CS 340 DATA STRUCTURESLecture: Everything is an Object

CS 340 2

OOP or… Object Oriented Programming

CS 340 3

Object-Oriented Languages• Smalltalk, C++, Java, etc…• You can make any kind of objects you want• How different from procedural languages?

• No different at all: Every (reasonable) language is “Turing complete”

• Very different: Make expression easier, less error-prone

CS 340 4

O-O Languages (Alan Kay)• Everything is an object. • A program is a bunch of objects telling each other what to

do, by sending messages. • Each object has its own memory, and is made up of other

objects. • Every object has a type (class). • All objects of the same type can receive the same

messages.

CS 340 5

Objects• An object has an interface, determined by its class.• A class is an abstract data type, or user-defined type.• Designing a class means defining its interface.

CS 340 6

Built-In Types• Think of an int…

• What is its interface?• How do you “send it messages”?• How do you make one?• Where does it go when you’re done with it?

• In solving a computational problem, the goal is to• Invent useful classes, and• Give them appropriate characteristics.

CS 340 7

Example• Suppose I’ve defined this class in Java:

• To make one, I typeBottleOfBeer myBeer = new BottleOfBeer();

• If I want myBeer opened, I saymyBeer.open();

BottleOfBeer

+open():void+lift(height:int):void+tilt(angle:int):void

CS 340 8

But Why Not Just…

• This is legal, but just makes a “reference variable” named myBeer

• This variable can refer to any BottleOfBeer object, but currently refers to nothing

• The operator new actually causes an object to be created, so we tell it what kind we want

BottleOfBeer myBeer;

CS 340 9

Designers Design, Users Use

• The interface is the critical part, but the details (implementation) are important too.

• Users use the interface (the “public part”); the implementation is hidden by “access control”.

BottleOfBeer

-topOn:Boolean

+open():void+lift(height:int):void+tilt(angle:int):void

CS 340 10

Two Ways of Reusing Classes• Composition: One class has another as a part (indicated

by the diamond “aggregation” symbol).

BottleOfBeer

+open():void+lift(height:int):void+tilt(angle:int):void

TavernCaseOfBeer

Patron

CS 340 11

Two Ways of Reusing Classes• Inheritance: One class is a specialized version of another

(indicated by the triangle “inheritance” symbol).

BottleOfBeer

+open():void+lift(height:int):void+tilt(angle:int):void

BottleOfRollingRock

-lowCalorie:Boolean

+isLowCalorie():Boolean

CS 340 12

Polymorphism• Different subclasses respond to the same message,

possibly with different actions.

Patron

+beerPlease():Polite

BritishPatronAmericanPatron

+beerPlease():Rude

GermanPatron

+beerPlease():InGerman

CS 340 13

Some Java CodePatron p1 = new Patron(); Patron p2 = new YankPatron(); Patron p3 = new BritPatron();

Patron p4 = new GermanPatron(); p1.BeerPlease() // polite request p2. BeerPlease() // rude request p3.BeerPlease() // polite request

p4.BeerPlease() // request in German (but polite)

• This is a bit of a trick: it requires late binding of the function call.

CS 340 14

QUESTIONS?

CS 340 15

Creating Objects• We usually assume this is free; with built-in types like int

or char, we just say

int i; char c;

• With user-defined types (the ones we make), we need to be explicit about what we want: • constructor function

• This is a very important issue!

CS 340 16

Destroying Objects• If an object goes “out of scope,” it can no longer be used

(its name is no longer known).• Java uses references and “garbage collection”.

CS 340 17

Example of Object Scope

• What happens to lect?• The LectureNotes object still exists, but the reference lect disappears (it’s out of scope after return).

• Eventually, the garbage collector removes the actual LectureNotes object.

public String getTitle(int lectureNumber) { LectureNotes lect; lect = syllabus.getLecture(lectureNumber); String s = lect.getLine(1); return s;}

CS 340 18

Java’s Use of Memory• Stack• Heap• Static variables• Constants• Non-RAM storage

CS 340 19

Java’s Primitive TypesType Size Wrapper type

boolean - Boolean

char 16-bit Character

byte 8-bit Byte

short 16-bit Short

int 32-bit Integer

long 64-bit Long

float 32-bit Float

double 64-bit Double

void - Void

CS 340 20

Wrapper Types• Variables of primitive types are “automatic”, i.e., they are

stored on the stack.• They are automatically deleted when they go out of

scope.• What if you want an object holding a primitive type?

Example:

char c = ‘x’;Character C = new Character(‘x’);

CS 340 21

Really Big Numbers

• BigInteger, BigDecimal• These are arbitrary precision, as big as they need to be.• You can’t use the usual operators (+-*/) since they are

objects. But there are methods (functions) to do these things.

CS 340 22

Creating New Types

class MyNewType {

// definition here

}• Now it’s legal to say

MyNewType m = new MyNewType();

CS 340 23

Class Members• Fields (a.k.a. member variables, data members)• Methods (a.k.a. member functions): they determine the

messages objects can receive• The method argument list specifies what information you pass into

the method

class MyClass { int a; YourClass b; float memberFunction(int x, float f) { return 0; }}

CS 340 24

Let’s Write Something// Our first program. File: HelloDate.java// Note that the file name is exactly the same// as the class name, including capitalization.import java.util.*;

public class HelloDate { public static void main(String[] args) { System.out.println(“Hello, it is ”); System.out.println(new Date()); }}

CS 340 25

QUESTIONS?

CS 340 26

Java design

CS 340 27

Java Design Goals• Simple, object oriented, and familiar• Robust and secure• Architecture neutral and portable• High performance• Interpreted, threaded, and dynamic

CS 340 28

Java abbreviations• JDK: Java Development Kit• JSDK: Java Servlet Development Kit• JVM: Java Virtual Machine• J2EE: Java Platform, Enterprise Edition. A widely used

platform for server programming.

