UML Class Diagrams and Accessibility

Estimated time to read: 3 minutes

Overview

CRC cards help to prototype the relationships between objects and their responsibilities whilst UML Cards enable us to create Class Diagrams. From the a Class Diagram, it is easy to develop code.

UML Cards

UML Cards contain the following information:

Class Name

This is the name of the class. In Java, an object is an instance of a class.

Properties

Attributes within a class are known as 'fields' in Java.

// <variable name> : <variable type> 
isWet: boolean;
age: int;

Operations

Operations are the 'methods' that the class contains.

// <name> ( <parameter list > ) : <return type>
isOnSale(): boolean;

is OnSale( date: Date): boolean; // If date matches Date, then return true

UML Card Layout

Class Name

Attributes(Attributes that are unique to this class)(attrName: attrType)

Methods(Implementation of an operation)(methodName(parameterList): returnType)

Access Modifiers

Access modifiers change which classes are able to access attributes and behaviours. They also determine which attribute and behaviours a superclass will share with its subclasses.

Modifier	Sign	Definition
Public	+	A public attribute can be accessed from anywhere.
Private	-	A private attribute is only visible from within the class. It can not be accessed from outside the class.
Protected	#	In Java, a protected attribute or method can only be accessed by: - The encapsulating class itself - All subclasses - All classes within the same package
Package	~	A package is a collection of packages that exist in the same namespace
Derived	/	A derived attribute is produced or calculated from other properties. It is common to mark it as read-only.
Class / Static	underlined	The static modifier makes an attribute or method of a class independent. This means that the value does not change across instances of the class.

Access Accessibility Matrix

Modifier	Class	Package	Subclass	Other Classes
Private	Yes	No	No	No
No Modifier	Yes	Yes	No	No
Protected	Yes	Yes	Yes	No
Public	Yes	Yes	Yes	Yes

UML Relationships

Association: A solid line between two classes. Add arrows at either end or both ends to show that the classes are aware (or unaware) of each other.

Dependency: A dashed line with an open arrow. If the definition of class 2 changes, it will change class 1, but not the other way around (depends on).

Aggregation: An open diamond at class 1. A special type of association that shows that class 2 is a part of class 1.

Composition: A solid diamond at class 1. A special type of aggregation that shows that class 2 can not exist without class 1.

Inheritance (generalisation): An open triangle at the parent class. This shows that the subclasses are specialisations of the parent class - they extend the parent class.

Implementation (realisation or execution): A dashed line with an open triangle at the blueprint class. This is used where a class implements the functionality of a ‘blueprint’ class, and may be implemented differently depending on each class that implements it.

Sites

https://www.uml-diagrams.org