1 Using Objects Chapter 3 Fall 2006 CS 101 Aaron Bloomfield.

1

Using Objects

Chapter 3Fall 2006CS 101Aaron Bloomfield

2

Getting classy Purpose of this chapter

Gain experience creating and manipulating objects from the standard Java types

Why Prepares you for defining your own classes and creating

and manipulating the objects of those classes

3

Values versus objects Numbers

Have values but they do not have behaviors In particular, each has only ONE value (or attribute)

Objects Have attributes and behaviors An object can have multiple values (or attributes)

4

Using objects First, we create an object:

Scanner stdin = new Scanner (System.in);

Most object creation lines look like this

Then we use the object stdin.nextInt(); stdin.nextDouble();

Note that we could have called the object foo, bar, or anything stdin is just what we chose to call it

5

Using Rectangle objects Let’s create some Rectangle objects

Rectangle creation: Rectangle r = new Rectangle (10, 20);

Objects have attributes (or properties): System.out.println (r.width); System.out.println (r.height);

Objects have behaviors (or methods): r.grow (10, 20); r.isEmpty(); r.setLocation (5,4);

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Using String objects Let’s create some String objects

String creation: String s = new String (“Hello world”);

Objects have attributes (or properties): But we can’t access them…

Objects have behaviors (or methods): s.substring(0,6); s.indexOf (“world”); s.toLowerCase();

7

The lowdown on objects Objects are “things” that have properties (attributes) and

behaviors (methods)

We first create one or more objects

We then manipulate their properties and call their methods

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So why bother with objects? Let’s say you want to do a lot of String manipulation

Once you create a String object, all the manipulation methods are contained therein Sun already wrote the methods for us

So we can use String objects instead of writing our own code to get the substring, indexOf, etc.

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More on Strings Strings are used very often

As a shortcut, you can use: String s = “Hello world”;instead of: String s = new String (“Hello world”);

It’s just a shortcut that Java allows

The two lines are almost the same There is a minor difference between the two

Which we’ll get to later

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Visualizing objects

Class (type) name

Attributes (properties)

Methods (behaviors)+ grow (int, int) : void+ isEmpty ( ) : void

+ setLocation ( int, int ) : void+ resize ( int, int ) : void

+ ...

Rectangle

- width = 10- height = 20- ...

11

How well do we understand How well do we understand using objects?using objects?

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For Valentine’s Day…For Valentine’s Day…

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Bittersweets: Dejected Bittersweets: Dejected sayingssayings

I MISS MY EXI MISS MY EX PEAKED AT 17PEAKED AT 17 MAIL ORDERMAIL ORDER TABLE FOR 1TABLE FOR 1 I CRY ON QI CRY ON Q U C MY BLOG?U C MY BLOG? REJECT PILEREJECT PILE PILLOW HUGGINPILLOW HUGGIN

ASYLUM BOUNDASYLUM BOUND DIGNITY FREEDIGNITY FREE PROG FANPROG FAN STATIC CLINGSTATIC CLING WE HAD PLANSWE HAD PLANS XANADU 2NITEXANADU 2NITE SETTLE 4LESSSETTLE 4LESS NOT AGAIN NOT AGAIN

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Bittersweets: Dysfunctional Bittersweets: Dysfunctional sayingssayings

RUMORS TRUERUMORS TRUE PRENUP OKAY?PRENUP OKAY? HE CAN LISTENHE CAN LISTEN GAME ON TVGAME ON TV CALL A 900#CALL A 900# P.S. I LUV MEP.S. I LUV ME DO MY DISHESDO MY DISHES UWATCH CMT UWATCH CMT

PAROLE IS UP!PAROLE IS UP! BE MY YOKOBE MY YOKO U+ME=GRIEFU+ME=GRIEF I WANT HALFI WANT HALF RETURN 2 PITRETURN 2 PIT NOT MY MOMMYNOT MY MOMMY BE MY PRISONBE MY PRISON C THAT DOOR? C THAT DOOR?

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Review Variables of primitive types

int, double, char, boolean, etc. Can assign a value to it Can read a value from it Can’t do much else!

Objects String, Rectangle, etc. Have many parts

Rectangle has width, length, etc. Like a complex type Have methods

String has length(), substring(), etc.

