Understanding UML

Basic Class Diagrams

1. Class Diagram

Visibility Markers: Visibility markers indicate the accessibility of attributes and methods within a class.

• + (Public): The attribute or method is accessible from any class.
• - (Private): The attribute or method is only accessible within the same class.
• # (Protected): The attribute or method is accessible within the same class and its subclasses.
• ~ (Package): The attribute or method is accessible within the same package.

2. Attributes

visibility name: type [multiplicity] = defaultValue

3. Methods

visibility name(parameterList): returnType

4. Interfaces

5. Abstract Class

6. Enums

2. Relationships

1. Association ($uses-a$)

$-$ “uses-a” relationship between two classes where one class uses or interacts with the other.

A Student class is associated with a Course class, as a student can enroll in multiple courses

2. Aggregation ($has-a,weak$)

$-$ Aggregation represents a "has-a" relationship $-$ where one class (the whole) contains another class (the part), but the contained $classcanexistindependently$.

A Car class has an Engine class but the Engine class can exist without the Car class.

3. Composition( $has-a,strong$)

$-$ Composition represents a strong “has-a” relationship where $-$ the part cannot exist without the whole. $-$ If the whole is destroyed, the parts are also destroyed.

A House class is composed of Room class but the Room class can not exist without the House class.

4. Inheritance ($is-a$)

$-$ Inheritance (or Generalization) represents an $is-a$ relationship $-$ where one class (subclass) inherits the attributes and methods of another class (superclass).

5. Realization ( $Implementation$ )

$-$ Realization or implementation represents a relationship between a class and an interface, $-$ where the class implements the methods declared in the interface.

6. Dependency ($uses$)

Dependency represents a uses” relationship where a change in one class (the supplier) may affect the other class (the client).

A Customer class uses an Order class to place order.