1. Chapter 4: Writing Classes

2. Writing Classes We've been using predefined classes.
Now we will learn to write our own classes to define new objects
Chapter 4 focuses on:
class declarations
method declarations
instance variables
encapsulation
method overloading
graphics-based objects

3. Objects An object has: For example, a particular bank account
state - descriptive characteristics
behaviors or actions - what it can do (or be done to it)
For example, a particular bank account
has an account number
has a current balance
can be deposited into
can be withdrawn from

4. Classes A class is a blueprint of an object Instantiation For example
A model or pattern from which objects are created
A class defines the methods and types of data associated with an object
Instantiation
Creating an object from a class is called instantiation;
an object is an instance of a particular class
For example
Account class could describe many bank accounts,
toms_savings is a particular bank account with a particular balance

5. Classes
The String class was provided for us by the Java standard class library
But we can also write our own classes that define specific objects that we need
For example, suppose we wanted to write a program that simulates the flipping of a coin
We could write a Coin class to represent a coin object

6. Classes A class contains data declarations and method declarations
int x, y;
char ch;
Data declarations
Method declarations

7. Defining Classes The syntax for defining a class is:
class class-name {
declarations
constructors
methods }
The variables, constructors, and methods of a class are generically called members of the class

8. Defining Classes class Account { int account_number; double balance;
Account (int account, double initial) {
account_number = account;
balance = initial;
} // constructor Account
void deposit (double amount) {
balance = balance + amount;
} // method deposit
} // class Account

9. Data Scope The scope of data
the area in a program in which that data can be used (referenced)
Data declared at the class level can be used by all methods in that class
Local data or variable
Data declared within a method can only be used in that method

10. Constructors A constructor:
is a special method that is used to set up a newly created object
often sets the initial values of variables
has the same name as the class
does not return a value
has no return type, not even void
The programmer does not have to define a constructor for a class

11. Constructors Constructs can take parameters
often used to initialize some variables in the object
For example, the Account constructor could be set up to take a parameter specifying its initial balance:
Account toms_savings = new Account (1,125.89);

12. Classes and Objects
A class defines the data types for an object, but a class does not store data values
Each object has its own unique data space(memory)
instance variables
The variables defined in a class because each instance of the class has its own
Methods are shared among all objects of a class
All methods in a class have access to all instance variables of the class

13. Classes and Objects Objects Class account_number balance 2908371
573.21
Class
int account_number
double balance
account_number
balance

14. Object References
The declaration of the object reference variable and the creation of the object can be separate activities:
Account toms_savings;
toms_savings = new Account (1, );
Once an object exists, its methods can be invoked using the dot operator:
toms_savings.deposit (35.00);

15. References An object reference holds the memory address of an object
Chess_Piece bishop1 = new Chess_Piece();
All interaction with an object occurs through a reference variable
References have an effect on actions such as assignment
bishop1

16. Assignment
The act of assignment takes a copy of a value and stores it in a variable
For primitive types:
num2 = num1;
Before
num1 5
num2 12
After
num1 5
num2

17. Reference Assignment
For object references, the value of the memory location is copied:
bishop2 = bishop1;
Before
bishop1
bishop2
After
bishop1
bishop2

18. Writing Methods
A method declaration specifies the code that will be executed when the method is invoked (or called)
When a method is invoked, the flow of control jumps to the method and executes its code
When complete, the flow returns to the place where the method was called and continues
The invocation may or may not return a value, depending on how the method was defined

19. Method Control Flow
The called method could be within the same class, in which case only the method name is needed
myMethod();
myMethod
compute

20. Method Control Flow
The called method could be part of another class or object
obj.doIt();
main
doIt
helpMe
helpMe();

21. Method Declarations A method declaration begins with a method header
char calc (int num1, int num2, String message)
method
name
parameter list
The parameter list specifies the type
and name of each parameter
The name of a parameter in the method
declaration is called a formal argument
return
type

22. Method Declarations The method header is followed by the method body
char calc (int num1, int num2, String message) {
int sum = num1 + num2;
char result = message.charAt (sum);
return result; }
sum and result
are local data
They are created each time the method is called, and are destroyed when it finishes executing
The return expression must be
consistent with the return type

