CHAPTER 2 OBJECTS AND CLASSES

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CHAPTER 2OBJECTS AND CLASSES

• Goals
• To understand the concepts of classes and objects
• To realize the difference between objects and
  object references
• To become familiar with the process of
  implementing classes
• To be able to implement simple methods
• To understand the purpose and use of constructors
• To understand how to access instance fields and
  local variables
• To appreciate the importance of documentation
  comments

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Objects and Classes

• Object entity that you can manipulate in your
  programs (by invoking methods)
• Each object belongs to a class
• Class Set of objects with the same behavior
• Class determines legal methods"Hello".println()
  // Error"Hello".length() // OK
• An example Rectangle Class

• Construct a rectanglenew Rectangle(5, 10, 20,
  30)new Rectangle()
• Use the constructed object
• System.out.println(new Rectangle(5,10,20,30))
• prints java.awt.Rectanglex5,y10,width20,he
  ight30

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Rectangle Shapes
A Rectangle Object
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Syntax 2.1 Object Construction

• Syntax new ClassName(parameters)
• Example
• new Rectangle(5, 10, 20, 30)
• new Car("BMW 540ti", 2004)
• Purpose To construct a new object, initialize it
  with the construction parameters, and return a
  reference to the constructed object.
• Object Variables
• Declare and optionally initialize
• Rectangle cerealBox new Rectangle(5, 10, 20,
  30)Rectangle crispyCrunchy
• Apply methodscerealBox.translate(15, 25)
• Share objectsr cerealBox

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Uninitialized and Initialized Variables
Uninitialized
Initialized
Two Object Variables Referring to the Same Object
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Syntax 2.2 Variable Definition

• Syntax
• TypeName variableName
• TypeName variableName expression
• Example
• Rectangle cerealBox
• String name "Dave"
• Purpose To define a new variable of a particular
  type and optionally supply an initial value

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Writing a Test Program

• Invent a new class, say MoveTest
• Supply a main method
• Place instructions inside the main method
• Import library classes by specifying the package
  and class nameimport java.awt.Rectangle
• You don't need to import classes in the java.lang
  package such as String and System

Syntax 2.3 Importing a Class from a Package

• Syntax import packageName.ClassName
• Example import java.awt.Rectangle
• Purpose To import a class from a package for use
  in a program

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File MoveRect.java

• import java.awt.Rectangle
• public class MoveTest
• public static void main(String args)
• Rectangle cerealBox new Rectangle(5, 10,
  20, 30)
• // move the rectangle
• cerealBox.translate(15, 25) // print the
  moved rectangle
• System.out.println(cerealBox)

public class Greeter public String
sayHello() String message
"Hello,World!" return message
A Simple Class
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Method Definition

• Method prototype (signature)
• access specifier (such as public)
• return type (such as String or void)
• method name (such as sayHello)
• list of parameters (empty for sayHello)
• Method body in

Example
public class Rectangle . . . public void
translate(int x, int y) method body
. . .
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Syntax 2.4 Method Implementation

• Example
• public class Greeter
• public String sayHello()
• String message
• "Hello,World!" return
  message
• Purpose To define the behavior of a method A
  method definition specifies the method name,
  parameters, and the statements for carrying out
  the method's actions

• Syntax
• public class ClassName
• ... accessSpecifier returnType
  methodName(parameterType parameterName,...)
  method body
• ...

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Syntax 2.5 The return Statement

• Example
• return expression
• or
• return
• Example
• return message
• Purpose To specify the value that a method
  returns, and exit the method immediately. The
  return value becomes the value of the method call
  expression.

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Testing a Class

• Test class a class with a main method that
  contains statements to test another class.
• Typically carries out the following steps
• Construct one or more objects of the class that
  is being tested.
• Invoke one or more methods.
• Print out one or more results
• A Test Class for the Greeter Class

public class GreeterTest public static void
main(String args)) Greeter
worldGreeter new Greeter()
System.out.println(worldGreeter.sayHello())

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Building a Test Program
Testing with the SDK Tools

• 1. Make a new subfolder for your program.
• 2. Make two files, one for each class.
• 3. Compile both files.
• 4. Run the test program.

