UML Class Diagram Tutorial

A class diagram is a static diagram that describes the structure of a system by showing its classes with their attributes and operations and the relationships among objects. This tutorial explains how you can draw UML class diagrams and what usage they have.

Class Diagram in UML

A class diagram is one of 7 structure diagrams in UML notation. It depicts a static structure of a modeled system, module, or its part. The basic building block of a class diagram is a class - a unit that represents a set of objects with the same attributes and logic. The diagram can also represent a data model of a specific domain.
Class Diagram in UML
Class Diagram in UML

Class Diagram Purpose

Class diagrams may be used in various scenarios primarily in software development, but also other areas for:

  • Static structure design and analysis
  • System responsibilities modeling
  • Software reverse engineering
  • Source code generation and scaffolding

UML Class Diagram Symbols

Software Ideas Modeler offers tools for following elements of a class diagram:

  • Package
  • Class
  • Interface
  • Enumeration
  • Structure
  • Table
  • Signal
  • Collaboration
  • Usage
  • Association
  • Unidirectional Association
  • Bidirectional Association
  • N-ary Association
  • Composition
  • Aggregation
  • Generalization
  • Realization
  • Dependency
  • Containment
  • Interface
UML Class Diagram Cheat Sheet (UML Class Diagram)
UML Class Diagram Cheat Sheet (Class Diagram Element Overview)

Class

A class is an extensible template for creating objects, it is a common form for all instances of the same type (e.g. concept, thing, person). A class that cannot have any instances is called an abstract class.

Each class can contain:

  • strong>Attributes
  • strong>Operations
  • strong>Template parameters
  • tereotypes
  • agged values
  • ocumentation

Visibility

Visibility determines the scope, in which can be attributes and operations seen and manipulated by other objects. UML defines these symbols for the visibility of members (attributes or operations)

Symbol Visibility Meaning
+ Public Defines a member that is accessible from the whole project.
~ Package Defines a member that is accessible from the whole package.
# Protected Defines a member that is accessible from the class and its subclasses.
- Private Defines a member that is accessible only from the class, where it is specified.

Attribute

An attribute is a named data field within a classifier, which determines allowed data using its type, multiplicity, and constraints. Each class can have zero or more attributes. Attributes are defined within an attribute compartment (placed after name compartment) of a classifier.

Attribute format: <<stereotypes>> visibility_symbol name : type [multiplicity ordering] = default_value {constraint}

Abstract attributes are indicated by italic.

Static attributes are indicated by underline.

Software Ideas Modeler offers several ways how to add, edit, and delete attributes of a class:

  • using menu Element / Add / Attribute
  • using context bar button Add Attribute
  • using Properties dialog
  • using Fast editor

Each attribute is defined by:

  • Name
  • ID
  • Type
  • Visibility
  • Default Value
  • Multiplicity
  • Ordering
  • Constraint
  • Modifiers
  • Stereotypes
  • Documentation
  • Tagged Values

Operation

Operation is a method, function, or query, which can be called by other objects. Each operation has its name and it can have any number of parameters (zero or more). Each operation should perform only one thing. Operations are defined within an operation compartment (placed after the attribute compartment) of a classifier.

Operation format: <<stereotypes>> visibility_symbol name (parameter1 ... parameterN) : return_type

Abstract operations are indicated by italic.

Static operations are indicated by underline.

Software Ideas Modeler offers several ways how to add, edit, and delete operations of a class:

  • using menu Element / Add / Operation
  • using context bar button Add Operation
  • using Properties dialog

Each operation is defined by:

  • Name
  • ID
  • Return type
  • Visibility
  • Modifiers
  • Stereotypes
  • Parameters
  • Body
  • Documentation
  • Tagged Values

Operation Parameter

Operation Parameter is usually an input argument, although it may be also an output or input-output argument.

Operation format: direction name : type [multiplicity]

Each operation parameter can be defined using:

  • Name
  • Type
  • Direction - it can be input (in), output (out), input-output (inout) or return
  • Default value
  • Multiplicity
  • Documentation
  • Tagged values

Association

What is Aggregation?

An aggregation is a kind of the "has-a" association relationship. It is an association that represents a part-whole or part-of relationship. Diamond arrow points from a part to a whole.

Aggregation Example

What is Composition?

A composition is a stronger kind of the "has-a" association relationship. It is similar to the aggregation, but it is more specific. The existence of a part (connected with composition) is dependent on the whole.

