Chapter 3 Introduction to Classes and Objects

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Chapter 3 Introduction to Classes and Objects

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Title: Chapter 3 Introduction to Classes and Objects

1
Chapter 3 - Introduction to Classes and Objects
Outline3.1 Introduction 3.2 Classes,
Objects, Methods, and Instance Variables 3.3
Declaring a Class with a Method and
Instantiating an Object of a Class 3.4
Declaring a Method With a Parameter 3.5
Instance Variables, set Methods and get
Methods 3.6 Primitive Types vs. Reference
Types 3.7 Initializing Objects with
Constructors 3.8 Floating-Point Numbers and type
double
2
3.1 Introduction

Previous chapter used simplest class
one method (main) and no data
Most applications use several classes to
accomplish program requirements
Very large industrial applications can have
thousands of classes
This chapter explores class basics
Classes
Objects
Methods and constructors
Instance variables

3
3.2 Classes, Objects, Methods, and Instance
Variables

What is a class?
A collection of data (attributes), plus
procedures that operate on that data
Class declaration is the programming unit that
ties the data and procedures together
What is an object?
A specific instance of a class
Classes are blueprints for actual objects

4
3.2 Classes, Objects, Methods, and Instance
Variables

Examples

5
3.2 Classes, Objects, Methods, and Instance
Variables

What is a method?
A procedure that manipulates data
Performs a task and returns a result
Used to hide implementation details from the user
(client) of the class
What is an instance variable?
the class data
specifically, a variable that represents an
attribute

6
3.2 Classes, Objects, Methods, and Instance
Variables

Examples

7
3.2 Classes, Objects, Methods, and Instance
Variables

What is a message?
A call to a method of a class
Tells the class to perform a task
A client sends a message (method call) to an
object
Example
currentBalance smith45729.checkBalance()
Reads as "send a checkBalance message to
smith45729, which returns the result to local
variable currentBalance"
More commonly read as "call checkBalance for
smith45729 and return result to currentBalance"

8
3.2 Classes, Objects, Methods, and Instance
Variables

import Target
// simple example of a storm trooper class
declaration
public class ImperialStormTrooper
// instance variables
private boolean isAlive // status
private Location currentLocation //location
in Deathstar
// method to determine if Storm Trooper is
still alive
public boolean isAlive()
return isAlive
// method to kill off Storm Trooper
public void die()
isAlive false
// method to have Storm Trooper chase a
specific target
public void chaseTarget( Target goodGuy)

9
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

// Fig 3.1 GradeBook.java
// Class declaration with one method.
public class GradeBook
// display a welcome message to the GradeBook
user
public void displayMessage()
System.out.println(" Welcome to the
GradeBook! ")
// end method displayMessage
// end class GradeBook

10
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

// Fig 3.2 GradeBookTest.java
// Create a GradeBook object and call its
displayMessage method
public class GradeBookTest
// main method begins program execution
public static void main( String args )
//create a GradeBook object and assign it
to myGradeBook
GradeBook myGradeBook new GradeBook()
// call myGradeBook's displayMessage
method
myGradeBook.displayMessage()
// end main
// end class GradeBookTest

11
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

Deitel uses two classes throughout Chapter 3
copy these files onto your local machine and
compile
javac .java (or javac GradeBook.java
GradeBookTest.java)
class GradeBook
simple class to keep track of information for a
single course
in Ch 3, we'll add attributes course name and
instructor name
in later chapters, we'll add grades, class
averages, etc
does NOT contain a main method
class GradeBookTest
an application program that contains only a main
method
we'll declare and use a GradeBook object for our
course here
could declare multiple GradeBook objects, one for
each course we want to track

12
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

Some observations about class GradeBook
Class name and file name are the same (GradeBook)
Keyword public is an access modifier
Determines who has visibility or access to the
class
Discussed in more detail in Chapter 8
For now, all classes are public - accessible by
anyone
Keyword class indicates we are declaring a class
GradeBook is the name of the class, and each word
is capitalized
Body of the class begins, ends with curly braces
GradeBook has one method, called displayMessage

13
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

Some observations about class GradeBookTest
Class name and file name are the same
(GradeBookTest)
This class also uses public access modifier
GradeBook class has only one method - main
Main method header is required to look like this
public static void main ( String args )
Keyword static is a method or variable modifier
it allows the method to be called without first
creating an object from the class
method is called a static method or a class
method
for now, only use static keyword with main method

14
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

Some general observations about methods
All methods in Java are declared within a class
Like a class, it starts with an access modifier
For now, all methods use public access modifier
Like procedures, methods have a return type
can be built-in type, like int or char
can be reference type, like String or Date
can be void, a special keyword that means "no
return type"
Method name starts with lower case letter, then
all subsequent words are capitalized
(displayMessage)