23. The return Statement
The return type of a method indicates the type of value that the method sends back to the calling location
A method that does not return a value has a void return type
The return statement specifies the value that will be returned
Its expression must conform to the return type

24. Parameters
Each time a method is called, the actual arguments in the invocation are copied into the formal arguments
ch = obj.calc (25, count, "Hello");
char calc (int num1, int num2, String message) {
int sum = num1 + num2;
char result = message.charAt (sum);
return result; }

25. Encapsulation You can take one of two views of an object:
internal - the structure of its data, the algorithms used by its methods
external - the interaction of the object with other objects in the program
From the external view, an object is an encapsulated entity, providing a set of specific services
These services define the interface to the object

26. Encapsulation An encapsulated object can be thought of as a black box
Its inner workings are hidden to the client, which only invokes the interface methods
Methods
Client
Data

27. Encapsulation An object should be self-governing;
any changes to the object's state (its variables) should be accomplished by that object's methods
We should make it difficult, if not impossible, for another object to "reach in" and alter an object's state
The user, or client, of an object
can request its services,
but it should not have to be aware of how those services are accomplished

28. Visibility Modifiers
We accomplish encapsulation through the appropriate use of visibility modifiers
A modifier
a Java reserved word that specifies particular characteristics of a programming construct
We've used the modifier final to define a constant
Java has three visibility modifiers:
public, private, and protected
We will discuss the protected modifier later

29. Visibility Modifiers public visibility private visibility
Members of a class that are declared with public can be accessed from anywhere
private visibility
Members of a class that are declared with private can only be accessed from inside the class
default visibility
Members declared without a visibility modifier can be accessed by any class in the same package
Java modifiers are discussed in detail in Appendix F

30. Visibility Modifiers
As a general rule, no object's data should be declared with public visibility
Methods that provide the object's services
usually declared with public visibility so that they can be invoked by clients
Public methods are also called service methods
Other methods, called support methods, can be defined that assist the service methods;
they should not be declared with public visibility

31. Writing Classes See BankAccounts.java (page 188)
See Account.java (page 189)
An aggregate object is an object that contains references to other objects
An aggregate object represents a has-a relationship
An Account object is an aggregate object
it contains a reference to a String object (that holds the owner's name)
A bank account has a name

32. BankAccouts.java import Account; public class BankAccounts {
public static void main (String[] args) {
Account acct1 = new Account ("Ted Murphy", 72354, );
Account acct2 = new Account ("Jane Smith", 69713, 40.00);
Account acct3 = new Account ("Edward Demsey", 93757, );
acct1.deposit (25.85);
double smithBalance = acct2.deposit (500.00);
System.out.println ("Smith balance after deposit: " + smithBalance);
System.out.println ("Smith balance after withdrawal: " +
acct2.withdraw (430.75, 1.50));
acct3.withdraw (800.00, 0.0); // exceeds balance
acct1.addInterest();
acct2.addInterest();
acct3.addInterest();
System.out.println ();
System.out.println (acct1);
System.out.println (acct2);
System.out.println (acct3); }

33. Account.java import java.text.NumberFormat; public class Account {
private NumberFormat fmt= NumberFormat.getCurrencyInstance();
private final double RATE = 0.045; // interest rate of 4.5%
private long acctNumber;
private double balance;
private String name;
public Account (String owner, long account, double initial) {
name = owner;
acctNumber = account;
balance = initial; }
public double deposit (double amount) {
if (amount < 0) { // deposit value is negative
System.out.println ();
System.out.println ("Error: Deposit amount is invalid.");
System.out.println (acctNumber + " " + fmt.format(amount));
else balance = balance + amount;
return balance;

34. Account.java public double withdraw (double amount, double fee) {
amount += fee;
if (amount < 0) { // withdraw value is negative
System.out.println ();
System.out.println ("Error: Withdraw amount is invalid.");
System.out.println ("Account: " + acctNumber);
System.out.println ("Requested: " + fmt.format(amount)); }
else if (amount > balance) { // withdraw value exceeds balance
System.out.println ("Error: Insufficient funds.");
System.out.println ("Available: " + fmt.format(balance));
else balance = balance - amount;
return balance;

