Java: Understanding Interfaces, Classes, and Exceptions in Complex Number Implementation, Study notes of Object Oriented Programming

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School of Computer Science

Principles of Software Construction:

Objects, Design, and Concurrency

Object-Oriented Programming in Java

Josh Bloch Charlie Garrod

Administrivia

• First homework due this Thursday, 11:59 PM

Collections usage example 3

Unfinished business from last lecture

Print index of first occurrence of each word

class Index {

public static void main(String[] args) {

Map<String, Integer> index = new TreeMap<>();

// Iterate backwards so first occurrence wins

for (int i = args.length - 1; i >= 0; i--) {

index.put(args[i], i);

System.out.println(index);

$ java Index if it is to be it is up to me to do it

{be=4, do=11, if=0, is=2, it=1, me=9, to=3, up=7}

Outline

I. Object-oriented programming basics

II. Information hiding

III. Exceptions

Classes

• Every object has a class

– A class defines methods and fields

– Methods and fields collectively known as members

• Class defines both type and implementation

– type ≈ where the object can be used

– implementation ≈ how the object does things

• Loosely speaking, the methods of a class are its

Application Programming Interface (API)

– Defines how users interact with instances

Class example – complex numbers

class Complex { private double re; // Real Part private double im; // Imaginary Part public Complex(double re, double im) { this.re = re; this.im = im; } public double realPart() { return re; } public double imaginaryPart() { return im; } public double r() { return Math.sqrt(re * re + im * im); } public double theta() { return Math.atan(im / re); } public Complex add(Complex c) { return new Complex(re + c.re, im + c.im); } public Complex subtract(Complex c) { ... } public Complex multiply(Complex c) { ... } public Complex divide(Complex c) { ... } }

Interfaces and implementations

• Multiple implementations of API can coexist

– Multiple classes can implement the same API

– They can differ in performance and behavior

• In Java, an API is specified by interface or class

– Interface provides only an API

– Class provides an API and an implementation

– A Class can implement multiple interfaces

An interface to go with our class

public interface Complex {

// No constructors, fields, or implementations!

double realPart();

double imaginaryPart();

double r();

double theta();

Complex plus(Complex c);

Complex minus(Complex c);

Complex times(Complex c);

Complex dividedBy(Complex c);

An interface defines but does not implement API

Modifying client to use interface

public class ComplexUser { public static void main(String args[]) { Complex c = new OrdinaryComplex(-1, 0); Complex d = new OrdinaryComplex(0, 1); Complex e = c.plus(d); System.out.println(e.realPart() + " + "

• e.imaginaryPart() + "i"); e = c.times(d); System.out.println(e.realPart() + " + "
• e.imaginaryPart() + "i"); } }

When you run this program, it still prints

• 1.0 + 1.0i
• 0.0 + - 1.0i

Interface permits multiple implementations class PolarComplex implements Complex { double r; double theta; public PolarComplex(double r, double theta) { this.r = r; this.theta = theta; } public double realPart() { return r * Math.cos(theta) ; } public double imaginaryPart() { return r * Math.sin(theta) ; } public double r() { return r; } public double theta() { return theta; } public Complex plus(Complex c) { ... } // Completely different impls public Complex minus(Complex c) { ... } public Complex times(Complex c) { ... } public Complex dividedBy(Complex c) { ... } }

Why multiple implementations?

• Different performance

– Choose implementation that works best for your use

• Different behavior

– Choose implementation that does what you want

– Behavior must comply with interface spec (“contract”)

• Often performance and behavior both vary

– Provides a functionality – performance tradeoff

– Example: HashSet, TreeSet

Java interfaces and classes

• Organize program program objects types

– Each type offers a specific set of operations

– Objects are otherwise opaque

• Interfaces vs. classes

– Interface: specifies expectations

– Class: delivers on expectations (the implementation)

Check your understanding

interface Animal {

void vocalize();

class Dog implements Animal {

public void vocalize() { System.out.println("Woof!"); }

class Cow implements Animal {

public void vocalize() { moo(); }

public void moo() {System.out.println("Moo!"); }

What Happens?

1. Animal a = new Animal();

a. vocalize();

2. Dog d = new Dog();

d.vocalize();

3. Animal b = new Cow();

b.vocalize();

4. b.moo();

Historical note: simulation and the origins

of OO programming

• Simula 67 was the first object-oriented language
• Developed by Kristin

Nygaard and Ole-Johan

Dahl at the Norwegian

Computing Center

• Developed to support discrete-event simulation
  • Application: operations research, e.g. traffic analysis
  • Extensibility was a key quality attribute for them
  • Code reuse was another