Chapter 3 Implementing Classes

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Chapter 3 Implementing Classes

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Title: Chapter 3 Implementing Classes

1
Chapter 3 Implementing Classes
2
Chapter Goals

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
3.1

What is a black box?
Why do we call it a black box?
What are some instances of a black box?

4
3.1 Black Boxes

A black box magically does its thing
Hides its inner workings
Encapsulation the hiding of unimportant details

Not everything is hidden in a black box
Ways to interact with it, but all done on the
outside
Interaction possibilities are well defined

6
Properties of Encapsulation

What is the right concept for each particular
black box?
Concepts are discovered through abstraction
Abstraction taking away inessential features,
until only the essence of the concept remains
How much does a user really need to know?

7
Example Cars

Black boxes in a car transmission, electronic
control module, etc

8
Example Car

Users of a car do not need to understand how
black boxes work
Interaction of a black box with outside world is
well-defined
Drivers interact with car using pedals, buttons,
etc.
Mechanic can test that engine control module
sends the right firing signals to the spark plugs
For engine control module manufacturers,
transistors and capacitors are black boxes
magically produced by an electronics component
manufacturer

9
Example Car

Encapsulation leads to efficiency
Mechanic deals only with car components (e.g.
electronic control module), not with sensors and
transistors
Driver worries only about interaction with car
(e.g. putting gas in the tank), not about motor
or electronic control module

10
Example Doorbell

What can we do to a doorbell?
Do we know how a doorbell works?
Models
Flintstones
Simpsons
Rube Goldberg Device
Noise

In object-oriented programming the black boxes
from which a program is manufactured are called
objects

12
Levels of abstraction Software Design
13

Old times computer programs only manipulated
primitive types such as numbers and characters
Gradually programs became more complex, vastly
increasing the amount of detail a programmer had
to remember and maintain

Solution Encapsulate routine computations to
software black boxes
Abstraction used to invent higher-level data
types
In object-oriented programming, objects are black
boxes

15
Important Concept

Who designs objects you use
Other programmers
What do these other objects contains
Other Objects
Developed by other programmers, who used other
objects, etc.
Lesson Multiple levels of abstraction
Almost always, you will be designing an
abstraction while simultaneously using another one

Encapsulation Programmer using an object knows
about its behavior, but not about its internal
structure
In software design, you can design good and bad
abstractions understanding what makes good
design is an important part of the education of a
software engineer
First, define behavior of a class then,
implement it

17
3.1

Chapter 2 you used objects
Analogous to engineer who puts final parts
together in car
Now you will learn how to design the different
parts of the car
NOTE software engineering more complex
No physical limitations to abstraction

18
3.2 Designing the Public Interface to a Class

Your task Develop a BankAccount class
First step Define essential features
Behavior of bank account (abstraction)
deposit money
withdraw money
get balance

19
Designing a Class

Remember you are designing a class that another
programmer could use.
Design your class as a black box

20
Designing 1 Methods

Methods of BankAccount class
deposit
withdraw
getBalance
Support method calls such as the following
harrysChecking.deposit(2000)
harrysChecking.withdraw(500)
System.out.println(harrysChecking.getBalance())
Which methods are accessors? Mutators?

21
Designing Method Definitions

COMPONENTS
access specifier (Ex. public)
return type (Ex. String or void)
method name (Ex. deposit)
list of parameters (Ex. amount for deposit)
method body in braces

Examples
public void deposit(double amount) ...
public void withdraw(double amount) ...
public double getBalance() ...

23
Syntax 3.1 Method Definition

accessSpecifier returnType methodName(parameterTyp
e parameterName,...)
method body
Example
public void deposit(double amount) . . .
Purpose
To define the behavior of a method

24
Designing 2 Constructor

A constructor initializes the instance variables
Constructor name class name
public BankAccount() // body--filled in later

25
Constructors

Constructor body is executed when new object is
created
Statements in constructor body will set the
internal data of the object that is being
constructed
How does the compile know which constructor to
call?

26
Constructor vs. Method

Constructors are a specialization of methods
Goal to set the internal data of the object
2 differences
All constructors are named after the class
Therefore all constructors of a class have the
same name
No return type listed EVER!

27
Syntax 3.2 Constructor Definition

accessSpecifier ClassName(parameterType
parameterName, . . .) constructor body
Example
public BankAccount(double initialBalance)
. . .
Purpose
To define the behavior of a constructor

28
BankAccount Public Interface

The public constructors and methods of a class
form the public interface of the class.
public class BankAccount
// private fields--filled in later
// Constructorspublic BankAccount() //
body--filled in later

public BankAccount(double initialBalance)
// body--filled in later
// Methods
public void deposit(double amount)
// body--filled in later
public void withdraw(double amount)
// body--filled in later
public double getBalance()
// body--filled in later

30
Syntax 3.3 Class Definition

accessSpecifier class ClassName
fields
constructors
methods
Example
public class BankAccountpublic
BankAccount(double initialBalance) ...public
void deposit(double amount) .... . .
PurposeTo define a class, its public interface,
and its implementation details

31
Remember

Public methods and constructors provide the
public interface to a class
They are how you interact with the black box
Our class is simple, but we can do many things
with it
Notice we havent defined the method body yet,
but know how we can use it

32
3.3 Commenting the Public Interface

Part of creating a well-defined public interface
is always commenting the class and methods
behaviors
The HTML pages from the API are created from
special comments in your program called javadoc
comments
Place before the class or method
/
..
/

33
Javadoc Comments

Begin with /
Ends with /
Put on lines in between as convention, makes it
easier to read
Javadoc tags - _at_ mark is one
_at_author, _at_param, _at_return
Benefits
Online documentation (Assignment 1)
Other java documents

34
Javadoc comments

First sentence is extracted for HTML page
Carefully explain method
_at_param for each parameter
Omit if no parameters
_at_return for the value returned
Omit if return type void

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

36
Class Comment

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

37
Convention

Seems repetitive, but is necessary
Very helpful if you comment before you code your
methods
Provide documentation comments for
every class
every method
every parameter
every return value.
When in doubt comment
But make sure comments are concise

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3.4 Instance Fields

Remember methods are the public interface of a
class
Instance Fields are part of the internal workings
- An object stores its data in instance fields
Field a technical term for a storage location
inside a block of memory
Instance of a class an object of the class
AKA Data Members

The class declaration specifies the instance
fieldspublic class BankAccount private
double balance ...

An instance field declaration consists of the
following parts
access specifier (usually private)
type of variable (such as double)
name of variable (such as balance)
Each object of a class has its own set of
instance fields
You should declare all instance fields as private

43
Access Specifiers

Access Specifiers defines the accessibility of
the instance field and methods
private only accessible within the class
methods
public accessible in outside class methods and
inside class methods
Private enforces encapsulation/black box
AKA Visibility Modifier

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Syntax 3.4 Instance Field Declaration

accessSpecifier class ClassName
. . .
accessSpecifier fieldType fieldName. . .
Example
public class BankAccount . . . private
double balance . . .
Purpose To define a field that is present in
every object of a class

46
Accessing Instance Fields

The deposit method of the BankAccount class can
access the private instance field
public void deposit(double amount) double
newBalance balance amount balance
newBalance

Other methods cannot
public class BankRobber public static void
main(String args) BankAccount
momsSavings new BankAccount(1000) . .
. momsSavings.balance -1000 // ERROR

Encapsulation Hiding data and providing access
through methods
By making data members private, we hide internal
workings of a class from a user
Note We can have public instance fields and
private methods, but commonly we do not

49
3.5 Implementing Constructors Methods

Constructors contain instructions to initialize
the instance fields of an object public
BankAccount() balance 0public
BankAccount(double initialBalance) balance
initialBalance

50
Constructor Call Example

BankAccount harrysChecking new
BankAccount(1000)
Create a new object of type BankAccount
Call the second constructor (since a construction
parameter is supplied)
Set the parameter variable initialBalance to 1000
Set the balance instance field of the newly
created object to initialBalance
Return an object reference, that is, the memory
location of the object, as the value of the new
expression
Store that object reference in the harrysChecking
variable

51
Why put instructions in a method?

or why not just have one long list of
instructions in our programs?
smaller ?? easier
test debug once, execute as often as needed
more readable code

52
Method Properties

Methods have several components that allow them
to be functional units of code.

53
Implementing Methods

Some methods do not return a value public void
withdraw(double amount) double newBalance
balance - amount balance newBalance
Some methods return an output value public double
getBalance() return balance

54
Implementing Methods

public void deposit(double amount)
double newBalance balance amount
balance newBalance

55
Method Call Example

harrysChecking.deposit(500)
Set the parameter variable amount to 500
Fetch the balance field of the object whose
location is stored in harrysChecking
Add the value of amount to balance and store the
result in the variable newBalance
Store the value of newBalance in the balance
instance field, overwriting the old value

56
Syntax 3.5 The return Statement

return expression or
return
Example
return balance
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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Wus Template
61
Template in action

/
This class is used to do the currency
conversion
between a foreign currency and the US Dollar
_at_author Ameet Soni
/

Class Comment
62
Defining the class
63
Template in action

/
This class is used to do the currency
conversion
between a foreign currency and the US Dollar
_at_author Dr. Caffeine
/
class CurrencyConverter

Class Name
64
Defining the class
65
Template in action

/
This class is used to do the currency
conversion
between a foreign currency and the US Dollar
_at_author Dr. Caffeine
/
class CurrencyConverter
/
how much 1 USD is worth in foreign currency
/
private double exchangeRate

Declaration (Data Members/Instance Fields
66
Defining the class
67
Template in action

/
This class is used to do the currency
conversion
between a foreign currency and the US Dollar
_at_author Dr. Caffeine
/
class CurrencyConverter
/
how much 1 USD is worth in foreign currency
/
private double exchangeRate
public void setExchangeRate (double rate )
exchangeRate rate

Methods
68
Not in Book

In this course, there are three types of classes
you will write

69
What's a Class? Defn 1

Source code that defines a new data type by
encapsulating
data members
constructors
method.
Ex. BankAccount class we just created
AKA Instantiable class designed to be created
as objects

/ Stores information about one pet. /
public class Pet
/ Name of the pet. /
private String name, kind
private int age
/ Initializes a new instance.
_at_param n The pet's name.
/
public Pet( String n, String k, int a )
name n kind k age a
/ Returns the name of this pet. /
public String getName() return name
/ Changes the name of this pet.
_at_param newName The new name of the pet.

Data member
Constructor
Methods
71
What's a Class? Defn 2

Code that is written to test another class.
Has a main method that
Creates instances of the class
Executes methods
Compares the actual with expected.
Classes should be tested before being used in
Java Applications.
Ex. Part of Assignment 1

/ A Test Program. /
public class TestPet
/
Test Program starts here.
_at_param args Unused in this program.
/
public static void main( String args )
// Declare and create a pet
Pet pet1 new Pet( "George", "bird", 14 )
// Test the pet's information
testGetName( pet1, George )
/ Tests Pet getName method. /
public static void testGetName( Pet pet,
String expectedName )

73
What's a Class? Defn 3

Java Application
Source code that uses instances of other classes
(objects) to solve problems.
Must have a main method.
Ex. Programs you completed in A0

/ A Java Application. /
public class PetApplication
/
Program starts here.
_at_param args Unused in this program.
/
public static void main( String args )
// Declare and create a pet
Pet pet1 new Pet( "George", "bird", 14 )
// Output the pet's information
display( pet1 )
/ Prints Pet information to screen. /
public static void display( Pet pet )
System.out.println(

75
3.6 Testing Classes

Previous section was designing a class
By itself, we cannot actually run a program
No main() method, like most classes
We create instances of it in other classes
Idea use a test class to make sure it works
properly before dispensing the product

76
3.6 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
Verify output is correct and program behaves as
expected

77
Youve done this

A0 asked you to make test classes
Perimeter.java tested getHeight() and getWidth()
methods
MoveTester tested the translate() method

Details for building the program vary. In most
environments, you need to carry out these steps
Write a test program for the methods
Include the class being tested in the project
Run the test program

/
A class to test the BankAccount class.
/
public class BankAccountTester
/
Tests the methods of the BankAccount class.
_at_param args not used
/
public static void main(String args)
BankAccount harrysChecking new
BankAccount()
harrysChecking.deposit(2000)
harrysChecking.withdraw(500)
System.out.println(harrysChecking.getBalance())

80
3.7 Categories of Variables

Categories of variables
Instance fields (balance in BankAccount)
Local variables (newBalance in deposit method)
Parameter variables (amount in deposit method)
Share the same properties of declaring and
creating
Differ in their lifetime (AKA scope)

81
Instance Field

An instance field belongs to an object
AKA Instance variable
Each object has its own copy of the instance
field
The fields stay alive until no method uses the
object any longer
More specifically, until the object no longer
exists

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Garbage

In Java, the garbage collector periodically
reclaims objects when they are no longer used
If no variables reference to the object anymore,
it can no longer be used and is collected

83
Local Variables

Local and parameter variables belong to a method
You declare them and create within the method
When method is done executing, variable is thrown
out
EVERYTIME the method is executed, a new copy of
any variables is created, values do not carry over

84
Initialization with scope

Instance fields are initialized to a default
value, but you must initialize local variables
Default values for numbers is 0
Objects is null
COMMON ERROR forgetting to create objects for
instance fields, declaration leaves it at null
Not an issue with parameter variables

85
Parameter variables

Parameter variables are initialized to the values
that are passed to them
Maintain same lifetime as local variables
Only difference is that they receive values
automatically

86
Lifetime Of Variables

harrysChecking.deposit(500)
double newBalance balance amount
balance newBalance

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3.8 Implicit and Explicit Method Parameters

The implicit parameter of a method is the object
on which the method is invoked
Why is this important?
When a method is invoked, and an instance field
is used, how does it know which object it belongs
to?

Use of an instance field name in a method denotes
the instance field of the implicit parameter
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 -
amountmomsSavings.balance newBalance

91
Implicit Parameters and this

Every method has one implicit parameter
The implicit parameter is always called this
The method knows what object called it based on a
reference, but does not know/care about the
identifier name
E.g. you cannot say momsSavings.balance in the
class, because momsSavings is a local variable of
another class
Exception Static methods do not have an implicit
parameter (more on Chapter 9)

92
Implicit Parameters and this

double newBalance balance amount// actually
meansdouble newBalance this.balance amount
When you refer to an instance field in a method,
the compiler automatically applies it to the this
parameter
momsSavings.deposit(500)

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Implicit Parameters and this
94
Advanced Info

this operator very powerful
Can call one constructor from another to avoid
redundancy
Important goal in programming, never have
duplicate code

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Example without this

public class BankAccount
double balance
public BankAccount(double initialBalance)
balance initialBalance
public BankAccount
balance 0

96
Example with this

public class BankAccount
double balance
public BankAccount(double initialBalance)
balance initialBalance
public BankAccount
this(0)

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HOW TO 3.1 Designing and Implementing a Class

Step 1 Find out what you are asked to do with an
object of the class
What do you want to be able to do with the
object?
Step 2 Specify the Public Interface
Convert the list from step 1 into methods, and
the parameters (think input) they need
Constructors how do I want to initialize this
object

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HOW-TO 3.1 (cont)