CS 340 29

Java vs C#Program Structure

Operators

Choices

Loops

30

Java vs C#: Program StructureJava C#

package hello;

public class HelloWorld { public static void main(String[] args) { String name = "Java"; // See if an argument was passed from the command line if (args.length == 1) name = args[0];

System.out.println("Hello, " + name + "!"); } }

using System;

namespace Hello { public class HelloWorld { public static void Main(string[] args) { string name = "C#";

// See if an argument was passed from the command line if (args.Length == 1) name = args[0];

Console.WriteLine("Hello, " + name + "!"); } } }

31

Java vs C#: CommentsJava C#

// Single line/* Multiple line *//** Javadoc documentation comments */

// Single line/* Multiple line *//// XML comments on a single line /** XML comments on multiple lines */

32

Java vs C#: Data TypesJava C#

Primitive Types boolean byte char short, int, long float, double

Reference Types Object (superclass of all other classes) Stringarrays, classes, interfaces

Value Types bool byte, sbyte char short, ushort, int, uint, long, ulong float, double, decimalstructures, enumerations

Reference Types object (superclass of all other classes) stringarrays, classes, interfaces, delegates

33

Java vs C#: Data TypesJava C#

Conversions

// int to String int x = 123; String y = Integer.toString(x); // y is "123"

// String to int y = "456"; x = Integer.parseInt(y); // x is 456

// double to int double z = 3.5; x = (int) z; // x is 3 (truncates decimal)

Conversions

// int to string int x = 123; String y = x.ToString(); // y is "123"

// string to int y = "456"; x = int.Parse(y); // or x = Convert.ToInt32(y);

// double to int double z = 3.5; x = (int) z; // x is 3 (truncates decimal)

34

Java vs C#: ConstantsJava C#

// May be initialized in a constructor final double PI = 3.14;

const double PI = 3.14;

// Can be set to a const or a variable. //May be initialized in a constructor. readonly int MAX_HEIGHT = 9;

35

Java vs C#: OperatorsJava C#

Comparison == < > <= >= !=

Arithmetic + - * / % (mod) / (integer division if both operands are ints) Math.Pow(x, y)

Assignment = += -= *= /= %= &= |= ^= <<= >>= >>>= ++ --

Bitwise & | ^ ~ << >> >>>

Comparison == < > <= >= !=

Arithmetic + - * / % (mod) / (integer division if both operands are ints) Math.Pow(x, y)

Assignment = += -= *= /= %= &= |= ^= <<= >>= ++ --

Bitwise & | ^ ~ << >>

36

Java vs C#: OperatorsJava C#

Logical && || & | ^ !

Note: && and || perform short-circuit logical evaluations

String Concatenation +

Logical && || & | ^ !

Note: && and || perform short-circuit logical evaluations

String Concatenation +

37

Java vs C#: ChoicesJava C#

greeting = age < 20 ? "What's up?" : "Hello";

if (x < y) System.out.println("greater");

if (x != 100) { x *= 5; y *= 2; } else z *= 6;

greeting = age < 20 ? "What's up?" : "Hello";

if (x < y) Console.WriteLine("greater");

if (x != 100) { x *= 5; y *= 2; } else z *= 6;

38

Java vs C#: ChoicesJava C#

int selection = 2;switch (selection) { // Must be byte, short, int, char, or enum case 1: x++; // Falls through to next case if no break case 2: y++; break; case 3: z++; break; default: other++; }

string color = "red";switch (color) { // Can be any predefined type case "red": r++; break; // break is mandatory; no fall-through case "blue": b++; break; case "green": g++; break; default: other++; break; // break necessary on default }

39

Java vs C#: LoopsJava C#

while (i < 10) i++;

for (i = 2; i <= 10; i += 2) System.out.println(i);

do i++; while (i < 10);

for (int i : numArray) // foreach construct sum += i;

while (i < 10) i++;

for (i = 2; i <= 10; i += 2) Console.WriteLine(i);

do i++; while (i < 10);

foreach (int i in numArray) sum += i;

40

Java vs C#: LoopsJava C#

// for loop can be used to iterate through any Collection import java.util.ArrayList; ArrayList<Object> list = new ArrayList<Object>();list.add(10); // boxing converts to instance of Integerlist.add("Bisons");list.add(2.3); // boxing converts to instance of Double

for (Object o : list) System.out.println(o);

// foreach can be used to iterate through any collection using System.Collections; ArrayList list = new ArrayList(); list.Add(10); list.Add("Bisons"); list.Add(2.3);

foreach (Object o in list) Console.WriteLine(o);

41

Java vs C#: ArraysJava C#

int nums[] = {1, 2, 3}; or int[] nums = {1, 2, 3};for (int i = 0; i < nums.length; i++) System.out.println(nums[i]);

String names[] = new String[5]; names[0] = "David";

float twoD[][] = new float[rows][cols]; twoD[2][0] = 4.5;

int[][] jagged = new int[3][]; jagged[0] = new int[5]; jagged[1] = new int[2]; jagged[2] = new int[3]; jagged[0][4] = 5;

int[] nums = {1, 2, 3};for (int i = 0; i < nums.Length; i++) Console.WriteLine(nums[i]);

string[] names = new string[5]; names[0] = "David";

float[,] twoD = new float[rows, cols]; twoD[2,0] = 4.5f;

int[][] jagged = new int[3][] { new int[5], new int[2], new int[3] };

jagged[0][4] = 5;

CS 340 42

QUESTIONS?