Composition Example

Generalization

A generalization specifies relationship between a superclass and its subclass. Arrow points from a subclass to a super class. A superclass represents a base for subclasses.

GeneralizationExample

Realization

A realization specifies a relationship between an interface and a class, which implements this interface. Arrow points from a class to an interface.

RealizationExample

Dependency

A dependency specifies a relationship between two dependent classes. The arrow points to the class that is used by the second class.

Dependency Example

Containment

A containment specifies a relationship between a parent class and a nested inner class. A containment relationship is depicted with a circle with a plus on one end. The circle end is attached to the container and the unmarked end of the connector is attached to the contained class. Containment is used to depict nested or inner classes - the classes defined within another class.

Containment Example

Package

A package is a general-purpose container element for grouping of elements, which represent a logical, semantical, or another unit. A package can be contained in other packages. Each package defines its namespace.

Interface

An interface is an abstract element that defines operations that must be implemented in a classifier. A classifier is connected to implemented interfaces using realization relationships.

Enumeration

An enumeration is a user-defined type, which consists of enumerated literals/named values. An enumeration element represents a type with enumerated literal values that may be used for attributes, operations and operation parameters.

N-ary Association

An n-ary association is depicted as a diamond in the class diagram. It connects more than two classes in a single association.

How to Draw a Class Diagram?

You can create a new class diagram in multiple ways:

  • Click on a Plus button in the tab switch bar, navigate to UML group and click on the UML Class Diagram item
  • Press CTRL+SHIFT+D, choose Class Diagram from the UML group, enter the name and click on the OK button.
  • Switch to Project tab in the ribbon and click on the Class Diagram item in the Content gallery.
  • Open Project sidebar, right-click on a project (or a folder) node, and in the Add Diagram submenu, choose UML Class Diagram.
Create a new UML class diagram
Create a new UML class diagram

How to Draw Class Diagram Elements?

How to Draw a Class?

  • Click on the Class button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new class by dragging on the diagram canvas.
  • If you want to add a class with the default size, just click on a diagram canvas and the class will be inserted to the specified position. You can also drag the Class button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple classes in a row, double click (or click twice) on the Class button in the toolbox and draw so many classes as you want in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

After creating a class in the diagram, you can add these fields to it:

  • attributes - click on the button Add Attribute in the context bar.
  • operations - click on the button Add Operation in the context bar.
  • rules (or constraints) - click on the button Add Rule in the context bar.
  • constructors - click on the button Add Constructor in the context bar.

How to Draw a Package?

  • Click on the Package button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new package by dragging on the diagram canvas. If the bounds cover other elements in the diagram, they will be added to the package as nested elements.
  • If you want to add a package with the default size, just click on a diagram canvas and the package will be inserted to the specified position. You can also drag the Package button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple packages in a row, double click (or click twice) on the Package button in the toolbox and draw as many packages as you want in the diagram editor.

How to Draw an Interface?

  • Click on the Interface button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new interface by dragging on the diagram canvas.
  • If you want to add an interface with the default size, just click on a diagram canvas and the interface will be inserted to the specified position. You can also drag the Interface button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple interfaces in a row, double click (or click twice) on the Interface button in the toolbox and draw desired number of interfaces in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

After creating an interface, you can add attributes, operations and rules to it using the context bar buttons.

How to Draw an Enumeration?

  • Click on the Enumeration button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new enumeration by dragging on the diagram canvas.
  • If you want to add an enumeration with the default size, just click on a diagram canvas and the enumeration will be inserted to the specified position. You can also drag the Enumeration button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple enumerations in a row, double click (or click twice) on the Enumeration button in the toolbox and draw desired number of enumerations in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

After creating an enumeration in the diagram, you can add enumeration items to it using the context bar button Add Item.

How to Draw a Structure?

  • Click on the Structure button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new structure by dragging on the diagram canvas.
  • If you want to add a structure with the default size, just click on a diagram canvas and the structure will be inserted to the specified position. You can also drag the Structure button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple structures in a row, double click (or click twice) on the Structure button in the toolbox and draw desired number of structures in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

How to Draw a Table?

  • Click on the Table button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new table by dragging on the diagram canvas.
  • If you want to add a table with the default size, just click on a diagram canvas and the table will be inserted to the specified position. You can also drag the Table button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple tables in a row, double click (or click twice) on the Table button in the toolbox and draw desired number of tables in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

How to Draw a Signal?

