UML - Class Diagram

Objects

Individuals of a system:

Object Diagram

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The objects of a system and their relationships, snapshot of objects at a specific moment in time:

Classes

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A class is a construction plan for a set of similar object of a system.

Object are instances of classes.

• Attributes: structural characteristics of a class.
• Operations: behaviour of a class.

Attribute Syntax

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Visibility

• +: public -> everybody.

• -: private -> only the object itself.

• #: protected -> class itself and subclasses.

• ~: package -> classes that are in the same package.

• /: attribute is derived from other attributes, for example /age as it is calculated from the date of birth.

Type

• User-defined classes.
• Data type:
  • Primitive data type
    • Pre-defined: Boolean, Integer, String.
    • User-defined: <<primitive>>
    • Composite data type: <<datatype>>
  • Enumerations: <<enumeration>>

Multiplicity

• Number of values an attribute may contain.
• Default value: 1
• Notation [min..max]
  • no upper limit: [*] or [0..*]

Properties

• Pre-defined properties:

  • {readOnly}: value cannot be changed.
  • {unique}: no duplicates permitted.
  • {non-unique}: duplicates permitted.
  • {ordered}: fixed order of the values.
  • {unordered}: no fixed order of the values.

• Attribute specification:

  • Set: {unordered, unique}
  • Multi-set: {unordered, non-unique}
  • Ordered set: {ordered, unique}
  • List: {ordered, non-unique}

Parameters

Similar to attributes.

• Direction of the parameter:
  • in: input parameter.
  • out: output parameter.
  • inout: combined.

Class Variable and Class Operations

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• Instance variable (instance attribute): attributes defined on instance level.
• Class variable (class attribute):
  • Defined only per class, shared by all instances of the class.
  • For example: counters for the number of instances of a class, constants, etc.
• Class operation:
  • Can be used if no instance was created, for example: constructors, counting operations, math functions etc.

Specification of Classes: Different Levels of Detail

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Association

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Models possible relationships between instances of classes.

Binary Association

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Connects two instances of two classes with one another.

Navigability

Object knows its partner objects and can therefore access their visible attributes and operations (open arrow head).

Non-Navigability

Indicated by a class, for example: A can access attributes and operations of B but B cannot access any attributes and operations of A.

Binary Association as Attribute

In Java:

class Professor()
 
class Student {
    public Professor[] lecturer;
    ...
}

Multiplicity and role

Multiplicity: number of objects that may be associated with exactly one objet of the opposite side.

Role: the way in which an object is involved in an association relationship.

Xor Constraint

An object of class A is to be associated with an object of class B or an object of class C but not with both.

Unary Association - Example

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n-ary Association

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More than two partner objects are involved in the relationship.

No navigation directions.

Association Class

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Assign attributes to the relationship between classes rather than to a class itself.

Necessary when modelling n:m Associations:

With 1:1 or 1:n possible but not necessary:

Association Class vs Regular Class

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Unique/Non-Unique

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Aggregation

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Special form of association used to express that a class is part of another class.

Properties of the aggregation association:

• Transitive: if B is part of A and C is part of B, C is also part of A.
• Asymmetric: it is not possible for A to be part of B and B to be part of A simultaneously.

Two types: shared aggregation and composition.

Shared Aggregation

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• Expresses a weak belonging of the parts to a whole, meaning they also exist independently of the whole.
• Multiplicity at the aggregating end may be > 1, meaning one element can be part of multiple other elements simultaneously.
• Spans a directed acyclic graph.
• Syntax: hollow diamond at the aggregating end.

Example:

• Student is part of LabClass
• Course is part of StudyProgram

Composition

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• Existence dependency between the composite object and its parts.
• Multiplicity at the aggregating end max of 1, the composite objects form a tree.
• If the composite object is deleted, its parts are also deleted.
• Syntax: solid diamond at the aggregating end.

Example:

• Beamer is part of LectureHall is part of Building.

Generalisation

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• Characteristics (attributes and operations), associations, and aggregations that are specified for a general class (superclass) are passed on to its subclasses.
• Every instance of a subclass is simultaneously an indirect instance of the superclass.
• Subclass inherits all characteristics, associations, and aggregations of the superclass except private ones.
• Subclass may have further characteristics, associations, and aggregations.
• Generalisations are transitive.

Abstract Class

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• Used to highlight common characteristics of their subclasses.
• Used to ensure that there are no direct instances of the superclass.
• Only its non-abstract subclasses can be instantiated.
• Useful in the context of generalisation relationships.
• Notation: keyword {abstract} or class name in italic font.

Multiple Inheritance

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• UML allows multiple inheritance.
• A class may have multiple superclasses.

Example:

With vs Without Generalisation

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Creating a Class Diagram

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• Not possible to completely extract classes, attributes and associations from a natural language text automatically.

• Guidelines:

  • Nouns often indicate classes.
  • Adjectives indicate attribute values.
  • Verbs indicate operations.

• Example: The library management system stores users with their unique ID, name and address as well as books with their title, author and ISBN number.

Notation

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Name	Notation	Description
Class		Description of the structure and behaviour of a set of objects.
Abstract Class		Class that cannot be instantiated.
Association		Relationship between classes where navigability is unspecified or bidirectional or one directional.
n-ary Association		Relationship between n (here 3) classes.
Association Class		More detailed description of an association.
xor Relationship		An object of C is in a relationship with A or with B but not both.
Shared aggregation		Parts-whole relationship (A is part of B)
Strong aggregation (composition)		Existence-dependant parts-whole relationship (A is part of B)
Generalisation		Inheritance relationship (A inherits from B)
Object		Instance of class.
Link		Relationship between objects.

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