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• can be withdrawn from

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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

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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

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Classes

• A class contains data declarations and method
  declarations

Data declarations
Method declarations
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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

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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

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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

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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

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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)

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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

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Classes and Objects
Objects
Class
int account_number double balance
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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)

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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
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Assignment

• The act of assignment takes a copy of a value and
  stores it in a variable
• For primitive types
• num2 num1

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Reference Assignment

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

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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

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Method Control Flow

• The called method could be within the same class,
  in which case only the method name is needed

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Method Control Flow

• The called method could be part of another class
  or object

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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
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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
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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

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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")
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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

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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
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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

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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

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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

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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

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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

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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

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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 ()

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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

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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))

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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)

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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

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Overloading Methods
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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

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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

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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

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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

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Die.java

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

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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

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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)

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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)

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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 ""

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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")

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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

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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)

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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

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LineUp.java

• public void paint (Graphics page)
• figure1.draw (page)
• figure2.draw (page)
• figure3.draw (page)
• figure4.draw (page)

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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

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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, top20, baseX,
  baseY-30) // trunk
• page.drawLine (baseX, baseY-30, baseX-15,
  baseY) // legs
• page.drawLine (baseX, baseY-30, baseX15,
  baseY)
• page.drawLine (baseX, baseY-70, baseX-25,
  baseY-70) // arms
• page.drawLine (baseX, baseY-70, baseX20,
  baseY-85)