Inheritance Like father like son Image from:

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:

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

5 Inheritance  Syntax: public class SubClass extends SuperClass { // 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 Class Hierarchies  A child class of one parent can be the parent of another child, forming a class hierarchy Business WebBasedMaAndPa ServiceBusiness HomeBasedService RetailBusiness

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

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 img/superman.gif

10 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(); Oval +void setLocation(int, int) +void setSize(int, int) +void setBackground(Color) +int getX() +int getY() +Color getBackground() +Oval(int, int, int, int) RedDot +RedDot(int,int,int) +void flatten() Example

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

Overriding 13 Oval +setSize(int w, int h) … Target +setSize(int w, int h) … Oval s1 = new Oval(x,y,w,h); … s1.setSize(30, 30); Oval s1 = new Oval(x,y,w,h); … s1.setSize(30, 30); Target t1 = new Oval(x,y,w,h); … t1.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 Overriding Example 1 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(); }

Consider A +A() +void jabberwock() C +C() +void jabberwock() B +B() +void jabberwock() D +D() +void jabberwock() public void foo(A jubJub) { jubJub.jabberwock(); } … foo(new A()); // OK? foo(new B()); // OK? foo(new C()); // OK? foo(new D()); // OK? 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! 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 Overriding Example 2 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(); }

18 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; } BasicCheckbook # balance : int «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + balanceInCents() : int Image from:

19 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; } 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()

20 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; } BasicCheckbook # int balance «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() CheckbookWithStrBalance «constructor» + CheckbookWith...( int, int) «query» + String toString() CheckbookWithTotals # int depositTot # int withdrawTot «constructor» + CheckbookWithTotals( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int deposits() + int withdraws()

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

22 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 ; } 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 ; } public String toString() { String str; if (balance > 0) { str = super.toString(); } else { balance = -balance; str = "(" + super.toString() + ")"; balance = -balance; } return str; } BasicCheckbook # int balance «constructor» + BasicCheckbook( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int balanceInCents() CheckbookWithStrBalance «constructor» + CheckbookWith...( int, int) «query» + String toString() CheckbookWithTotals # int depositTot # int withdrawTot «constructor» + CheckbookWithTotals( int, int) «update» + deposit( int, int) + withdraw( int, int) «query» + int deposits() + int withdraws()

23 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()); } 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

24 Container  This class is meant to be extended to create customized components  Probably want to override setSize for example  Probably don’t need to override getX, 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

Object  All classes descend from Object  Two methods that should often be overridden Object +boolean equals(Object other) +String toString() …others not shown 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

Fraction Class  Write a Fraction class  Represents a fraction (i.e. numerator / denominator)  Support addition and multiplication  Support equality checking and conversion to text Fraction +Fraction() +Fraction(int n,int d) +String toString() +boolean equals(Object other) +Fraction add(Fraction other) +Fraction multiply(Fraction other) // 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)); // 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));

Fraction class 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) { … } } 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) { … } }  What does a Fraction object look like textually?  When are two Fraction objects the same?

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); 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);

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.