35. Account.java public double addInterest () {
balance += (balance * RATE);
return balance; }
public double getBalance () {
public long getAccountNumber ()
return acctNumber;
public String toString ()
return (acctNumber + "\t" + name + "\t" + fmt.format(balance));

36. Writing Classes
Sometimes an object has to interact with other objects of the same type
For example, we might add two Rational number objects together as follows:
r3 = r1.add(r2);
One object (r1) is executing the method and another (r2) is passed as a parameter
See RationalNumbers.java (page 196)
See Rational.java (page 197)

37. Overloaded Methods Method overloading Signature
the process of using the same method name for multiple methods
Signature
The signature of each overloaded method must be unique
The signature is based on the number, type, and order of the parameters
The compiler must be able to determine which version of the method is being invoked by analyzing the parameters
The return type of the method is not part of the signature

38. Overloading Methods float tryMe (int x) { return x + .375; } Version 1
float tryMe (int x, float y) {
return x*y; }
Version 2
result = tryMe (25, 4.32)
Invocation

39. Overloaded Methods The println method is overloaded: etc. The lines
println (String s)
println (int i)
println (double d)
etc.
The lines
System.out.println ("The total is:");
System.out.println (total);
invoke different versions of the println method

40. Overloaded Methods
Constructors are often overloaded to provide multiple ways to set up a new object
Account (int account) {
account_number = account;
balance = 0.0;
} // constructor Account
Account (int account, double initial) {
balance = initial;
} // constructor Account

41. SnakeEyes.java import Die; public class SnakeEyes {
public static void main (String[] args) {
final int ROLLS = 500;
int snakeEyes = 0, num1, num2;
Die die1 = new Die(); // creates a six-sided die
Die die2 = new Die(20); // creates a twenty-sided die
for (int roll = 1; roll <= ROLLS; roll++) {
num1 = die1.roll();
num2 = die2.roll();
if (num1 == 1 && num2 == 1) // check for snake eyes
snakeEyes++; }
System.out.println ("Number of rolls: " + ROLLS);
System.out.println ("Number of snake eyes: " + snakeEyes);
System.out.println ("Ratio: " + (float)snakeEyes/ROLLS);

42. Die.java public class Die { private final int MIN_FACES = 4;
private int numFaces; // number of sides on the die
private int faceValue; // current value showing on the die
public Die () {
numFaces = 6;
faceValue = 1; }
public Die (int faces) {
if (faces < MIN_FACES)
else
numFaces = faces;

43. Die.java public int roll () {
faceValue = (int) (Math.random() * numFaces) + 1;
return faceValue; }
public int getFaceValue ()

44. The StringTokenizer Class
The next example makes use of the StringTokenizer class, which is defined in the java.util package
A StringTokenizer object separates a string into smaller substrings (tokens)
By default, the tokenizer separates the string at white space
The StringTokenizer constructor takes the original string to be separated as a parameter
Each call to the nextToken method returns the next token in the string

45. Method Decomposition
A method should be relatively small, so that it can be readily understood as a single entity
A potentially large method should be decomposed into several smaller methods as needed for clarity
Therefore, a service method of an object may call one or more support methods to accomplish its goal
See PigLatin.java (page 207)
See PigLatinTranslator.java (page 208)

46. PigLatin.java import PigLatinTranslator; import cs1.Keyboard;
public class PigLatin {
public static void main (String[] args) {
String sentence, result, another;
PigLatinTranslator translator = new PigLatinTranslator();
do {
System.out.println ();
System.out.println ("Enter a sentence (no punctuation):");
sentence = Keyboard.readString();
result = translator.translate (sentence);
System.out.println ("That sentence in Pig Latin is:");
System.out.println (result);
System.out.print ("Translate another sentence (y/n)? ");
another = Keyboard.readString();
} while (another.equalsIgnoreCase("y")); }

