CS%20112%20Introduction%20to%20Programming

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CS 112 Introduction to Programming

• Design Good Classes Encapsulation and OOP
  Analysis
• Yang (Richard) Yang
• Computer Science Department
• Yale University
• 308A Watson, Phone [phone]
• Email yry_at_cs.yale.edu

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Admin

• Updated class schedule

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Recap Encapsulating Data
Client can see only the external API
Methods
Client
Data/state
Client should not see the internal state or
behaviors
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Recap Encapsulating Data

• Benefit
• Consistency so that it is impossible for others
  to "reach in" and directly alter another object's
  state
• Flexibility so that you can change the state
  representation later without worrying about
  breaking others code

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Ask, Don't Touch

• Encapsulated data types.
• Don't touch data and do whatever you want.
• Instead, ask object to manipulate its data.
• Lesson. Limiting scope makes programs easier to
  maintain and understand.

"Ask, don't touch."
Adele GoldbergFormer president of
ACMCo-developed Smalltalk
"principle of least privilege"
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Recap Encapsulation/How

• access modifiers enforce encapsulation
• public members (data and methods can be accessed
  from anywhere
• private members can be accessed from a method
  defined in the same class
• Members without an access modifier default
  private accessibility, i.e., accessible in the
  same package otherwise, not accessible.

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Examples Set the Access Modifiers

• Coin
• Ball
• BankAccount
• Point

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Class Diagram
class name
attributes
methods
Above is a class diagram representing the Coin
class. - indicates private data or method
indicates public data or method
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Outline

• Admin and recap
• Defining classes
• Motivation and basic syntax
• Examples
• The encapsulation principle
• An OOP example geomap visualization

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Data Visualization
If I can't picture it, I can't understand it.
Albert Einstein
Edward Tufte. Create charts with high data
density that tell the truth.
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Visualization based on Geography
Classes?
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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon
Point
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Major Classes and Relationship
GeoMap
Region
Region

String
Polygon
Color
Color
Point

Point
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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon
Design question - Immutable objects?
Point
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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon
Design question - What is the basic controller
(of GeoMap) structure?
Point
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Coloring Controller Structure

• Retrieve region
• Batch retrieve list of all regions
• Specific retrieve one specific region
• Coloring
• Map properties of a region to a color

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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon

• Discussion
• Public methods (API) of Point

Point
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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon

• Discussion
• Public methods (API) of Polygon

Point
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Polygon
public class Polygon private final int N
// number of boundary points private
final Point points // the points // read
from input stream public Polygon(Scanner
input) N input.nextInt()
points new PointN1 for (int i 0 i
lt N i) pointsi new Point (
input ) pointsN
points0 public void draw()
public void fill() public boolean
contains(Point p) public Point
centroid()
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Major Classes and Relationship
GeoMap
Region
Color
A composition relationship
An association relationship
Polygon

• Discussion
• Public methods (API) of Region

Point
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Region
Q Should Region have a method that returns its
internal Polygon?
public class Region private final String
regionName // name of region private final
String mapName private final Polygon poly
// polygonal boundary private Color
fillColor, drawColor public Region(String
mName, String rName, Polygon poly)
regionName rName mapName mName
this.poly poly setDefaultColor()
public void setDrawColor (Color c) drawColor
c public void draw() setDrawColor()
poly.draw () public void fill()
public boolean contains(Point p) return
poly.contains(p) public Point centroid()
return poly.centroid()
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Example Controllers

• GeoMap.java
• RandomColorMap.java
• ClickColorMap.java
• RedBlueMap.java

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Purple America (PurpleMap.java)

• Idea. Robert J. Vanderbei Assign color based
  on number of votes.
• a1 Rep. votes.
• a2 Other votes.
• a3 Dem. votes.

100 Rep
55 Dem, 45 Rep
100 Dem
100 Other
public Color getColor() int dem
tally.dem(), rep tally.rep(), ind
tally.ind() int tot tally.dem tally.rep
tally.ind return new Color((float) rep/tot,
(float) ind/tot, (float) dem/tot)
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Cartograms

• Cartogram. Area of state proportional to number
  of electoral votes.

Michael Gastner, Cosma Shalizi, and Mark
Newman www-personal.umich.edu/mejn/election
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Cartograms

• Cartogram. Area of country proportional to
  population.

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Summary

• Encapsulation is a key problem solving technique
  for large, complex problems
• A good way to learn more is to read about designs
  of large-scale systems