  • Click on the Signal button in the Class Diagram group in the toolbox.
  • Specify the bounds of the new signal by dragging on the diagram canvas.
  • If you want to add a signal with the default size, just click on a diagram canvas and the signal will be inserted to the specified position. You can also drag the Signal button from the toolbox and drop it to the desired position on the diagram.
  • If you want to insert multiple signals in a row, double click (or click twice) on the Signal button in the toolbox and draw desired number of signals in the diagram editor. If you do not want to add others, press the ESC key or click to the Selection tool button.

How to Draw a Collaboration?

  • Click on the Collaboration button in the Class Diagram group in the toolbox.
  • If you want to add a collaboration with the default size, just click on a diagram canvas and the collaboration will be inserted to the specified position.
  • If you want to add a collaboration with a custom size, click on the desired position on the diagram and drag to define the size of the new collaboration.
  • If you want to insert multiple collaborations in a row, double click (or click twice) on the Collaboration button in the toolbox and draw as many collaborations as you want in the diagram editor.

How to Draw a Generalization?

  • Click on the Generalization button in the Class Diagram group in the toolbox.
  • Drag from a sub-class to a super-superclass.
  • Another way you can insert a generalization relationship between two classes is to click on the Generalization button in the toolbox and drag it in between two class elements. The diagram editor shows you which two classes will be connected when you release the button.
  • If you want to insert multiple generalizations in a row, double click (or click twice) on the Generalization button in the toolbox and draw the desired number of generalizations in the diagram editor.

How to Draw a Realization?

  • Click on the Realization button in the Class Diagram group in the toolbox.
  • Drag from a classifier to an interface. The arrow points from a classifier to an interface.
  • Another way you can insert a realization relationship between a classifier and an interface is to click on the Realization button in the toolbox and drag it in between an interface and a classifier. The diagram editor shows you which two elements will be connected when you release the button.
  • If you want to insert multiple realizations in a row, double click (or click twice) on the Realization button in the toolbox and draw the desired number of realizations in the diagram editor.

How to Draw an Association?

  • Click on the Association button in the Class Diagram group in the toolbox.
  • Drag from an element to another element. The inserted association is undirected.
  • Another way you can insert an association between two elements (e.g. classes) is to click on the Association button in the toolbox and drag it in between two elements. The diagram editor shows you which two elements will be connected when you release the button.
  • If you want to insert multiple associations in a row, double click (or click twice) on the Association button in the toolbox and draw the desired number of associations in the diagram editor.

How to Draw a Containment?

  • Click on the Containment button in the Class Diagram group in the toolbox.
  • Drag from an element (e.g. class, instance specification) that should be contained to another element (e.g. class, instance specification) that contains the start element. The inserted relationship points from contained element to its container.
  • Another way you can insert a containment between two elements is to click on the Containment button in the toolbox and drag it in between two elements. The diagram editor shows you which two elements will be connected in the containment relationship when you release the button.
  • If you want to insert multiple associations in a row, double click (or click twice) on the Containment button in the toolbox and draw the desired number of containments in the diagram editor.

Class Diagram Examples

You may find some examples of UML class diagram here:

Comments

KC 24 March 2016 23:39:36

Attributes of attributes

I am trying to figure out how to set the attributes of the attributes such as foreign-key, primary-key,auto-increment. How do you set them in the app?

Dusan Rodina - softwareideas.net 25 March 2016 2:12:50

RE: Attributes of attributes

The best way is to choose Entity Relationship Diagram (Project/Add Diagram/Other/Entity Relationship Diagram). Insert an entity to your diagram, add an attribute, right-click on it and choose Primary Key, Foreign Key, Nullable or Auto Increment from context menu.

If you want to use UML Class Diagram, you can add an attribute with custom stereotypes to a class. You can create custom stereotypes for foreign key and auto increment (right click on attribute / Field Properties / Stereotypes tab). You can use (UML) standard modifier 'ID' for primary key (right click on attribute / Field Properties / Modifiers group)

Bruno Santos 30 October 2017 17:46:29

Association multiplicity

I can't select the association multiplicity in the Class diagrams, it opens and closes very fast in the upper left corner. It looks like a bug. Can somebody help me?

Dusan Rodina - softwareideas.net 30 October 2017 18:40:51

RE: Association multiplicity

This problem may occur at some zoom levels. It will be fixed in the next release this week.

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