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Chapter 4: Writing Classes

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Title: Chapter 4: Writing Classes

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
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can be withdrawn from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
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

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
int account_number double 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, 125.89)
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

17
Reference Assignment

For object references, the value of the memory
location is copied
bishop2 bishop1

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

20
Method Control Flow

The called method could be part of another class
or object

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")
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
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, 102.56)
Account acct2 new Account ("Jane Smith",
69713, 40.00)
Account acct3 new Account ("Edward
Demsey", 93757, 759.32)
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

33
Account.java

import java.text.NumberFormat
public class Account
private NumberFormat fmt NumberFormat.getCurre
ncyInstance()
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 lt 0) // deposit value is
negative
System.out.println ()

34
Account.java

public double withdraw (double amount, double
fee)
amount fee
if (amount lt 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 gt balance) // withdraw
value exceeds balance
System.out.println ()
System.out.println ("Error
Insufficient funds.")
System.out.println ("Account "
acctNumber)
System.out.println ("Requested "
fmt.format(amount))
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 ()
return balance
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
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
39
Overloaded Methods

The println method is overloaded
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)
account_number account
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 lt ROLLS roll)
num1 die1.roll()
num2 die2.roll()
if (num1 1 num2 1) // check
for snake eyes
snakeEyes

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 lt MIN_FACES)
numFaces 6
else
numFaces faces
faceValue 1

43
Die.java

public int roll ()
faceValue (int) (Math.random()
numFaces) 1
return faceValue
public int getFaceValue ()
return faceValue

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()
System.out.println ()
result translator.translate
(sentence)
System.out.println ("That sentence in
Pig Latin is")
System.out.println (result)

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)
String result ""

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")

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