15
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

Back to GradeBookTest class
Two statements in Fig 3.2
A lot going on in the first statement
GradeBook myGradeBook new GradeBook()
First declare a variable named myGradeBook
Its type is GradeBook, the class we declared
earlier
Java allows new types to be created via classes
This is known as extensibility
Next we initialize myGradeBook
keyword new creates a new object of the class
type
in this case, we create a new GradeBook object
the parentheses are required - they call a
constructor (discussed in detail in a little bit)

16
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

The second statement is a method call
myGradeBook.displayMessage()
The format for this call is always the object's
name, followed by a dot separator, followed by
the method name
This calls the displayMessage method using the
myGradeBook object
Cannot call the displayMessage method without
first creating a GradeBook object to use

17
3.3 Declaring a Class with a Method and
Instantiating an Object of a Class

UML class diagram
Graphically represents a class
Has three components (from top to bottom)
Class name
Instance variables
Methods
Plus sign () means public, minus sign (-)
means private
(Pound sign () means protected, but
we'll discuss this in Ch 8)

GradeBook
displayMessage()
18
3.4 Declaring a Method with a Parameter

Update the displayMessage method in GradeBook to
accept a parameter
// Fig 3.4 GradeBook.java
// Class declaration with a method that has a
parameter.
public class GradeBook
// display a welcome message to the GradeBook
user
public void displayMessage( String courseName
)
System.out.println( "Welcome to the
gradebook for "
courseName )
// end method displayMessage
// end class GradeBook

19
3.4 Declaring a Method with a Parameter

Update GradeBookTest application to use the new
method
// Fig 3.5 GradeBookTest.java
// Create a GradeBook object and pass a String to
displayMessage method
import java.util.Scanner // program uses Scanner
public class GradeBookTest
// main method begins program execution
public static void main( String args )
Scanner input new Scanner ( System.in )
//create a GradeBook object and assign it
to myGradeBook
GradeBook myGradeBook new GradeBook()
// prompt for and input course name
System.out.println(" Please enter the
course name ")
String nameOfCourse input.nextLine()
// read a line of text
System.out.println() // outputs a blank
line
// call myGradeBook's displayMessage
method and pass nameOfCourse
myGradeBook.displayMessage( nameOfCourse
)

20
3.4 Declaring a Method with a Parameter

In summary, methods
Take an argument list, within a set of
parentheses
If no arguments, just an empty set of parentheses
Examples of method headers
public void setCourseName( String name )
public String getCourseName()
Methods also use curly braces to start, end the
body
Use a return statement in the body to return a
value
return is a keyword
methods with a void return type do not need a
return statement

21
3.4 Declaring a Method with a Parameter

In summary, method calls
Always have the format variable name, followed
by a dot separator, followed by the method name
Example myGradeBook.displayMessage()
Must first instantiate an object to call a method
on it
(except for static methods, which we'll cover
later)
Argument types in the method call must match
those in the method header

22
3.4 Declaring a Method with a Parameter

Notes on import declarations
Let you abbreviate a class that you want to use
Are not required, but make code easier to read
java.util.Scanner input new java.util.Scanner (
System.in )
This is the fully qualified class name
Don't need to import classes compiled in the same
directory
Considered to be part of the default package
Classes in the java.lang package are implicitly
imported
Brief description of packages
A way to group related classes together
Java has a package statement to specify which
package a class is in

23
3.5 Instance Variables, set and get Methods

Each class has attributes associated with it
Instance variables are variable declarations
within a class
Class variables are static variable declarations,
discussed later
Instance and class variables are also known as
fields
Example
public class GradeBook
private String courseName // this is an
instance variable
// course name
for this GradeBook
public void displayMessage()
System.out.println("Welcome to the grade
book for class "
courseName)
// end method displayMessage
// end class GradeBook

24
3.5 Instance Variables, set and get Methods

All methods of a class can use that classs
fields
For now, all fields use private access modifier
No one outside of the class's methods can read or
write a private field
Fields can be either built-in or reference types
I.e., we can use another class as a type
Example private courseName String

25
3.5 Instance Variables, set and get Methods

Each time an object is instantiated, it gets its
own set of instance variables declared in the
class
GradeBook CSC142GradeBook new GradeBook()
GradeBook CSC143GradeBook new GradeBook()

26
3.5 Instance Variables, set and get Methods

Question If fields are private, how are they
accessed?
Answer with public set and get methods
Set and get methods allow the class to control
its own data
Known as data-hiding
Set and get methods sometimes perform
computations or error-checking
Naming convention is to precede the method with
either "set" or "get"

