Admin. CS 112 Introduction to Programming. Recap: OOP Analysis. Software Design and Reuse. Recap: OOP Analysis. Inheritance

Transcription

1 Admin CS 112 Introduction to Programming q Class project Inheritance Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: yry@cs.yale.edu 2 Recap: OOP Analysis Recap: OOP Analysis: Controller Structure GeoMap A composition relationship 1 m m 2 Region 1 1 Polygon 1 m Point An association relationship Color q Retrieve region Batch: retrieve list of all regions Specific: retrieve one specific region q Coloring Map properties of a region to a color 3 4 Recap: OOP Analysis q Encapsulation is a key problem solving technique for large, complex problems q A good way to learn more is to read about designs of large-scale OOP software systems Software Design and Reuse q Question: What programming language feature(s) have we covered to allow software reuse? 6 1

2 Outline q Admin and recap q Class inheritance o why and how? A Law Firm Problem: Setting q Consider the following law firm: Work time policy: Employees work 40 hours / week. 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. 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. 7 8 A Law Firm Problem: Setting An Employee class 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 public int hours() { return 40; // works 40 hours / week 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 Secretary class: Attempt 1 q Secretaries are employees who can prepare documents. 11 public class Secretary { public int hours() { return 40; // works 40 hours / week 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 2

3 Desire for code-sharing Inheritance preparedoc is the only unique behavior in Secretary. We'd like to be able to say: // A class to represent secretaries. public class Secretary { <copy all the contents from the Employee class> public void preparedoc(string text) { System.out.println( Work on Document: " + text); q Inheritance: A way to allow a software developer to reuse classes by deriving a new class from an existing one The existing class is called the parent class, or superclass, or base class The derived class is called the child class or subclass. q As the name implies, the child inherits characteristics of the parent The child class inherits every method and every data field defined for the parent class Inheritance q Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class Animal - weight : int + getweight() : int Bird - flyspeed : int + fly() : void Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent Inheritance q The child class inherits all methods and data defined for the parent class Animal - weight : int + getweight() : int Bird - flyspeed : int + fly() : void an animal object weight = 120 getweight() a bird object weight = 100 flyspeed = 30 getweight() fly() Deriving Subclasses: Syntax Exercise: Implement Secretary public class <name> extends <superclass> { For example: class Animal { // class contents private int weight; public int getweight() { class Bird extends Animal { private int flyspeed; public void fly() {;

4 Improved Secretary code Outline // A class to represent secretaries. public class Secretary extends Employee { public void preparedoc(string text) { System.out.println( Working on document: " + text); q Admin and recap q Class inheritance o why and how? o inheritance and object construction q 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) q Now we only write the parts unique to each type Inheritance and Constructor Example q 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() ); public class Secretary extends Employee { public Secretary() { // super() is automatically inserted System.out.println( In Secretary() ); public Employee() { System.out.println( In Employee() ); public static void main(string[] args) { Secretary seth = new Secretary(); 21 Output: In Employee() In Secretary() 22 Exercise: Add Name to Employee Problem with constructors private String name; public Employee(String name) { this.name = name; q Now that we've added the constructor to the Employee class, our subclasses do not compile. q 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. q The long explanation: (next couple slides) 23 4

5 The explanation q Constructors aren't inherited. The Employee subclasses don't inherit the public Employee(String name) constructor. q 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 q But public Secretary() { // super() is automatically inserted but not defined // in Employee System.out.println( In Secretary() ); super and Constructor q 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() ); private String name; public Employee(String name) { System.out.println( In Employee() ); this.name = name; Example: Picture Type q Raster graphics. Basis for image processing. q Set of values. 2D array of Color objects (pixels). q API. Exercise: Extending the Picture Type q Although the Picture class is quite useful already, it misses some useful filtering functions such as grayscale, scaling q 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 Grayscale Filter q 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; mandrill.jpg 5

6 y Color Compatibility Scaling Filter q Q. Which font colors will be most readable with which background colors on computer and cell phone screens? q A. Rule of thumb: difference in luminance should be public static boolean compatible(color a, Color b) { return Math.abs(lum(a) - lum(b)) >= 128.0; mandrill.jpg (298-by-298) Image Scaling Scaling Filter q Goal. Shrink or enlarge an image to desired size. q Scaling filter. Creates two Picture objects and two windows. q Assume uniform strategy to convert from w s -by-h s to w t -by-h t : (x w s / w t, y h s / h t )? (x,y) source image (w s -by-h s ) target image (w t -by-h t ) mandrill.jpg (298-by-298) % java Scale mandrill.jpg 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!"); Problem q 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

7 Defining Methods in the Child Class: Overriding Methods q A child class can (have the option to) override the definition of an inherited method in favor of its own that is, a child can redefine a method that it inherits from its parent the new method must have the same signature as the parent's method, but can have different code in the body 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"; q The method invoked is always the one defined in the child class, if the child class refines (overrides) a method public void sue() { System.out.println("I'll see you in court!"); Overloading vs. Overriding Marketer class q Overloading deals with multiple methods in the same class with the same name but different signatures q Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature Exercise: Complete the Marketer class. Marketers can advertise and make 20% more than the base ($60,000 total). q Overloading lets you define a similar operation in different ways for different data q Overriding lets you define a similar operation in different ways for different object types Marketer class A Problem // A class to represent marketers. public class Marketer extends Employee { public void advertise() { System.out.println("Act while supplies last!"); public class Marketer extends Employee { return ;... // override return ; // $60, / year 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. Problem of design?

8 Changes to Common Behavior q Imagine a company-wide change affecting all employees. q 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. q We must modify our code to reflect this policy change. Modifying the superclass // A class to represent employees in general (20-page manual). public int hours() { return 40; // works 40 hours / week return ;... // $60, / year q Issue: the Marketer subclass is still incorrect. It has overridden pay to return another value. q Requirement: derived behavior is based on base behavior Calling overridden methods Subclasses can call overridden methods with super super.<method>(<parameters>) Improved subclasses public class Marketer extends Employee { public void advertise() { System.out.println("Act now while supplies last!"); Exercise: Modify Marketer to derive pay for marketers from base pay. // override and invoke the parent s version return super.pay() * 1.2; Exercise: Revise Lawyer to Maintain Consistency 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!");