47. PigLatinTranslator.java import java.util.StringTokenizer;
public class PigLatinTranslator {
public String translate (String sentence) {
String result = "";
sentence = sentence.toLowerCase();
StringTokenizer tokenizer = new StringTokenizer (sentence);
while (tokenizer.hasMoreTokens()) {
result += translateWord (tokenizer.nextToken());
result += " "; }
return result;
private String translateWord (String word) {
if (beginsWithVowel(word))
result = word + "yay";
else if (beginsWithPrefix(word))
result = word.substring(2) + word.substring(0,2) + "ay";
else result = word.substring(1) + word.charAt(0) + "ay";

48. PigLatinTranslator.java
private boolean beginsWithVowel (String word) {
String vowels = "aeiouAEIOU";
char letter = word.charAt(0);
return (vowels.indexOf(letter) != -1); }
private boolean beginsWithPrefix (String str) {
return ( str.startsWith ("bl") || str.startsWith ("pl") ||
str.startsWith ("br") || str.startsWith ("pr") ||
str.startsWith ("ch") || str.startsWith ("sh") ||
str.startsWith ("cl") || str.startsWith ("sl") ||
str.startsWith ("cr") || str.startsWith ("sp") ||
str.startsWith ("dr") || str.startsWith ("sr") ||
str.startsWith ("fl") || str.startsWith ("st") ||
str.startsWith ("fr") || str.startsWith ("th") ||
str.startsWith ("gl") || str.startsWith ("tr") ||
str.startsWith ("gr") || str.startsWith ("wh") ||
str.startsWith ("kl") || str.startsWith ("wr") ||
str.startsWith ("ph") );

49. Applet Methods
In previous examples we've used the paint method of the Applet class to draw on an applet
The Applet class has several methods that are invoked automatically at certain points in an applet's life
The init method, for instance, is executed only once when the applet is initially loaded
The Applet class also contains other methods that generally assist in applet processing

50. Graphical Objects
Any object we define by writing a class can have graphical elements
The object must simply obtain a graphics context (a Graphics object) in which to draw
An applet can pass its graphics context to another object just as it can any other parameter
See LineUp.java (page 212)
See StickFigure.java (page 215)

51. LineUp.java import StickFigure; import java.applet.Applet;
import java.awt.*;
public class LineUp extends Applet {
private final int APPLET_WIDTH = 400;
private final int APPLET_HEIGHT = 150;
private final int HEIGHT_MIN = 100;
private final int VARIANCE = 30;
private StickFigure figure1, figure2, figure3, figure4;
public void init () {
int h1, h2, h3, h4; // heights of stick figures
h1 = (int) (Math.random() * VARIANCE) + HEIGHT_MIN;
h2 = (int) (Math.random() * VARIANCE) + HEIGHT_MIN;
h3 = (int) (Math.random() * VARIANCE) + HEIGHT_MIN;
h4 = (int) (Math.random() * VARIANCE) + HEIGHT_MIN;
figure1 = new StickFigure (100, 150, Color.red, h1);
figure2 = new StickFigure (150, 150, Color.cyan, h2);
figure3 = new StickFigure (200, 150, Color.green, h3);
figure4 = new StickFigure (250, 150, Color.yellow, h4);
setBackground (Color.black);
setSize (APPLET_WIDTH, APPLET_HEIGHT); }

52. LineUp.java public void paint (Graphics page) { figure1.draw (page);}

53. StickFigure.java import java.awt.*; public class StickFigure {
private int baseX; // center of figure
private int baseY; // floor (bottom of feet)
private Color color; // color of stick figure
private int height; // height of stick figure
public StickFigure (int center, int bottom, Color shade, int size)
baseX = center;
baseY = bottom;
color = shade;
height = size; }

54. StickFigure.java public void draw (Graphics page) {
int top = baseY - height; // top of head
page.setColor (color);
page.drawOval (baseX-10, top, 20, 20); // head
page.drawLine (baseX, top+20, baseX, baseY-30); // trunk
page.drawLine (baseX, baseY-30, baseX-15, baseY); // legs
page.drawLine (baseX, baseY-30, baseX+15, baseY);
page.drawLine (baseX, baseY-70, baseX-25, baseY-70); // arms
page.drawLine (baseX, baseY-70, baseX+20, baseY-85); }