1 COS 260 DAY 12 Tony Gauvin. 2 Agenda Questions? 5 th Mini quiz –Chapter 5 40 min Assignment 3...
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Transcript of 1 COS 260 DAY 12 Tony Gauvin. 2 Agenda Questions? 5 th Mini quiz –Chapter 5 40 min Assignment 3...
1
COS 260 DAY 12
Tony Gauvin
2
Agenda• Questions?• 5th Mini quiz
– Chapter 5 40 min • Assignment 3 Due • Assignment 4 will be posted later (next
week) – If you are bored, Exercises 6.34, 6.35, 6.36 and
6.37 will be part of assignment 4 • Capstone Proposals Over Due • Discuss Designing Classes
Designing classes
How to write classes in a way that they are easily
understandable, maintainable and reusable
5.0
4
Main concepts to be covered
• Responsibility-driven design• Coupling• Cohesion• Refactoring
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
5
Software changes• Software is not like a novel that is
written once and then remains unchanged.
• Software is extended, corrected, maintained, ported, adapted, …
• The work is done by different people over time (often decades).
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
6
Change or die• There are only two options for
software:– Either it is continuously maintained– or it dies.
• Software that cannot be maintained will be thrown away.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
7
World of Zuul
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Explore zuul-bad
8
The Zuul Classes• Game: The starting point and main
control loop.• Room: A room in the game.• Parser: Reads user input.• Command: A user command.• CommandWords: Recognized
user commands.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
9
Code and design quality• If we are to be critical of code
quality, we need evaluation criteria.
• Two important concepts for assessing the quality of code are:– Coupling– Cohesion
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
10
Coupling• Coupling refers to links between
separate units of a program.• If two classes depend closely on
many details of each other, we say they are tightly coupled.
• We aim for loose coupling.• A class diagram provides (limited)
hints at the degree of coupling.Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
11
Cohesion• Cohesion refers to the number
and diversity of tasks that a single unit is responsible for.
• If each unit is responsible for one single logical task, we say it has high cohesion.
• We aim for high cohesion.• ‘Unit’ applies to classes, methods
and modules (packages).Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
12
Code Duplication• Examine Class Game for code
duplications• How can we fix the problem?• Elimination of duplication fixes
Bad Cohesion – One task per method– One entity per class
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
13
An example to test quality• Add two new directions to the 'World
of Zuul':• “up”• “down”
• What do you need to change to do this? – Which classes
• How easy are the changes to apply thoroughly?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
14
Up/down Extension• Use of a HashMap for each room to store exits
(directions and adjacent rooms )– Key is a direction– Value is a (another) room
• exits.get(north) would return room that is north of the current room
• We then can add as many exist as want by add to the “exits” HashMap
• Would require changes to many classes (tight coupling)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Designing classes
Coupling, cohesion, and responsibility-driven design
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Coupling (reprise)• Coupling refers to links between
separate units of a program.• If two classes depend closely on
many details of each other, we say they are tightly coupled.
• We aim for loose coupling.• A class diagram provides (limited)
hints at the degree of coupling.Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
17
Loose coupling• We aim for loose coupling.• Loose coupling makes it possible to:
– understand one class without reading others;
– change one class with little or no effect on other classes.
• Thus: loose coupling increases maintainability.
• Encapsulation helps reduce coupling
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
18
Tight coupling• We try to avoid tight coupling.• Changes to one class bring a
cascade of changes to other classes.
• Classes are harder to understand in isolation.
• Flow of control between objects of different classes is complex.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
19
Cohesion (reprise)• Cohesion refers to the number
and diversity of tasks that a single unit is responsible for.
• If each unit is responsible for one single logical task, we say it has high cohesion.
• We aim for high cohesion.• ‘Unit’ applies to classes, methods
and modules (packages).Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
20
High cohesion• We aim for high cohesion.• High cohesion makes it easier to:
– understand what a class or method does;
– use descriptive names for variables, methods and classes;
– reuse classes and methods.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
21
Loose cohesion• We aim to avoid loosely cohesive
classes and methods.• Methods perform multiple tasks.• Classes have no clear identity.• Every part of class should refer to
ONE thing
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
22
Cohesion applied at different levels
• Class level:– Classes should represent one single,
well defined entity.• Method level:
– A method should be responsible for one and only one well defined task.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
23
Code duplication• Code duplication
– is an indicator of bad design,– makes maintenance harder,– can lead to introduction of errors during
maintenance.• Very frustrating to Trouble Shoot to
maintain code that has duplications. You can’t just fix one part you have find all the duplications
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
24
Responsibility-driven design
• Question: where should we add a new method (which class)?
• Each class should be responsible for manipulating its own data.
• The class that owns the data should be responsible for processing it.• encapsulation!
• RDD leads to low coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
25
Localizing change• One aim of reducing coupling and
responsibility-driven design is to localize change.
• When a change is needed, as few classes as possible should be affected.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
26
Implicit coupling • Occurs when one class depends on the
internal information of another class• Example add a new command to the
game world-of-zuul – Add to CommandWord class– Modify Game class
• Add new method for command• Add new command method to
processCommand method logic • What else?
Objects First with Java - A Practica l Introduction using BlueJ, © David J. Barnes, Michael Kölling
27
Thinking ahead• When designing a class, we try to
think what changes are likely to be made in the future.
• We aim to make those changes easy.
• One pattern often seen• Text based game becomes GUI
based Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
28
Model View Controller • Originates from Smalltalk (basis
for most modern object oriented programming languages)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
29
Cohesion in Depth • Cohesion of Methods
– One method one task• Cohesion of Classes
– One class one well defined entity• Cohesion for readability & reuse
– Proper naming and class Division provides for easier to understand and reuse code.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
30
Refactoring• When classes are maintained,
often code is added.• Classes and methods tend to
become longer. (bloat code) • Every now and then, classes and
methods should be refactored to maintain cohesion and low coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
31
Refactoring and testing• When refactoring code, separate
the refactoring from making other changes.
• First do the refactoring only, without changing the functionality.
• Test before and after refactoring to ensure that nothing was broken.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
32
Design questions• Common questions:
– How long should a class be?– How long should a method be?
• These can now be answered in terms of cohesion and coupling.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
33
Design guidelines• A method is too long if it does more
then one logical task.• A class is too complex if it represents
more than one logical entity.
• Note: these are guidelines - they still leave much open to the designer.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
34
Enumerated Types• A language feature.• Uses enum instead of class to
introduce a type name.• Their simplest use is to define a
set of significant names.– Alternative to static int constants.– When the constants’ values would
be arbitrary.Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
35
A basic enumerated type
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public enum CommandWord{ // A value for each command word, // plus one for unrecognised commands. GO, QUIT, HELP, UNKNOWN;} • Each name represents an object of the
enum type, e.g., CommandWord.HELP. • Enum objects are not created directly.• Enum definitions can also have fields,
constructors and methods.
36
Review• Programs are continuously
changed.• It is important to make this
change possible.• Quality of code requires much
more than just performing correct at one time.
• Code must be understandable and maintainable.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
37
Review• Good quality code avoids duplication,
displays high cohesion, low coupling.• Coding style (commenting, naming,
layout, etc.) is also important.• There is a big difference in the
amount of work required to change poorly structured and well structured code.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling