AN INTRODUCTION TO OBJECTS AND CLASSES

1
CHAPTER 2

• AN INTRODUCTION TO OBJECTS AND CLASSES

2
Chapter 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

3
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

4
Rectangle Class

• Construct a rectanglenew Rectangle(5, 10, 20,
  30)new Rectangle()
• Use the constructed objectSystem.out.println(new
  Rectangle(5, 10, 20, 30))prints
  java.awt.Rectanglex5,y10,width20,height30

5
Rectangle Shapes
6
A Rectangle Object
7
Syntax 2.1 Object Construction

• 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.

8
Object Variables

• Declare and optionally initializeRectangle
  cerealBox new Rectangle(5, 10, 20,
  30)Rectangle crispyCrunchy
• Apply methodscerealBox.translate(15, 25)
• Share objectsr cerealBox

9
Uninitialized and Initialized Variables
Uninitialized
Initialized
10
Two Object Variables Referring to the Same Object
11
Syntax 2.2 Variable Definition

• 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

12
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

13
Syntax 2.3 Importing a Class from a Package

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

14
File MoveRect.java

• 1 import java.awt.Rectangle
• 2
• 3 public class MoveTest
• 4
• 5 public static void main(String args)
• 6
• 7 Rectangle cerealBox new Rectangle(5,
  
• 10, 20, 30)
• 8 // move the rectangle
• 9 cerealBox.translate(15, 25)
• 10 // print the moved rectangle
• 11 System.out.println(cerealBox)
• 12

15
A Simple Class

• public class Greeter public String
  sayHello() String message
  "Hello,World!" return message

16
Method Definition

• 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

17
Method Parameters

• public class Rectangle . . . public void
  translate(int x, int y) method body
  . . .

18
Syntax 2.4 Method Implementation

• public class ClassName ...
  accessSpecifier returnType methodName(parameterTyp
  e parameterName,...) method body
  ...
  Continue

19

• Continue
• 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

20
Syntax 2.5 The return Statement

• 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.

21
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

22
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())
  

23
Building a Test Program

• 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.

24
Testing with the SDK Tools

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

25
Testing with BlueJ
26
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)

27
Instance Fields
28
Accessing Instance Fields

• The sayHello method of the Greeter class can
  access the private instance fieldpublic String
  sayHello()String message "Hello, " name
  "!"return message

29

• Other methods cannotpublic class
  GreeterTestpublic static void main(String
  args). . .System.out.println(daveGreeter.name)
  // ERROR
• Encapsulation Hiding data and providing access
  through methods

30
Syntax 2.6 Instance Field Declaration

• accessSpecifier class ClassName ...
  accessSpecifier fieldType fieldName ...

31

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

32
Constructors

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

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Syntax 2.7 Constructor Implementation

• accessSpecifier class ClassName ...
  accessSpecifier ClassName(parameterTypeparameterN
  ame ...) constructor implementation
  ...

34

• 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

• 1 public class Greeter
• 2
• 3 public Greeter(String aName)
• 4
• 5 name aName
• 6
• 7
• 8 public String sayHello()
• 9

36

• 10 String message "Hello, " name "!"
• 11 return message
• 12
• 13
• 14 private String name
• 15

37
File GreeterTest.java

• 1 public class GreeterTest
• 2
• 3 public static void
• main(String args)
• 4
• 5 Greeter worldGreeter new Greeter("World")
• 6System.out.println(worldGreeter.sayHello())
• 7

38

• 8 Greeter daveGreeter new Greeter("Dave")
• 9System.out.println(daveGreeter.sayHello())
• 10
• 11

39
Designing the Public Interface

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

40
BankAccount Public Interface

• public BankAccount()
• public BankAccount(double initialBalance)
• public void deposit(double amount)
• public void withdraw(double amount)
• public double getBalance()

41
Using the Public Interface

• Transfer balance
• double amt 500momsSavings.withdraw(amt)harry
  sChecking.deposit(amt)
• Add interest
• double rate 5 // 5double amt
  acct.getBalance() rate / 100acct.deposit(am
  t)

42
Commenting the Public Interface

• / Withdraws money from the bank account.
  _at_param the amount to withdraw / public void
  withdraw(double amount) implementation filled
  in later

43

• / Gets the current balance of the bank
  account. _at_return the current balance / public
  double getBalance() implementation filled in
  later

44
Class Comment

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

45
Javadoc Method Summary
46
Javadoc Method Detail
47
BankAccount Class Implementation

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

48
File BankAccount.java

• 1 /
• 2 A bank account has a balance that can be
  changed by
• 3 deposits and withdrawals.
• 4 /
• 5 public class BankAccount
• 6

49

• 7 /
• 8 Constructs a bank account with a zero balance
• 9 /
• 10 public BankAccount()
• 11
• 12 balance 0
• 13
• 14

50

• 15 /
• 16 Constructs a bank account with a given balance
  
• 17 _at_param initialBalance the initial balance
• 18 /
• 19 public BankAccount(double initialBalance)
• 20
• 21 balance initialBalance
• 22

51

• 23
• 24 /
• 25 Deposits money into the bank account.
• 26 _at_param amount the amount to deposit
• 27 /
• 28 public void deposit(double amount)
• 29
• 30 double newBalance balance amount
• 31 balance newBalance
• 32
• 33

52

• 34 /
• 35 Withdraws money from the bank account.
• 36 _at_param amount the amount to withdraw
• 37 /
• 38 public void withdraw(double amount)
• 39
• 40 double newBalance balance - amount
• 41 balance newBalance
• 42
• 43

53

• 44 /
• 45 Gets the current balance of the bank account.
• 46 _at_return the current balance
• 47 /
• 48 public double getBalance()
• 49
• 50 return balance
• 51
• 52
• 53 private double balance
• 54

54
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 /

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• 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

56
Calling a Method in BlueJ
57
The Method Return Value in BlueJ
58
Variable Types

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

59
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 dot
• momsSavings.withdraw(500)
• means
• double newBalance momsSavings.balance -
  amount momsSavings.balance newBalance