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String methods

length(): returns the String’s length (duh!)

String s = “hello world”;String t = “goodbye”;System.out.println (s.length());System.out.println (t.length());

Prints 11 and 7

Note that calling s.length() is different than calling t.length()! Both return the length But of different Strings

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More String methods Consider

String weddingDate = "August 21, 1976";String month = weddingDate.substring(0, 6);System.out.println("Month is " + month + ".");

What is the output?Month is August.

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More String methods Consider

String fruit = "banana";String searchString = "an";

int n1 = fruit.indexOf(searchString, 0);int n2 = fruit.indexOf(searchString, n1 + 1);int n3 = fruit.indexOf(searchString, n2 + 1);

System.out.println("First search: " + n1);System.out.println("Second search: " + n2);System.out.println("Third search: " + n3);

What is the output?First search: 1Second search: 3Third search: -1

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String program examplesString program examples

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Program WordLength.java

public class WordLength {

public static void main(String[] args) {Scanner stdin = new Scanner(System.in);

System.out.print("Enter a word: ");String word = stdin.next();

int wordLength = word.length();

System.out.println("Word " + word + " has length "

+ wordLength + ".");}

}

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Program demoProgram demo

WordLength.javaWordLength.java

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More String methods

trim() Returns the String without leading and trailing whitespace Whitespace is a space, tab, or return

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Date translation Goal: to translate the date from American format to standard

format

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DateTranslation.java// Convert user-specified date from American to standard format

import java.util.*;

class DateTranslation {

// main(): application entry point static public void main(String args[]) {

// produce a legend (Step 1) // prompt the user for a date in American format (Step 2)

// acquire the input entered by the user (Step 3) // echo the input back (Step 4) // get month entered by the user (Step 5) // get day entered by the user (Step 6) // get year entered by the user (Step 7) // create standard format version of input (Step 8) // display the translation (Step 9)

}}

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DateTranslation.javaDateTranslation.java

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Today’s demotivatorsToday’s demotivators

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Classes vs. ObjectsClasses vs. Objects

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Variables vs. Types The type is the recipe or template for how to create a variable

Examples: int, double, char, boolean, etc. There are only 8 primitive types

There are only a few things you can do with a type: Declare a variable

int x; Use it as a cast

x = (int) 3.5; There is only one of each type

The variable is the actual instance of a type in memory It’s a spot in memory where you store a value You choose the name: width, x, thatThemThereValue, etc. You can have as may variables as you want – but only one type!

Like the difference between a recipe and a bunch of cookies

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How well do we understand How well do we understand variables versus types?variables versus types?

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Classes vs. Objects A class is a user-defined “thing”

Examples: String, Scanner, Rectangle, etc. We’ll start defining our own classes later this semester

Classes are more complex than the primitive types A class is analogous to a type

It’s just more complex and user-defined There can be only one class of each name

An object is an instance of a class There is only one String class, but you can have 100

String objects A object is analogous to a variable

It just is a reference instead

A class is a “template” used for creating objects

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More on classes vs. objects

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How well do we understand How well do we understand classes versus objects?classes versus objects?

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Lots of piercings…Lots of piercings…

This may be a bit disturbing…This may be a bit disturbing…

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ReferencesReferences

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Java and variables Consider:

int x = 7;double d;char c = ‘x’;

The variable name is the actual spot in memory where the value is stored

Note that d does not have a value

7

int x

-

double d

‘x’

char c

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What is a reference A reference is a memory address

References are like pointers in C/C++ But they are not the exact same thing! C++ has references also (in addition to pointers) You may hear me call them pointers instead of references

All objects in Java are declared as references

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References 1 Consider:

int j = 5;String s = “Hello world”;

Java translates that last line into:String s = new String (“Hello world”);

(Not really, but close enough for now)

Note that there is no “new” here

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0x0d4fe1a8

What’s happening in memoryint j = 5;String s = “Hello world”;

Primitive types are never references; only objects

References 2

5

int j

Hello world

String s

Takes up 32 bits(4 bytes) of memory Takes up 32 bits

(4 bytes) of memory

Takes up 12 bytes of memory

At memory location 0x0d4fe1a8

int j = 5;String s = “Hello world”;

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Representation

message

+ length () : int + charAt ( int i ) : char

+ subString ( int m, int n ) : String+ indexOf ( String s, int m ) : int

+ ...

