CS 112 Introduction to Programming Object Relationship Analysis: Composition, Association, Inheritance Yang (Richard) Yang Computer Science Department Yale University 208A Watson, Phone:

2 Admin  Project next step

Recap: Math.random() and Random Objects r Random generator must be stateful and hence is natural to use objects r Design lesson: One can implement static Math.random() using singleton and delegation. 3 public class Math { private static Random rand; public static double random() { if (rand == null) rand = new Random(); return rand.nextDouble(); }.. } A static variable, also called a singleton. A delegation implementation pattern.

4 Recap: Complex Number Objects r Design lesson: we design behaviors of complex numbers to be consistent with those of standard numbers (complex numbers are numbers after all): immutable objects public Complex plus(Complex b) { double real = re + b.re; double imag = im + b.im; return new Complex(real, imag); }

5 Outline  Admin and recap  Defining classes o Motivation and basic syntax o Simple examples o The encapsulation principle o Object examples o DrawingPanel objects vs StdDraw o Random objects vs Math.random o Complex number objects and fractal graphics  Object-oriented design  Geo-map visualization and has/associate relationship

Data Visualization “ If I can't picture it, I can't understand it. ” — Albert Einstein 6 Edward Tufte. Create charts with high data density that tell the truth.

Domain of Visualization based on Geography

Example Use Cases  RandomColorMap.java  RedBlueMap.java  ClickColorMap.java  GeoMap.java

Domain of Visualization based on Geography Classes and their relationships?

10 Major Classes and Relationship GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship

11 Major Classes and Relationship GeoMap Region String … Polygon Color Point …

12 Major Classes and Relationship GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship Design question: - What is the basic controller structure?

 Retrieve region (standard) m Batch: retrieve list of all regions m Specific: retrieve one specific region  Coloring (customized) m Map properties of each region to a color 13 Coloring Controller Structure

14 Major Classes and Relationship GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship Discussion: -Public methods (API) of Point

15 Major Classes and Relationship GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship Discussion: -Public methods (API) of Polygon

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 Point[N+1]; for (int i = 0; i < N; i++) { points[i] = new Point ( input ); } points[N] = points[0]; } … public void draw() { … } public void fill() { … } public boolean contains(Point p) { … } public Point centroid() { … } … }

17 Major Classes and Relationship GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship Discussion: -Public methods (API) of Region

Region 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() } … } Q: Should Region have a method that returns its internal Polygon? Even though most complexity is in Polygon, Polygon is not exposed. Region delegates tasks internally to Polygon.

Example Controllers  GeoMap.java  RandomColorMap.java  ClickColorMap.java  RedBlueMap.java

Extension: Purple America (PurpleMap.java)  Idea. [Robert J. Vanderbei] Assign color based on number of votes. m a 1 = Rep. votes. m a 2 = Other votes. m a 3 = Dem. votes. 100% Dem 100% Rep 100% Other 55% Dem, 45% Rep 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); } PurpleAmerica.java

Cartograms  Cartogram. Area of state proportional to number of electoral votes. Michael Gastner, Cosma Shalizi, and Mark Newman www-personal.umich.edu/~mejn/election

Cartograms  Cartogram. Area of country proportional to population.

23 Outline  Admin and recap  Defining classes  Object-oriented design o Geo-map visualization and has/associate relationship  Class inheritance

A Law Firm Problem: Setting  The firm has 5 types of employees m Standard employee m Secretary knows how to prepare ordinary documents m Legal secretary knows how to prepare both ordinary documents and legal documents. m Marketer knows how to advertise m Lawyer knows how to sue 24

A Law Firm Problem: Policies m Work time policy: Employees work 40 hours / week. m Pay policy: Employees make a base salary of $50,000 per year, except that o legal secretaries make 10% extra over base per year, o marketers make 20% extra over base per year, o lawyers who reach partner level get bonus. m Vacation policy: Employees have 2 weeks of paid vacation leave per year, except that o lawyers get an extra week on top of base, o employees should use a yellow form to apply for leave, except for lawyers who use a pink form. 25 Classes and their relationships?

26 Major Classes and Relationship StaffEmployee A composition relationship Secretary 1m m LegalSec Marketer Lawyer m m m

An Employee class public class Employee { public int hours() { return 40; // works 40 hours / week } public double pay() { return ; // $50, / year } public int vacationDays() { return 10; // 2 weeks' paid vacation } public String vacationForm() { return "yellow"; // use the yellow form } public String toString() { String result = "Hours: " + hours() + "\n"; result += "Pay: " + pay() + "\n"; result += "Vacation days: " + vacationDays() + "\n"; result += "Vacation Form: " + vacationForm() + "\n"; return result; } 27

Question: Writing class Secretary  Secretaries are employees who can prepare documents. 28

Secretary class: Attempt 1 public class Secretary { public int hours() { return 40; // works 40 hours / week } public double pay() { return ; // $50, / year } public int vacationDays() { return 10; // 2 weeks' paid vacation } public String vacationForm() { return "yellow"; // use the yellow form } public String toString() { String result += "Hours: " + hours() + "\n"; result += "Pay: ” + pay() + "\n"; result += "Vacation days: " + vacationDays() + "\n"; result += "Vacation Form: " + vacationForm() + "\n"; return result; } public void prepareDoc(String text) { System.out.println(“Working on Document: " + text); } } 29

Desire for code-sharing prepareDoc is the only unique behavior in Secretary. We'd like to be able to say: // A class to represent secretaries. public class Secretary { public void prepareDoc(String text) { System.out.println(“Work on Document: " + text); } 30

Inheritance  Inheritance: A way to allow a software developer to reuse classes by deriving a new class from an existing one m The existing class is called the parent class, or superclass, or base class m The derived class is called the child class or subclass.  As the name implies, the child inherits characteristics of the parent m The child class inherits every method and every data field defined for the parent class 31

32 Inheritance  Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent Animal - weight : int + getWeight() : int Bird + fly() : void - flySpeed : int

33 Inheritance  The child class inherits all methods and data defined for the parent class Animal - weight : int + getWeight() : int Bird - flySpeed : int + fly() : void weight = 120 getWeight() weight = 100 flySpeed = 30 getWeight() fly() an animal object a bird object

34 Deriving Subclasses: Syntax public class extends { } For example: class Animal { // class contents private int weight; public int getWeight() {…} } class Bird extends Animal { private int flySpeed; public void fly() {…}; }

Exercise: Implement Secretary 35

Summary: Improved Secretary code // A class to represent secretaries. public class Secretary extends Employee { public void prepareDoc(String text) { System.out.println(“Working on document: " + text); } }  By extending Employee, each Secretary object now: receives methods hours, pay, vacationDays, vacationForm, toString from Employee ’s definition automatically can be treated as an Employee by client code (seen later)  Now we only write the parts unique to each type. 36