1 Using Objects Chapter 3 Fall 2005 CS 101 Aaron Bloomfield.

Using Objects

Chapter 3Fall 2005CS 101Aaron Bloomfield

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

Values versus objects Numbers

Have values but they do not have behaviors

Objects Have attributes and behaviors

System.in References an InputStream

Attribute: keyboard Behaviors: reading

System.out References an OutputStream

Attribute: monitor Behaviors: printing

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

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.length); System.out.println (r.width);

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

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()

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

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.

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

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- ...

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

format

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

Sidewalk chalk guySidewalk chalk guy

Source: http://www.gprime.net/images/sidewalkchalkguy/Source: http://www.gprime.net/images/sidewalkchalkguy/

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

double d

‘x’

char c

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

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 this lecture)

Note that there is no “new” here

0x0d4fe1a8

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

Primitive types are never references; only objects

References 2

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

Other Java object types String

Rectangle

JFrame

Representation

peasPerPod

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!“

Representation

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);

message

peasPerPod

"Don't look behind the door!"

Shorthand representation Statements

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

Examples Consider

String a = "excellence“;String b = a;

What is the representation?

"excellence"a

Uninitialized versus null Consider

String dayOfWeek;Scanner inStream;

-dayOfWeek

-inStream

String fontName = null;Scanner fileStream = null;

nullfontName

nullfileStream

fontName

fileStreamOR

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

The null reference Consider:

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

What happens?

Java prints out 11

String

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

Computer bugsComputer bugs

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: …

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

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!

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

length = 12

“first string”

References 4

String s1

String s2

“second string”

length = 13

What happensto this?

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

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

Assignment Consider

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

Initial representation

Garbage collection

"luminous"word1

"graceful"word2

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

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

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?

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 + ".");}

Program demoProgram demo

An optical illusionAn optical illusion

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.

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

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.

Variable declaration Consider:

int x = 5; int x = 7;

Java won’t allow this You can only declare a variable once At the int x=7; line, Java already has a x spot in memory

It can’t have two

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

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

What is the output?Month is August.

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

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 + ".");}

PhoneNumberFun.javaimport java.util.*;

public class PhoneNumberFun {

// main(): demonstrates a simple String manipulation public static void main(String[] args) {

// determine the area code

// determine the local number

// arrange result

// display input and result

Lots of piercings…Lots of piercings…

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

More String methods

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

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)

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

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

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

Examples: String, Scanner, Rectangle, etc. We’ll start defining our own classes next chapter

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

More on classes vs. objects

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

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

ReferencesReferences

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

double d

‘x’

char c

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

References 1 Consider:

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

(Not really, but close enough for this lecture)

Note that there is no “new” here

0x0d4fe1a8

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

Primitive types are never references; only objects

References 2

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

Representation

peasPerPod

message

String

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

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

Representation

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);

message

peasPerPod

"Don't look behind the door!"

Shorthand representation Statements

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

Examples Consider

String a = "excellence“;String b = a;

"excellence"a

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

length = 12

“first string”

References 3

String s1

String s2

“second string”

length = 13

What happensto this?

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

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

Warn your grandparents!Warn your grandparents!

Historically, this class has been lethal Historically, this class has been lethal to grandparents of students in the to grandparents of students in the classclass– More often grandmothersMore often grandmothers

This happens most around test timeThis happens most around test time– Although occasionally around the times Although occasionally around the times

a big assignment is duea big assignment is due

String dayOfWeek;Scanner inStream;

-dayOfWeek

-inStream

String fontName = null;Scanner fileStream = null;

nullfontName

nullfileStream

fontName

fileStreamOR

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

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

What happens?

Java prints out 11

String

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

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: …

What happens in Windows…What happens in Windows…

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

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!

Why speling is not so Why speling is not so important…important…

I cdnuolt blveieetaht I cluod aulaclty uesdnatnrd I cdnuolt blveieetaht I cluod aulaclty uesdnatnrd waht I was rdanieg. The phaonmneal pweor of waht I was rdanieg. The phaonmneal pweor of thehmuan mind. Aoccdrnig to a rscheearch at thehmuan mind. Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht thefrist and lsat ltteer be iprmoatnt tihng is taht thefrist and lsat ltteer be in the rghit pclae. The rset can be a taotl mses in the rghit pclae. The rset can be a taotl mses andyou can sitll raed it wouthit a porbelm. Tihs andyou can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deosnot raed ervey is bcuseae the huamn mnid deosnot raed ervey lteter by istlef, but the wrod as a wlohe. lteter by istlef, but the wrod as a wlohe. Amzanig huh? yaeh and I awlyas thought Amzanig huh? yaeh and I awlyas thought slpeling was ipmorantt.slpeling was ipmorantt.

Assignment Consider

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

Initial representation

Garbage collection

"luminous"word1

"graceful"word2

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

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

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?

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

v1 -12

v2 3.14

v3 ‘a’

Just in time for Valentine’s Just in time for Valentine’s Day…Day…

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

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?

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

Final variables

objecttype

constant

In general, these attributes can bemodified through member methods

The reference cannot bemodified once it is established

Rectangle

Rectangle:

5width

height 2

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);

Final variables Consider

final String LANGUAGE = "Java";

The reference cannot bemodified once it is

established

"Java"LANGUAGE

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

2(6, 9)

Rectangle:BLOCK

3(1, 4)

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"

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"

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

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

variable uvariable u

Rectangle method usage

Consider:

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

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

+ 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

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

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

- x = 0- y = 0

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();

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();

103103

Today’s demotivatorsToday’s demotivators

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 3rd to the 5th character

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

More on 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

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

s.valueOf (3) The parameter is an int

s.valueOf (3.5) The parameter is a double

s.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

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

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

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);

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

Circle

- radius = 0.0- PI = 3.14159…- …

+ Circle()+ Circle (double r)+ …

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

Circle

- radius = 0.0- PI = 3.14159…- …

Circle

- radius = 2.0- PI = 3.14159…- …

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)

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

1 Using Objects Chapter 3 Fall 2005 CS 101 Aaron Bloomfield.

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