CS 112 Introduction to Programming Inheritance Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone:

2 Admin  Class project

3 Recap: OOP Analysis GeoMapRegion Polygon Color Point 1mm2 111m A composition relationship An association relationship

 Retrieve region m Batch: retrieve list of all regions m Specific: retrieve one specific region  Coloring m Map properties of a region to a color 4 Recap: OOP Analysis: Controller Structure

 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 OOP software systems Recap: OOP Analysis

6 Software Design and Reuse  Question: What programming language feature(s) have we covered to allow software reuse?

7 Outline  Admin and recap  Class inheritance o why and how?

A Law Firm Problem: Setting  Consider the following law firm: 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. 8

A Law Firm Problem: Setting Each type of employee has some job functions: Lawyers know how to sue. Marketers know how to advertise. Secretaries know how to prepare ordinary documents. Legal secretaries know how to prepare both ordinary documents and legal documents. 9

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; } 10

Question: Writing class Secretary  Secretaries are employees who can prepare documents. 11

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); } } 12

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); } 13

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 14

15 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

16 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

17 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 18

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. 19

20 Outline  Admin and recap  Class inheritance o why and how? o inheritance and object construction

21 Inheritance and Constructor  When constructing an object, Java makes sure that the constructor of the parent is first called  If no parent constructor called, Java automatically inserts super() as the first statement in the constructor of a child class: public class Secretary extends Employee { public Secretary () { // super() is automatically inserted System.out.println(“In Secretary()”); } … }

Example 22 public class Secretary extends Employee { public Secretary() { // super() is automatically inserted System.out.println(“In Secretary()”); } … } public class Employee { public Employee() { System.out.println(“In Employee()”); } … } public static void main(String[] args) { Secretary seth = new Secretary(); } In Employee() In Secretary() Output:

Exercise: Add Name to Employee public class Employee { private String name; public Employee(String name) { this.name = name; } … } 23

Problem with constructors  Now that we've added the constructor to the Employee class, our subclasses do not compile.  The short explanation: Once we write a constructor (that requires parameters) in the superclass, we must now write constructors for our employee subclasses as well.  The long explanation: (next couple slides)

25 The explanation  Constructors aren't inherited.  The Employee subclasses don't inherit the public Employee(String name) constructor.  After defining public Employee(String), Java sees that we have a constructor, and will no longer provide the default Employee constructor.  i.e., public Employee() is not defined unless we define it explicitly  But public Secretary() { // super() is automatically inserted but not defined // in Employee System.out.println(“In Secretary()”); }

26 super and Constructor  If you insert super(…) as the first statement in child’s constructor, Java will not insert the default parent constructor: public class Secretary extends Employee { public Secretary(String name) { super(name); System.out.println(“In Secretary()”); } … } public class Employee { private String name; public Employee(String name) { System.out.println(“In Employee()”); this.name = name; } … }

Example: Picture Type  Raster graphics. Basis for image processing.  Set of values. 2D array of Color objects (pixels).  API.

Exercise: Extending the Picture Type  Although the Picture class is quite useful already, it misses some useful filtering functions such as grayScale, scaling  Goal: create a new type InstaPic with all of the existing methods defined in Picture, with two additional methods: gray and scale 28

Monochrome Luminance  Monochrome luminance. Effective brightness of a color (NTSC formula): Y = 0.299r g b. import java.awt.Color; public class Luminance { public static double lum(Color c) { int r = c.getRed(); int g = c.getGreen(); int b = c.getBlue(); return.299*r +.587*g +.114*b; }

Grayscale Filter mandrill.jpg

Color Compatibility  Q. Which font colors will be most readable with which background colors on computer and cell phone screens?  A. Rule of thumb: difference in luminance should be  128. public static boolean compatible(Color a, Color b) { return Math.abs(lum(a) - lum(b)) >= 128.0; }

Scaling Filter mandrill.jpg (298-by-298)

Image Scaling  Goal. Shrink or enlarge an image to desired size.  Assume uniform strategy to convert from w s -by- h s to w t -by- h t : ? source image (w s -by-h s ) target image (w t -by-h t ) y (x,y) y  hs / ht)y  hs / ht)(x  w s / w t,

Scaling Filter  Scaling filter. Creates two Picture objects and two windows. % java Scale mandrill.jpg mandrill.jpg (298-by-298)

Back to Law Firm: Implementing the Lawyer class: Attempt 1 // A class to represent lawyers. public class Lawyer extends Employee { public void sue() { System.out.println("I'll see you in court!"); } 35

Problem  We want lawyers to inherit most behaviors from employee, but we want to replace parts with new behavior: o Lawyers get an extra week of paid vacation over base vacation (a total of 3). o Lawyers use a pink form when applying for vacation leave. 36

37 Defining Methods in the Child Class: Overriding Methods  A child class can (have the option to) override the definition of an inherited method in favor of its own m that is, a child can redefine a method that it inherits from its parent m the new method must have the same signature as the parent's method, but can have different code in the body  The method invoked is always the one defined in the child class, if the child class refines (overrides) a method

Lawyer class // A class to represent lawyers. public class Lawyer extends Employee { // overrides getVacationDays from Employee class public int vacationDays() { return 15; // 3 weeks vacation } // overrides getVacationForm from Employee class public String vacationForm() { return "pink"; } public void sue() { System.out.println("I'll see you in court!"); } 38

39 Overloading vs. Overriding  Overloading deals with multiple methods in the same class with the same name but different signatures  Overloading lets you define a similar operation in different ways for different data  Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature  Overriding lets you define a similar operation in different ways for different object types

Marketer class – Exercise: Complete the Marketer class. Marketers can advertise and make 20% more than the base ($60,000 total). 40

Marketer class // A class to represent marketers. public class Marketer extends Employee { public void advertise() { System.out.println("Act while supplies last!"); } // override public double pay() { return ; // $60, / year } 41 Problem of design?

A Problem public class Marketer extends Employee { public double pay() { return ; }... } Problem: The Marketer‘s salaries are based on the Employee’s base salary (20% more than base), but the pay code does not reflect this. 42

Changes to Common Behavior  Imagine a company-wide change affecting all employees.  Example: Everyone is given a $10,000 raise due to inflation. The base employee salary is now $60,000. Marketers should now make $72,000.  We must modify our code to reflect this policy change. 43

Modifying the superclass // A class to represent employees in general (20-page manual). public class Employee { public int hours() { return 40; // works 40 hours / week } public double pay() { return ; // $60, / year }... }  Issue: the Marketer subclass is still incorrect. It has overridden pay to return another value.  Requirement: derived behavior is based on base behavior 44

Calling overridden methods Subclasses can call overridden methods with super super. ( ) – Exercise: Modify Marketer to derive pay for marketers from base pay. 45

Improved subclasses public class Marketer extends Employee { public void advertise() { System.out.println("Act now while supplies last!"); } // override and invoke the parent’s version public double pay() { return super.pay() * 1.2; } 46

Exercise: Revise Lawyer to Maintain Consistency 47

Solution public class Lawyer extends Employee { public String vacationForm() { return "pink"; } public int vacationDays() { return super.vacationDays() + 5; } public void sue() { System.out.println("I'll see you in court!"); } 48