1 / 29

Inheritance

Inheritance. Like father like son. Image from: http://www.latimes.com/includes/ramirez/ramirez_20031125.gif. Inheritance. Sometimes classes have commonalities Consider a checking/savings/ cd account Consider a janitor/sales clerk/manager

brennan-greene
Télécharger la présentation

Inheritance

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Inheritance Like father like son Image from: http://www.latimes.com/includes/ramirez/ramirez_20031125.gif

  2. Inheritance • Sometimes classes have commonalities • Consider a checking/savings/cd account • Consider a janitor/sales clerk/manager • Inheritance is a way of writing common code once, and using it in many classes • Commonalities are captured in a super-class • Also known as a base class or parent class • Differences are captured in sub-classes • Also known as a child class Image from: http://www.glenbard.net/Glenbard_East/Activities/media/GIFs/Businessman%20yelling%20into%20b.gif

  3. Inheritance • Software reuse is at the heart of inheritance • The sub-class class inherits all properties of the parent • all methods • all variables • Inheritance relationships can be depicted in a UML class diagram • boxes represent classes • lines represent relationships between classes • an arrow with an open arrowhead pointing to the parent class indicates inheritance

  4. Vehicle Car Motorcycle Inheritance Inheritance creates an is-a relationship The child is a more specific kindof the parent class Vehicle { // data and methods } class Car extends Vehicle { // more data and methods } class Motorcycle extends Vehicle { // more data and methods } UML Diagram indicating inheritance Java code indicating inheritance

  5. Inheritance • Syntax: public class SubClassextendsSuperClass { // data and methods } • Read “SubClass is a specialized version of SuperClass” • All Oval’s have width and height • Circle is a specialized oval since the width and height are always the same.

  6. Business RetailBusiness ServiceBusiness WebBased MaAndPa HomeBasedService Class Hierarchies • A child class of one parent can be the parent of another child, forming a class hierarchy

  7. The super Reference • Constructors are NOT inherited • A child’s constructor is responsible for calling the parent’s constructor • The word super • can be used as a method to mean “the parent classes constructor” • can be used as a direct reference to the super class • The first line of a child’s constructor should use the super reference to call the parent’s constructor

  8. The super Reference class Account { protected double balance; Account(double openingBalance) { balance = openingBalance; } } class MoneyMarketAccount extends Account { protected double annualInterestRate; MoneyMarketAccount(double openingBalance, double rate) { balance = openingBalance; // could you do this annualInterestRate = rate; // could you do this? } } image from http://www.gamasutra.com/features/designers_notebook/superman.gif

  9. The super Reference class Account { protected double balance; Account(double openingBalance) { balance = openingBalance; } } class MoneyMarketAccount extends Account { protected double annualInterestRate; MoneyMarketAccount(double openingBalance, double rate) { super(openingBalance); // should do this annualInterestRate = rate; // should do this } } image from http://www.menzies.us/ img/superman.gif

  10. Example import java.awt.Color; public class RedDot extends Oval { public RedDot( int x, int y, int d ) { super(x, y, d, d); setBackground( Color.red ); } public void flatten() { setSize(getWidth()+1, getHeight()-1); repaint(); } } // assume a Jframe named window RedDot dotty = new RedDot(10, 20, 5); window.add(dotty,0); dotty.repaint(); dotty.setLocation(20, 30); dotty.flatten();

  11. Conformance • When performing assignment “x = y;” • Y must conform to X • If X and Y are primitives then the type of y must • be the identical to the type of X • OR widen to the type of X • Otherwise • The class of Y must be identical to the class of X • OR the class of Y must be a subclass of X

  12. Overriding Methods • A method in a super-class can be overridden in a sub-class. • The sub-class method must have exactly the same signature • same access, return type, name, formal parameter list • The sub-classes method is able to re-define the behavior of the super-class method http://gladstone.uoregon.edu/~edetzler/n.w.riding/images/fly.gif

  13. Overriding Oval s1 = new Oval(x,y,w,h); … s1.setSize(30, 30); Target t1 = new Oval(x,y,w,h); … t1.setSize(30, 30);

  14. Dynamic Dispatch • Mechanism used to determine which method to actually call • Determined at run time • The class of the actual calling object is determined • If that class contains the specified method, then execute it, otherwise look in the super-class for the method. Repeat if needed!

  15. class A { int i, j; A(int a, int b) { i = a; j = b; } void show() { System.out.println(“i and j: “ + i + “,” + j); } } class B extends A { int k; B(int a, int b, int c) { super(a, b); k = c; } void show() { System.out.println(“k: “ + k); } } class Example { public static void main(String[] args) { B aBThing = new B(1,2,3); aBThing.show(); } } OverridingExample 1

  16. Consider public void foo(A jubJub) { jubJub.jabberwock(); } … foo(new A()); // OK? foo(new B()); // OK? foo(new C()); // OK? foo(new D()); // OK? Compile time: an object is treated as it’s declared type. Compiler ensures that the runtime code will work – not what the runtime code exactly means! Runtime: an object is treated according to it’s actual (most specific) type!

  17. class A { public void foo() { System.out.println(“inside foo A”); } } class B extends A { public void foo() { System.out.println(“inside foo B”); } } class C extends B { public void foo() { System.out.println(“inside foo C”); } } class Demo { public static void main(String[] args) { A a = new A(); B b = new B(); C c = new C(); A reference; reference = a; reference.foo(); reference = b; reference.foo(); reference = c; reference.foo(); } } OverridingExample 2

  18. BasicCheckbook # balance : int «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + balanceInCents() : int public class BasicCheckbook { protected int balance; // in cents /* postcondition balance == bd*100 + bc */ public BasicCheckbook(int bd, int bc) { balance = bd*100 + bc; } /* postcondition balance == old balance + dd*100 + dc */ public void deposit(int dd, int dc) { balance = balance + dd*100 + dc; } /* postcondition balance == old balance - (wd*100 + wc) */ public void withdraw(int wd, int wc) { balance = balance - (wd*100 + wc); } /* postcondition result == balance */ public int balanceInCents() { return balance; } } Image from: http://www.southernunited.com/sufi_images/page1/checkbook.jpg

  19. BasicCheckbook # balance : int «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() CheckbookWithStrBalance «constructor» + CheckbookWithStr...( int, int) «query» + String toString() public class CheckbookWithStrBalance extends BasicCheckbook { public CheckbookWithStrBalance(int bd, int bc) { super(bd, bc); } public String toString() { String dollarStr, centStr; int cents; dollarStr = "" + (balance / 100); cents = balance % 100; if (cents < 10) { centStr = "0" + cents; } else { centStr = "" + cents; } return "$" + dollarStr + "." + centStr; } }

  20. BasicCheckbook # int balance «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() CheckbookWithTotals # int depositTot # int withdrawTot «constructor» + CheckbookWithTotals( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int deposits() + int withdraws() CheckbookWithStrBalance «constructor» + CheckbookWith...( int, int) «query» + String toString() public class CheckbookWithTotals extends CheckbookWithStrBalance { protected int depositTot, withdrawTot; public CheckbookWithTotals(int bd, int bc) { super(bd, bc); depositTot = 0; withdrawTot = 0; } public void deposit(int dd, int dc) { super.deposit(dd, dc); depositTot = depositTot + dd*100 + dc; } public void withdraw(int wd, int wc) { super.withdraw(wd, wc); withdrawTot = withdrawTot - (wd*100 + wc); } public int deposits() { return depositTot; } public int withdraws() { return withdrawTot; } }

  21. Permit overdraft (negative balance) (charge $10 for each transaction in the red)

  22. BasicCheckbook # int balance «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() CheckbookWithTotals # int depositTot # int withdrawTot «constructor» + CheckbookWithTotals( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int deposits() + int withdraws() CheckbookWithStrBalance «constructor» + CheckbookWith...( int, int) «query» + String toString() public class CheckbookWithRedInk extends CheckbookWithTotals { public CheckbookWithRedInk(int bd, int bc) { super(bd, bc); } public void deposit(int dd, int dc) { super.deposit(dd, dc); if (dd*100+dc < 0 && balance < 0) { System.out.println("$10 surcharge"); balance = balance - 1000; } } public void withdraw(int wd, int wc) { super.withdraw(wd.wc); if (wd*100+wc > 0 && balance < 0) { System.out.println("$10 surcharge"); balance = balance - 1000; } } public String toString() { String str; if (balance > 0) { str = super.toString(); } else { balance = -balance; str = "(" + super.toString() + ")"; balance = -balance; } return str; } }

  23. CheckbookWithRedInk # int balance # int depositTot # int withdrawTot «constructor» + CheckbookWithRedInk( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() + String toString() + int deposits() + int withdraws() A flattened version of the class A client public class Director { private CheckbookWithRedInk checkbook; public Director() { checkbook = new CheckbookWithRedInk( 100, 0 ); checkbook.deposit( 20, 0 ); checkbook.withdraw( 125, 99 ); System.out.println("Final Balance: " + checkbook.toString()); } }

  24. Container • This class is meant to be extended to create customized components • Probably want to overridesetSize for example • Probably don’t need to overridegetX, getY, setLocation, getWidth, getHeight • Example: write a TrafficLight class! • Lights are centered horizontally and distributed vertically • Lights are 3/4 of the width of the light and ¼ of the light height

  25. Object • All classes descend from Object • Two methods that should often be overridden boolean equals(Object other) : returns true if the calling object is equal to the other one and false otherwise String toString() : returns the textual representation of the calling object

  26. Fraction Class • Write a Fraction class • Represents a fraction (i.e. numerator / denominator) • Support addition and multiplication • Support equality checking and conversion to text // Code fragment showing usage Fraction f1 = new Fraction(3,5); Fraction f2 = new Fraction(6,3); Fraction f3 = f1.add(f2); Fraction f4 = f1.multiply(f2); System.out.println(f4.toString()); System.out.println(f4.equals(f3));

  27. Fraction class • What does a Fraction object look like textually? • When are two Fraction objects the same? public class Fraction { private int num, denom; public Fraction(int n, int d) { … } public Fraction() { … } public Fraction multiply(Fraction other) { … } public Fraction add(Fraction other) { … } public String toString() { …} public boolean equals(Object other) { … } }

  28. Equality • Equality can be defined in two ways • Identity equality: two objects are equal iff they are the same object • Content equality: two objects are equal if they have the same content • Both definitions are supported in Java • ==: this operator always means identity equality. Can be applied to objects. • equals method: this method usually means content equality but is defined as identity equality by default. Fraction f1 = new Fraction(3,5); Fraction f2 = new Fraction(3,5); Fraction f3 = new Fraction(5,3); boolean b1 = f1 == f2; boolean b2 = f1 == f3; boolean b3 = f1.equals(f2); boolean b4 = f1.equals(f3);

  29. Summary • Most classes should override toString and equals • The equals method allows a class to define what equality means • The toString method allows a class to define it’s textual look.

More Related