String

- text = "Don't look behind the door!"- length = 27- ...

Statementsint peasPerPod = 8;String message = "Don't look behind the door!“

peasPerPod 8

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Representation

s



+ ...

String

- text = “I love CS 101"- length = 13- ...

String s = “I love CS 101”;int l = s.length();char c = s.charAt (3);String t = s.subString(1,2);int t = s.indexOf (t, 0);

A period means “follow the reference”

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Shorthand represntation Consider:

String s = “Hello world”;

Takes up a lot of space on my slides…

So we’ll use a shorthand representation:

s



+ ...

String

- text = “Hello world"- length = 11- ...

“Hello world" s

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Examples Consider

String a = "excellence“;String b = a;

What is the representation?

"excellence"a

b

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Consider:String s1 = “first string”;String s2 = “second string”;s2 = s1;System.out.println (s2);

“first string”

References 3

String s1

String s2

“second string”

What happensto this?

String s1 = “first string”;String s2 = “second string”;s2 = s1;System.out.println (s2);

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Java’s garbage collection If an object in memory does not have a reference pointing to

it, Java will automagically delete the object

This is really cool!

In C/C++, you had to do this by yourself

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An optical illusionAn optical illusion

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The null referenceThe null reference

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Uninitialized versus null Consider

String dayOfWeek;Scanner inStream;


-dayOfWeek

-inStream

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Uninitialized versus null Consider

String fontName = null;Scanner fileStream = null;


nullfontName

nullfileStream

fontName

fileStreamOR

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The null reference Sometimes you want a reference to point to nothing

Use the null reference:String s = null;

The null reference is equivalent to a memory address of zero (0x00000000) No user program can exist there

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The null reference Consider:

String s = “Hello world”;System.out.println (s.length());

What happens?

Java prints out 11

s



+ ...

String

- text = “Hello world"- length = 11- ...

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The null reference Consider:

String s = null;System.out.println (s.length());

This is called accessing (or following) a null pointer/reference

What happens? Java: java.lang.NullPointerException C/C++: Segmentation fault (core dumped) Windows: …

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What happens in Windows…What happens in Windows…

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So what is a null reference good for?

Let’s say you had a method that returned a String when passed some parameters Normally it returns a valid String

But what if it can’t? How to deal with that?

Return a null reference

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References and memory Most modern computers are 32-bit computers

This means that a reference takes up 32 bits 232 = 4 Gb

This means that a 32-bit machine cannot access more than 4 Gb of memory! Well, without doing some “tricks”, at least Most machines come with 1 Gb memory these days Will come with 4 Gb in a year or so

64-bit machines will have a maximum of 16 exabytes of memory Giga, Tera, Peta, Exa That’s 16 billion Gb!

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MedicineMedicine PhysicsPhysics Public HealthPublic Health

ChemistryChemistry EngineeringEngineering LiteratureLiterature PsychologyPsychology

EconomicsEconomics PeacePeace

BiologyBiology

The 2004 Ig Nobel PrizesThe 2004 Ig Nobel Prizes

"The Effect of Country Music on Suicide.“"The Effect of Country Music on Suicide.“

For explaining the dynamics of hula-hoopingFor explaining the dynamics of hula-hooping

Investigating the scientific validity of the Investigating the scientific validity of the Five-Second RuleFive-Second Rule

The Coca-Cola Company of Great BritainThe Coca-Cola Company of Great Britain

For the patent of the comboverFor the patent of the combover

The American Nudist Research LibraryThe American Nudist Research Library

It’s easy to overlook things – even a man in a It’s easy to overlook things – even a man in a gorilla suit.gorilla suit.

The Vatican, for outsourcing prayers to IndiaThe Vatican, for outsourcing prayers to India

The invention of karaoke, thereby providing The invention of karaoke, thereby providing an entirely new way for people to learn to an entirely new way for people to learn to tolerate each othertolerate each other

For showing that herrings apparently For showing that herrings apparently communicate by fartingcommunicate by farting

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Using object examplesUsing object examples

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Assignment Consider

String word1 = "luminous";String word2 = "graceful";word1 = word2;

Initial representation

Garbage collection

time!

