LECTURE 3:DESIGNING PROGRAMS & FINDING CLASSES

CSC 213 –Large Scale Programming

Today’s Goal

Improve design skills to make usable designs Noun extraction & UML class diagram

reviewed Connections to use for fields in diagrams

detailed Describe process to find classes’ public

methods Using textual analysis to start our search for

methods Learn to test designs & find potential

errors Discover reasons for iterative nature of this

process Indentify & fix common errors that occur in

designs Learn about using CRC cards to check

design validity

Designing Programs

Designing programs just like making music How do you get to Carnegie Hall?

Designing Programs

Designing programs just like making music How do you get to Carnegie Hall? (old

joke)

Good News For The Term

Designing program ultimately like any other skill

Get many opportunities at this over term

Why is this important? Why should you care?

Good News For The Term

Designing program ultimately like any other skill

Get many opportunities at this over term

Why is this important? Why should you care?

First Step In Design: Classes Find (& underline) nouns in requirements

Go through & eliminate any external to solution Select entity classes (main classes in program)

Look at remaining nouns that not abstract & singular Find boundary classes (I/O & recording states)

From options, use singular form of abstract nouns Adjectives describing entity classes could work, too

Determine control classes (structs. & algorithms) Needed for any plurals lacking specific number Important algorithms that do not have a name

Next Step: Fields

Look for possessives during reading of text Should also look for text suggesting data

possession Consider boundary classes; often used for

fields May rely on common understandings: use

your brain Can now publicize design for entire

team to use You’ve done your part: let suckers do the

hard work Use UML class diagrams to avoid writing

lots of words

UML Class Diagrams

Classes drawn in 3-part box Class name written in top portion of box Attributes (fields) written in middle

portion Operations (methods) written in bottom

portion All variables written using following

format :name : type

Show important relationships between classes Inheritence & important fields normally

shown here Solid lines for classes and dashed lines for

interfaces

Optional Relationships

Aggregation Used when target instances used outside of

source Car aggregates CarParts (e.g., tires, engine,

lights…) Composition

Use when objects from target used only within source

Chessboard composes Squares Association

Source object has field of target type Often used to imply source calls target's

methods

Generalization (“inheritance”) Shows subclass-superclass relationship Does not involve fields, multiplicities, or

anything else Makes this a different type of connection

than others Must draw all generalizations within the

diagram For implementation to be correct, these are

critical Cannot specify any other way, so vital to be

shown

Required Connection

Elevator's UML Class Diagram

Actions Are Verbs

Now need to find methods for all of our classes At this point in time, many not know all

methods Important public methods described in text,

however To find methods, look for verbs in

requirements Circle each verb since it is a possible method Since often not helpful, check all “to be” &

“has” uses Circle algorithm names and longer action

descriptions Looking through text, only public

methods found Find more methods later when looking at

algorithms

Looking for verbs

1. Circle all the verbs in the text

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Looking for verbs

1. Circle all the verbs in the text

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Looking for verbs

2. Double-check all uses of “to be” and “has”

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Looking for verbs

2. Double-check all uses of “to be” and “has”

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Looking for verbs

3. Circle algorithm names & action descriptions

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Looking for verbs

3. Circle algorithm names & action descriptions

Buttons in elevators and on the floors control the movement of n elevators in a building with m floors. Buttons illuminate when pressed to request the elevator stop at a specific floor; the illumination is canceled when the request has been satisfied. When an elevator has no requests, it remains at its current floor with its doors closed.

Which Class Gets the Method? Look for SRP violations for path to true

laziness

Which Class Gets the Method? Look for SRP violations for path to true

laziness

Single Responsibility Principle

Goal of every class & method written Make sure that class represents single

concept Methods perform single action & not

“bucket of mud” Many benefits accrue when code

follows SRP Easier to write code, since design becomes

simpler More focused results make debugging a

snap Modifying design quicker when changes

needed

No Free Cheeseburgers

Achieving SRP means 1 last step in design More classes & methods often created by

this process But also create smaller, simpler classes &

methods Less total time required even with this

"extra" step To get SRP, people found CRC cards are

easiest 3x5 cards & a brain are all this process

takes

CRC Cards Explained

Divide 3x5 Card into 3 parts Class name from UML design document Responsibilities are the methods class will

need Collaborations are classes it will interact

with

Responsibilities

Initially you should start with actions you found Add facts class knows, since these are

getter methods If class should not be responsible,

move action Instead, the action placed in class where

makes sense Make classes collaborators after action

moved Pays off in long run, but may make new

classes now

Responsibilities

Initially you should start with actions you found Add facts class knows, since these are

getter methods If class should not be responsible,

move action Instead, the action placed in class where

makes sense Make classes collaborators after action

moved Pays off in long run, but may make new

classes now

How to know if responsibility logical?

Seriously!

Go Yourself

If the Responsibility Fits

Try each responsibility into following statement Will find statement is valid only if

responsibility fits

The (class name) (responsibility) itself

Move actions to class that works in statement

The (new class) (responsibility) (old class)

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

Let’s Try It For Car

Which of these responsibilities belong to carStarts engineKills engineChanges tiresGets drivenGets washedChanges oilWarns oil is low Reports on fuel level

So Remember

So Remember

Go Yourself

For Next Lecture

Reading for Wednesday available on Angel How can we solve bugs BEFORE we code? Can we prevent getting trapped and lost? Prevent your name from becoming swear

word! Weekly assignment problem due in

class Wed. Can be found on Angel as problem #3 for

Week #1