CompSci 230
Software Construction
Class Diagrams
Version 1.1 of 2014-03-12: added learning objectives and DuckTestApp
Version 1.2 of 2014-03-14: added slide 6 on default values in UML
Agenda
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Topics:
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Reading
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Leigh Dodd’s discussion of the UML examples in his introductory lecture
slides.
If you actually want to learn this material…
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2
Examples of class diagrams
Navigation, visibility, named associations, and multiplicity in UML
Interfaces and implementation, in UML
Find your own examples of class diagrams on the web. Try to interpret
them. Look for patterns.
Talk about it within your study group.
COMPSCI 230: S4
Learning Objectives
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Students will have a strong conceptual foundation for their future
uses of the OO features of Java
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Students will be able to interpret a class diagram
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Worked examples in this lecture
Students will be competent at the basic OO design patterns
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3
Interfaces
Implementing interfaces, composing classes, extending classes
COMPSCI 230: S3
Example of Inheritance
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Source: http://www.ldodds.com/lectures/intro2java/Lesson1.ppt
COMPSCI 230: S4
Too much detail?
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Source: http://www.ldodds.com/lectures/intro2java/Lesson1.ppt
COMPSCI 230: S4
Simplify! (with a little more notation)
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Multiplicity:
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The arrowheads indicate that the
taughtBy association is navigable in
both directions, telling us that
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6
There is exactly one teacher per
course, as indicated by the 1.
A lecturer can teach any number of
courses, as indicated by 0..*.
We can also write 1..* in a UML
diagram.
Course has an instance variable
teacher, of type Lecturer, and
Lecturer has the instance variable
Vector<Course> course.
COMPSCI 230: S4
Simplify even more, with defaults
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Associations have default multiplicity 1
Association endpoints have a default name.
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Getters, setters may be implied.
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Unimportant members might not be shown.
Defaults may be well-defined by an
organisation’s stylesheet, or (more
commonly) by their UML-drawing software
package.
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Course has an instance variable myLecturer of
type Lecturer
Lecturer has an instance variable myCourse of
type Vector<Course>
See e.g. http://msdn.microsoft.com/enus/library/dd323861.aspx: “Properties of
Attributes in UML Diagrams” for VS 2013.
COMPSCI 230: S4
One-way Navigation
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These courses have a Vector of their Lecturers and Students.
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(They might have a List; they might have an array; these are
implementation decisions.)
These lecturers don’t know what they are teaching!
These students have no idea of what course they are taking!
COMPSCI 230: S4
Creating new classes by generalising
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Lecturers and students have some
attributes in common.
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We write these methods once for the
Person class, and we can reuse them in the
Lecturer and Student class.
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9
A public name
An email address that is revealed to everyone
in our University
A secret password
But we have complicated our design by adding
another class.
Do we really need so many classes?
COMPSCI 230: S4
Interfaces
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Interfaces are a way to specify
behaviour (a public contract) without
data or implementation.
Interfaces are classed with an extra
label next to their name:
<<Interface>>
A dotted open-triangle arrow, from a
class to an interface means that “the
class implements this interface”.
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We also say that “the class fulfils the
contract specified by this interface”, or
that it “realizes the interface.”
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Note that interfaces
define methods but
not attributes.
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A password allows a
secureLogin().
COMPSCI 230: S4
Can you understand this design?
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Adapted from http://www.ldodds.com/lectures/intro2java/Lesson1.ppt
COMPSCI 230: S4
Can you understand this design?
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Source: http://commons.wikimedia.org/wiki/File:UML_diagram_
of_composition_over_inheritance.svg, available 2014-03-12.
COMPSCI 230: S4
Don’t believe everything you read!
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In Quiz 1, over the weekend, you’ll be exploring the design space of
the DuckTestApp on the previous slide.
I hope this quiz inspires one or more of you to improve
http://en.wikipedia.org/wiki/Composition_over_inheritance, so that
its Example section discusses two class diagrams.
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The first class diagram could show a simple design for a DuckTestApp
which relies solely on inheritance to model its ducks.
The second diagram could show a more complicated but more extensible
design which makes appropriate use of composition and realisation.
Ideally, your discussion would mention that the Adapter design pattern
was employed when the app was extended to handle turkey-testing as
well as duck-testing.
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Warning: the Wikipedia article on the Adapter design pattern is also in need of
improvement.
COMPSCI 230: S4
Learning Objectives (review)
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Students will have a strong conceptual foundation for their future
uses of the OO features of Java


Students will be able to interpret a class diagram


Worked examples in this lecture
Students will be competent at the basic OO design patterns

14
Interfaces
Implementing interfaces, composing classes, extending classes
COMPSCI 230: S3