"luminous"word1

"graceful"word2

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Using objects Consider

Scanner stdin = new Scanner(System.in);System.out.print("Enter your account name: ");String response = stdin.next();

Suppose the user interaction isEnter your account name: artiste

Scanner:stdin

"artiste"reponse

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String representation Consider

String alphabet = "abcdefghijklmnopqrstuvwxyz";

Standard shorthand representation

Truer representation

alphabet

a b c d e f g h i j k l m n o p q r s t u v w y z

"abcdefghijklmnopqrstuvwxyz"alphabet

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String representation Consider

String alphabet = "abcdefghijklmnopqrstuvwxyz"; char c1 = alphabet.charAt(9); char c2 = alphabet.charAt(15); char c3 = alphabet.charAt(2);

What are the values of c1, c2, and c3? Why?

'j'c1

'p'c2

'c'c3

"abcdefghijklmnopqrstuvwxyz"alphabet

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Considerint v1 = -12;double v2 = 3.14;char v3 = 'a';String s1 = String.valueOf(v1);String s2 = String.valueOf(v2);String s3 = String.valueOf(v3);

int v1 = -12;double v2 = 3.14;char v3 = 'a';String s1 = String.valueOf(v1);String s2 = String.valueOf(v2);String s3 = String.valueOf(v3);

More String methods

"-12"s1

"3.14"s2

"a"s3

v1 -12

v2 3.14

v3 ‘a’

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Final variables Consider

final String POEM_TITLE = “Appearance of Brown";final String WARNING = “Weather ball is black";


"Appearance of Brown"POEM_TITLE

"Weather ball is black"WARNING

The locks indicate the memory locations holds constants

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Final variables Consider

final String LANGUAGE = "Java";

The reference cannot bemodified once it is

established

"Java"LANGUAGE

6666


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Rectangle

3x

4y

Rectangle:

5width

height 2

r

5

2(3, 4)

The dimensions ofthe new Rectangle

The upper-left-handcorner of the new Rectangle

int x = 3;int y = 4;int width = 5;int height = 2;Rectangle r = new Rectangle(x, y, width, height);

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Considerfinal Rectangle BLOCK = new Rectangle(6, 9, 4,

2);BLOCK.setLocation(1, 4);BLOCK.resize(8, 3);

final Rectangle BLOCK = new Rectangle(6, 9, 4, 2);

BLOCK.setLocation(1, 4);BLOCK.resize(8, 3);

Rectangle

Rectangle:BLOCK

4

2(6, 9)

Rectangle:BLOCK

8

3(1, 4)

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s

t

u

Consider:String s = "Halloween";String t = "Groundhog Day";String u = "May Day";String v = s.substring(0,6);int x = t.indexOf ("Day", 0);int y = u.indexOf ("Day");s = t;u = null;

+ length () : int + subString ( int m, int n ) : String+ indexOf ( String s, int m ) : int+ indexOf ( String s ) : int

+ ...

String

- ...

String method usage

String s = "Halloween";String t = "Groundhog Day";String u = "May Day";String v = s.substring(0,6);int x = t.indexOf ("Day", 0);int y = u.indexOf ("Day");s = t;u = null;

“Groundhog Day"

“May Day"

- text = “Halloween"- length = 9

“Halloween"

- text = “Groundhog Day"- length = 13- text = “May Day"- length = 7

x 10 y 4

“Hallow"

v

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s

t

u

Consider:String s = "Halloween";String t = "Groundhog Day";final String u = "May Day";String v = s.substring(0,6);int x = t.indexOf ("Day", 0);int y = u.indexOf ("Day");s = t;u = null;

String method usage

“Groundhog Day"

“May Day"

“Halloween"

x 10 y 4

“Hallow"

v

s = t;u = null; Java error:Java error:

cannot assign a cannot assign a value to final value to final

variable uvariable u

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Consider:

Rectangle r = new Rectangle();final Rectangle s = new

Rectangle (3, 4, 1, 2);r.setWidth(5);r.setHeight(6);s.setWidth (7);r = new Rectangle (10,11,8,9);s = new Rectangle (12,13,14,15);

Rectangle r = new Rectangle();final Rectangle s = new

Rectangle (3, 4, 1, 2);r.setWidth(5);r.setHeight(6);s.setWidth (7);r = new Rectangle (10,11,8,9);s = new Rectangle (12,13,14,15);

s

Rectangle method usage

r

+ setWidth ( int w )+ setHeight ( int wh )+ setX ( int x )+ setY ( int y )+ ...

