram
    direction LR
    classA --o classB : Aggregation

Intuition: The relationship between the whole and the parts. The parts cannot survive without the whole.
Iff: class A depends on class B
- object A can visit info in object B
- object A is constructed by object B
- object A controls the life-circle of object B (object B can not live without object A)
E.g: A car (A) needs to have tires, an engine, and oil (B) to work correctly.

classDiagram
    direction LR
    classA --* classB : Composition

Intuition: The relationship between the interface and class implimentation.
Iff: class A depends on class B
- object A can be seen as object B
- method A (Methods defined in class A) is declared by interface B
E.g: An interface (B) defines a set of methods that an implementation class implements (A).

classDiagram
    direction LR
    classA ..|> classB : Realization

Intuition: The relationship between the interface and class implimentation.
Iff: class A depends on class B
- object A can be seen as object B
- class A inherits interfaces and implementations in class B and can extend them.
E.g: An animal (B) as a parent, dogs and cats as a subclass (A) inherit some of the characteristics and behaviors of animals, while having their own unique characteristics and behaviors

classDiagram
    direction LR
    classA --|> classB : Inheritance

Reference⚓︎

-UML Class Diagram Tutorial