1. Lecture Notes – Interface and Polymorphism (Ch 9-10)
Yonglei Tao

2. Polymorphism
A reference of a super class can refer to an object of any descendent class
Allowing objects of different classes to respond to the same method call in different ways
Benefits
No need to use conditional logic
Simplify the client code
Extensible

3. Abstract Classes Represent an abstract concept Cannot be instantiated
public abstract class Employee {
protected String name;
protected double basePay; …
public abstract double calcBiweeklyPay (); }
Represent an abstract concept
Cannot be instantiated
A subclass must define all of its parent’s abstract methods or itself is an abstract class

4. An Example // in class Employee
public abstract double calcBiWeeklyPay ();
// in class Worker
public double calcBiWeeklyPay () { return hours * basePay; }
// in class Manager
public double calcBiWeeklyPay () { return basePay / 26; }
// in client code
Employee list[] = new Employee[30];
List [0] = new Worker (“Tom”, 12.5);
List [1] = new Manager (“John”, 36000);
List [2] = new Worker (“Ed”, 11.25); …
for (int i = 0; i < list.length; i++) {
System.out.print ( list[i] );
System.out.println ( list[i].calcBiWeeklyPay () ); }

5. Dynamic Binding Employee p; Worker w = new Worker(“Tom”, 7.95, 40);
Manager m = new Manager(“John”, 36000);
… // either p = w or p = m
w.calcBinweeklyPay(); // static/early binding
m.calcBinweeklyPay();
p.calcBiweeklyPay(); // dynamic/late binding
The binding of a method call to its definition is performed at runtime for a polymorphic reference

6. Class Shape public abstract class Shape { private String name;
public abstract double area ();
public Shape( String shapeName ) {
name = shapeName; }
final public boolean lessThan ( Shape other ) {
return area() < other.area();
final public String toString () {
return name + " of area " + area();

7. Circle Rectangle
public class Rectangle extends Shape { private double length, width; public Rectangle ( double len, double wid ) { super ( "rectangle" ); length = len; width = wid; } public double area () { return length * width;

8. Class Circle
public class Circle extends Shape { private double radius; public Circle( double radius ) { super ( "circle" ); this.radius = radius; } public double area () { return Math.PI * radius * radius;

9. Class Square public class Square extends Rectangle {
public Square ( double side ) {
super ( side, side ); }

10. public class ShapeTest { final static int MaxSize = 50; public static void main ( String[] args ) { Shape[] shapeList = new Shape [ MaxSize ]; int size = 0, choice; for ( int i = 0; i < 15; i++ ) { // object construction choice = (int) ( Math.random() * 3 ); switch ( choice ) { case 0: shapeList[size++] = new Circle( size*1.25 ); break; case 1: shapeList[size++] = new Rectangle( size+2.5, size*7.0 ); case 2: shapeList[size++] = new Square ( size*5.0 ); }

11. What needs to be done if a new subclass is added?
// processing of objects
for ( int i = 0; i < size; i++ ) {
System.out.println( shapeList[i] ); }
double total = 0;
total += shapeList[i].area();
System.out.println( "The total are is " + total );
What needs to be done if a new subclass is added?

12. Interfaces interface is a reserved word None of the methods in
an interface are given
a definition (body)
public interface Doable {
public void doThis();
public int doThat(); }
A semicolon immediately
follows each method header

13. Interfaces public class CanDo implements Doable {
public void doThis ()
// whatever }
public void doThat ()
// instance variables and methods
implements is a
reserved word
Each method listed
in Doable is
given a definition

14. Interfaces

15. Interface and Its Implementation
public interface Measurable { double getMeasure(); } public class Coin implements Measurable { private double value; private String name; public double getMeasure() { return value; public String getName() { … } ... public class BankAccount implements Measurable { private double balance; public double getMeasure() { return balance; } … }

16. DataSet for Measurable Objects
public class DataSet {
private double sum; private Measurable max; private int count; public void add(Measurable x) { sum = sum + x.getMeasure(); if (count==0 || max.getMeasure()< x.getMeasure()) max = x; count++; }
public double getAverage() { return sum/count; } public Measurable getMaximum() { return max; } }
A reference of an interface can refer to an object of any implementation class

17. public class DataSetTester {
public static void main(String[] args) {
DataSet bankData = new DataSet();
bankData.add(new BankAccount(0));
bankData.add(new BankAccount(10000));
bankData.add(new BankAccount(2000));
System.out.println("Average balance: " + bankData.getAverage());
System.out.println("Expected: 4000");
Measurable max = bankData.getMaximum();
System.out.println("Highest balance: " + max.getMeasure());
System.out.println("Expected: 10000");
DataSet coinData = new DataSet();
coinData.add(new Coin(0.25, "quarter"));
coinData.add(new Coin(0.1, "dime"));
coinData.add(new Coin(0.05, "nickel"));
System.out.println("Average coin value: “ + coinData.getAverage());
System.out.println("Expected: 0.133");
max = coinData.getMaximum();
System.out.println("Highest coin value: " + max.getMeasure());
System.out.println("Expected: 0.25"); }

18. Interfaces vs. Classes
An interface is similar to a class, but there are several important differences:
All methods in an interface are abstract; they don’t have an implementation
All methods in an interface are automatically public
Instance variables in an interface are always static or final
A class may implement multiple interfaces

19. Type Conversion Implicit conversion Explicit conversion
intVar = charExpr;
floatVar = intExpr;
doubleVar = floatExpr;
Worker w = new Worker ();
Employee p = w;
Explicit conversion
char ch = (char) anInt; // anInt = 65
long num = (long) aDouble; // aDouble = 7.99

20. Explicit Conversion
Employee p; Worker w = new Worker(“Tom”, 7.95, 40); Manager m = new Manager(“John”, 36000); … // either p = w or p = m w = p; ? public void aMethod ( Employee p ) { if ( p instanceof Worker ) Worker w = (Worker) p; … }