Rectangle

+ setWidth ( int w )+ setHeight ( int wh )+ setX ( int x )+ setY ( int y )+ ...

Rectangle

- width = 7- height = 2

- x = 3- y = 4

+ setWidth ( int w )

Rectangle


- x = 10- y = 11


- x = 3- y = 4


- x = 0- y = 0


- x = 0- y = 0


- x = 0- y = 0

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Scanner review To initialize a Scanner object:

Scanner stdin = new Scanner (System.in); Scanner stdin = Scanner.create (System.in);

This one will not work!

To read an int from the keyboard: stdin.nextInt();

To read a double from the keyboard: stdin.nextDouble();

To read a String from the keyboard: stdin.next();

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Scanner usage examples Consider:

Scanner stdin = new Scanner (System.in);int x = stdin.nextInt();double d = stdin.nextDouble();String s = stdin.next();

Scanner:stdin

“hello world”s

d 3.5x 5

Scanner stdin = new Scanner (System.in);int x = stdin.nextInt();double d = stdin.nextDouble();String s = stdin.next();

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Beware!!!Beware!!!

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OverloadingOverloading

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Overloading Consider the ‘+’ operator

It can mean integer addition: 3+5 = 8 It can mean floating-point addition: 3.0+5.0 = 8.0 It can mean string concatenation: “foo” + “bar” =

“foobar”

The ‘+’ operator has multiple “things” it can do a.k.a. the ‘+’ operator is overloaded

78

More on overloading We’ve seen a number of methods

In the String class: substring(), charAt(), indexOf(), etc. In the Rectangle class: setLocation(), translate()

Consider the substring() method in the String class One version: s.substring(3)

This will return a string from the 4th character on Another version: s.substring (3,6)

This version will return a string from the character at index 3 up to (but not including!) the character at index 6

There are multiple versions of the same method Differentiated by their parameter list

The substring method can take one OR two parameters This is called overloading

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More on more on overloading Consider the valueOf() method in the String class

String.valueOf (3) The parameter is an int

String.valueOf (3.5) The parameter is a double

String.valueOf (‘3’) The parameter is a char

There are multiple versions of this method Differentiated by their parameter list Thus, the valueOf() method is overloaded

8080

More on methodsMore on methods

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Accessors Some methods allow us to find out information about an

object In the Rectangle class: getWidth(), getHeight() These methods are called accessors

They allow us to access attributes of the object An accessor is a method that allows us to find out

attributes of object Usually start with get in the method name I won’t use this terminology much, but the book uses it

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Mutators

Some methods allow us to set information about the object In the Rectangle class: setLocation(), setBounds() These methods are called mutators

They allow us to change (or mutate) the attributes of an object

A mutator is a method that allows us to set attributes of object

Usually start with set in the method name I won’t use this terminology much, but the book uses it

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Constructors A constructor is a special method called ONLY when you are

creating (or constructing) and object The name of the constructor is ALWAYS the exact same

name as the class

Scanner stdin = new Scanner (System.in); String foo = new String (“hello world”);

There can be overloaded constructors Rectangle r = new Rectangle(); Rectangle s = new Rectangle (1, 2, 3, 4);

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Calling the Circle constructor To create a Circle object:

Circle c1 = new Circle();

This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point

to the Circle object Calls the constructor with no

parameters (the ‘default’ constructor)

The constructor is always the first method called when creating (or ‘constructing’) an object

c1

Circle

- radius = 0.0- PI = 3.14159…- …

+ Circle()+ Circle (double r)+ …

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Calling the Circle constructor To create a Circle object:

Circle c1 = new Circle(2.0);

This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point

to the Circle object Calls the constructor with 1

double parameters (the ‘specific’constructor)

The constructor is always the first method called when creating (or ‘constructing’) an object

c1

Circle

- radius = 0.0- PI = 3.14159…- …


Circle

- radius = 2.0- PI = 3.14159…- …


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Constructor varieties The default constructor usually sets the attributes of an

object to default values But that’s not why it’s called default (we’ll get to that

later) The default constructor ALWAYS takes in zero parameters

Thus, there can be only one

A specific constructor sets the attributes of the object to the passed values We’ll get to why it’s called a specific constructor later The specific constructor takes in one or more parameters There can be more than one (via overloading)

