UML Class Diagrams

• The most basic UML class diagram consists of one class.
• In each class the following are specified:
  • The class name
  • Attributes (a.k.a., fields or characteristics) of the class or instance from the class.
  • Behaviors (a.k.a., methods or member functions) of the class or instance from the class.

Associations

• Often there are relationships between classes and/or objects.
• One very general relationship is known as an association which links two objects.

• Here we denote the relationship between a Student instance and a City instance (representing the student's hometown).
• The role name (found next to the arrow) corresponds to the attribute name, hometown (in the Student class).
• Since the hometown attribute in the Student class can hold a reference to a City object, it provides a way to navigate from a student to his/her home town.
• Here the arrow head on the association indicates a one-way navigation from student to hometown.
• The diagram does not denote a way to navigate from a city to all the students who consider it their home town.
• Some associations may involve objects from the same class.
• For example:

• Here the buddy attribute in one Student instance can hold a reference to another Student instance.

Aggregation

• Some associations imply a "whole-part" relationship.
• Consider the relationship between Section objects and Student objects:

• The unfilled (open) diamond on the Section side of the association line denotes that the students belong to (or make up) the section object.
• This kind of association is called an aggregation.
• The 0..* on the Student side indicates that there can be zero or more students in a section.
• This is called the multiplicity of the participation of the Student object(s) in this aggregation.
• Aggregation is made up of objects that can be shared or exchanged (they are not "owned" by the aggregating object).

Composition

• Some "whole-part" relationships represent an even stronger link.

• The filled (closed) diamond on the Course side of the association line denotes that the sections belong exclusively to a course and cannot exist independently.
• This kind of association is called a composition.

Dependency

• Often times one class may make use of another class.
• This is known as a dependency since one class is dependent upon another class.
• We represent such a dependency with a dashed line with an arrow. The arrow points to the class that is depended upon.
• For example, if a BankAccount class makes use of methods from the Math class in order to calculate interest, we could represent that as follows:

Inheritance

• The inheritance relationship is shown in UML class diagrams using an open arrow from the subclass to the superclass.
• The open arrow signifies that the superclass is a generalization of the subclass.
• Here is the UML class diagram for the Shape and Circle classes described on the Inheritance page.
• In addition, a Rectangle class is shown.
• The two fields of the Rectangle class are declared as protected which is signified with the # symbol.

Abstract Classes/Methods

• The name of an abstract class is italicized.
• In addition, abstract methods are italicized.
• For example, draw, erase, and zoom are abstract methods in the abstract Shape class below.

Interfaces

• The name of an interface is italicized and «interface» is placed above the interface name.
• A dashed line going from a class to an interface, terminating with an open arrow signifies that the class implements the interface.
• For example, the following diagram indicates that the LoginScreen class implements the Serializable interface.

Example Diagram

• The name of an abstract class appears in italics. See DesktopItem.
• The name of an interface appears in italics and is preceded by «interface». See Group.
• The name of an abstract method appears in italics. See DesktopItem.erase().
• The name of a class (static) attribute is underlined. See MyApp.main(String[]).
• The inheritance relationship is shown using a line with an open arrow pointing from the subclass to the superclass.
• A class implementing an interface is shown using a dotted line with an open arrow pointing from the class to the interface it implements.
• A class may indicate that it makes use of another class/interface using a dotted line with an arrow pointing to the class that is used.
• A class indicates that it is contains object(s) from another class as a field using a line. (See line from Folder to DesktopItem)
  • At the end of the line next to the class that contains the field should be a solid diamond.
  • At the end of the line next to the class that is contained, a number, or range of numbers, indicates how many of the objects are contained. In this example, the Folder can contain zero or more DesktopItems.