• mkdir greeter
• cd greeter
• edit Greeter.java
• edit GreeterTest.java
• javac Greeter.java
• javac GreeterTest.java
• java GreeterTest

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Testing with BlueJ
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Instance Fields

• public class Greeter
• ...private String name

• access specifier (such as private)
• type of variable (such as String)
• name of variable (such as name)

Accessing Instance Fields

• The sayHello method of the Greeter class can
  access the private instance field, others can not

• public String sayHello() String message
  "Hello, " name "!" return message

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Syntax 2.6 Instance Field Declaration

• Syntax
• accessSpecifier class ClassName ...
  accessSpecifier fieldType fieldName ...
• Example
• public class Greeter ... private
  String name ...
• Purpose To define a field that is present in
  every object of a class

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Constructors
Syntax 2.7 Constructor Implementation

• A constructor initializes the instance variables
• Constructor name class name
• Invoked in new expressionnew Greeter("Dave")

• Syntax
• accessSpecifier class ClassName ...
  accessSpecifier
• ClassName(parameterType
• parameterName ...) constructor
  implementation ...

• Example
• public class Greeter ... public
  Greeter(String aName) name aName
  ...

• Purpose To define the behavior of a constructor,
  which is used to initialize the instance fields
  of newly created objects

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File Greeter.java
File GreeterTest.java
1 public class GreeterTest 2 3 public static
void main(String args) 4 5
Greeter worldGreeter new Greeter("World") 6
System.out.println( worldGreeter.sayHello())
7 8 Greeter daveGreeter new Greeter("Dave")
9 System.out.println(daveGreeter. sayHello())
10 11

• 1 public class Greeter
• 2
• 3 public Greeter(String aName)
• 4
• 5 name aName
• 6
• 7
• 8 public String sayHello()
• 9
• 10 String message "Hello, "
• name "!"
• 11 return message
• 12
• 13
• 14 private String name
• 15

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Designing A Class Bank Account

• Designing Interface
• Constructor
• public BankAccount()
• public BankAccount(double initialBalance)
• Methods
• public void deposit(double amount)
• public void withdraw(double amount)
• public double getBalance()

• Analysis
• Behavior of bank account
• deposit money
• withdraw money
• get balance
• Methods of BankAccount class
• deposit
• withdraw
• getBalance

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Commenting
Commenting interface
/ Gets the current balance of the bank
account. _at_return the current balance / public
double getBalance() implementation filled in
later
/ Withdraws money from the bank account.
_at_param the amount to withdraw / public void
withdraw(double amount) implementation
filled in later

• / A bank account has a balance that can be
  changed by deposits and withdrawals.
• /
• public class BankAccount

Commenting the class
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Javadoc Method Summary
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Javadoc Method Detail
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BankAccount Class Implementation
Using the Interface

• Add interest
• double rate 5 // 5
• double amt
• acct.getBalance() rate / 100
• acct.deposit(amt)

• Transfer balance
• double amt 500
• momsSavings.withdraw(amt)
• harrysChecking.deposit(amt)

• Determine instance variables to hold object state
• private double balance
• Implement methods and constructors

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File BankAccount.java
/ Deposits money into the bank account.
_at_param amount the amount to deposit / public
void deposit(double amount) double
newBalancebalanceamount balance
newBalance / Withdraws money from the
bank account. _at_param amount the amount to
withdraw / public void withdraw(double amount)
double newBalancebalance-amount balance
newBalance / Gets the current balance.
_at_return the current balance / public double
getBalance() return balance

• /
• A bank account has a balance that can be
  changed by deposits and withdrawals.
• /
• public class BankAccount
• // instance field
• private double balance
• /
• Constructs a bank account with a zero
  balance
• /
• public BankAccount()
• balance 0

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File BankAccountTest.java

• 1 /
• 2 A class to test the BankAccount class.
• 3 /
• 4 public class BankAccountTest
• 5
• 6 /
• 7 Tests the methods of the BankAccount class.
• 8 _at_param args not used
• 9 /
• 10 public static void main(String args)
• 11
• 12 BankAccount harrysChecking new
  BankAccount()
• 13 harrysChecking.deposit(2000)
• 14 harrysChecking.withdraw(500)
• 15 System.out.println(harrysChecking.getBalance(
  ))
• 16
• 17

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Calling a Method in BlueJ
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The Method Return Value in BlueJ
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Variable Types

• Instance fields (balance in BankAccount)
• Local variables (newBalance in deposit method)
• Parameter variables (amount in deposit method)

Explicit and Implicit Parameters

• public void withdraw(double amount) double
  newBalance balance - amount balance
  newBalance
• balance is the balance of the object to the left
  of the dotbalance is an implicit parameter,
  while amount is an explicit parameter
• momsSavings.withdraw(500)
• means
• double newBalance momsSavings.balance -
  amount momsSavings.balance newBalance