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Method types review With the exception of constructors, these names are purely

for human categorization

Accessor: allows one to access parts of the object Mutator: allows one to change (mutate) a part of an object Constructor: used to create a object

Default constructor: takes in no parameters Specific constructor: takes in one or more parameters

Facilitator Any method that is not one of the above

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Java documentationJava documentation

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Java documentation

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Java packages Group similar classes together

Packages we will use: java.lang: automatically imported by Java

Contains the clases needed by the Java language java.util: contains Scanner, Vector, etc.

Contains various utility classes java.text: we will use it later in the semester

Contains classes used to manipulate text

Any package (other than java.lang) must be imported to use the classes within it

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Not going over remaining Not going over remaining slides in this setslides in this set

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Example: last semester’s HW J2Example: last semester’s HW J2

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Last semester’s HW J2 Found online at

http://www.cs.virginia.edu/~asb/teaching/cs101-fall05/hws/hwj2/index.html The HW listed 10 steps to be performed

Used the StringBuffer class Which can be found at

http://java.sun.com/j2se/1.5.0/docs/api/java/lang/StringBuffer.html

Strings are immutable Meaning that once you create a String, you can never

change it There are no mutator methods

You can change what the String reference points to, but not the String itself

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Preliminaries

import java.util.*;

public class StringBufferManipulator { public static void main (String args[]) {

// Preliminaries System.out.println ("StringBuffer manipulator\n"); Scanner stdin = new Scanner (System.in);

// Code for steps 1 to 10 will go here

}}

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Step 1 The user needs to enter two strings: one long string (say, 10

or so characters at a minimum) and a shorter string that is contained within the longer string. This input should be obtained via the nextLine() method,

as using the next() method will not read in a string that contains spaces.

// Step 1System.out.println ("Enter a long string");String longString = stdin.nextLine();System.out.print ("\nEnter a shorter string within “);System.out.println (“the long string");String shortString = stdin.nextLine();System.out.println ();

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Step 2 Create a StringBuffer object from the longer string -- this is

the StringBuffer that you will manipulate for the rest of the homework. There are two ways to do this: create a default constructred StringBuffer, and append() the long string to that, or use the StringBuffer with the appropriate specific constructor.

// Step 2StringBuffer buffer = new StringBuffer(longString);

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Step 3 Include, as a comment in your program, the code for creating

the StringBuffer in the other way from step 2.

// Step 3// StringBuffer buffer = new StringBuffer();// buffer.append(longString();

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Step 4 Find the position of the small string within the StringBuffer,

and save that position.

// Step 4int pos = buffer.indexOf(shortString);

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These images are not These images are not animated…animated…

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Step 5 Delete the small string from the StringBuffer, and print out

the result.

// Step 5int shortLength = shortString.length();buffer.delete (pos, pos+shortLength);System.out.println (buffer);

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Step 6 Insert "CS101" into the position of the StringBuffer where the

small string was originally found (from step 3), and print out the result

// Step 6buffer.insert (pos, "CS101");System.out.println (buffer);

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Step 7 Remove the last word from the string. You can assume that

everything from the last space (found via lastIndexOf()) to the end of the String is the last word. Print out the result.

// Step 7pos = buffer.lastIndexOf(" ");int bufferLength = buffer.length();buffer.delete(pos, bufferLength);System.out.println (buffer);

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Step 8 Append " rocks" to the end of the StringBuffer, and print out

the result. Note that there is a space before the work 'rocks'.

// Step 8buffer.append (" rocks");System.out.println (buffer);

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Step 9 Delete the character at position n/2, where n is the length of

the StringBuffer. Print out the result.

// Step 9int n = buffer.length();buffer.deleteCharAt (n/2);System.out.println (buffer);

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Step 10 Reverse the StringBuffer, and print out the result.

// Step 10buffer.reverse();System.out.println (buffer);

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StringBufferManipulator.javaStringBufferManipulator.java

1 Using Objects Chapter 3 Fall 2006 CS 101 Aaron Bloomfield.

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