27
3.5 Instance Variables, set and get Methods

Set methods take parameters and return void type
Get methods don't, and return a specific
attribute's value

28
3.6 Primitive Types vs Reference Types

Eight primitive types in Java
boolean, byte, char, short, int, long, float,
double
Primitive types are always lower case
Note Boolean is a class, not a primitive type
Instance variables initialized to 0 by default
Except boolean, which is initialized to false
Local variables are not initialized by default
One variable, one location in memory

29
3.6 Primitive Types vs Reference Types

Everything else is a reference type
Classes are reference types
So are arrays (covered in Chapter 7)
Reference type variables are called references
References store the location of an object in
memory and not the actual object
Many references can point to one object
Reference types are initialized to null
null is a keyword meaning "reference to nothing"

30
3.6 Primitive Types vs Reference Types

Example
GradeBook csc142GradeBook new GradeBook()
GradeBook currentGradeBook csc142GradeBook
Need a reference to invoke an object's method
csc142GradeBook.setCourseName("Intro to Java 1")
Thus primitive variables cannot invoke methods
For those familiar with C or C
A reference is like a pointer, but you can't
manipulate it
Java does not have pointers

31
3.6 Primitive Types vs Reference Types

More examples
int counter
int countdown 10
boolean isAlive true
String name1
String name2 "Tracey Sconyers"
String name3 new String( "Tracey Sconyers" )
Complex Number complex1
ComplexNumber complex2 new ComplexNumber( 1.0,
2.0 )
ComplexNumber complex3 new ComplexNumber(
complex2 )

32
3.7 Initializing Objects with Constructors

Question If a reference is initialized to null
when its created, how is memory allocated for its
object?
Answer With new keyword, followed by a
constructor
GradeBook csc142GradeBook new GradeBook()
A constructor is a method that initializes the
instance variables of the object
Can only be called during object creation
new GradeBook() is the class instance creation
expression

33
3.7 Initializing Objects with Constructors

Constructors
Always have the same name as the class
Can be added to any class
May contain parameters, to initialize instance
variables
Can have multiple constructors in a class
must have unique parameter list (signature)
Multiple methods with same name called
overloading
Do not have a return type, not even void
Do not have return statement

34
3.7 Initializing Objects with Constructors

Compiler will provide a default constructor if
programmer doesn't include one
Otherwise, compiler won't create default
constructor
Good programming practice to include constructors

35
3.7 Initializing Objects with Constructors

//class to represent complex numbers, such as 2
3i
public class ComplexNumber
private double myRealPart, myImagPart
public ComplexNumber()
myRealPart 0.0
myImagPart 0.0
public ComplexNumber( double realPart )
myRealPart realPart
myImagPart 0.0
public ComplexNumber( double realPart, double
imagPart )
myRealPart realPart
myImagPart imagPart
public ComplexNumber( ComplexNumber copy)
myRealPart copy.getRealPart()
myImagPart copy.getImagPart()

36
3.7 Initializing Objects with Constructors

//class to test our Complex Number class
public class ComplexNumberTest
// the number 0
ComplexNumber zero new ComplexNumber()
// the real number 42
ComplexNumber fortyTwo new ComplexNumber(
42.0 )
// the imaginary number 2i
ComplexNumber twoI new ComplexNumber(0.0,
2.0 )
// the complex number 2 3i
ComplexNumber twoPlusThreeI new
ComplexNumber( 2.0, 3.0 )
// end class ComplexNumberTest

37
3.8 Floating-Point Numbers and Type double

Two primitive floating point types
float
represent single-precision floating point numbers
have 7 significant digits
32 bits
double
represent double-precision floating point numbers
have 15 significant digits
64 bits
literals in Java are stored as double values
double is preferred over float because of greater
precision

38
Variable Scope

The scope of a variable refers to the body of
code in which the variable can be used
A class or instance variable has a class-wide
scope
Local variables have a scope that is local to the
body of code () in which they are declared

39
Class Variables and Class Methods

The static keyword means the variable or method
belongs to the class and an object
Class variables
Only one copy per class
Initialized when the class is loaded by the JVM
Example private static int totalCount 0
Class methods (static methods)
Called using class name, not an object
Example In class Math
public static double sqrt( double a )
double root Math.sqrt( 64.0 )

40
Constants

Use both the static and final modifiers
Are class variables
Cannot be modified
Naming convention is all uppercase letters
Example
public static final double PI
3.14159265358979323846

41
Freeing Memory for Objects

Objects are created with new keyword, but there
is no corresponding delete or free method
Handled automatically with garbage collection
Generally objects are marked for collection when
No longer referenced by any variable
Go out of scope
JVM handles garbage collection during slow times
More details on garbage collection in Chapter 8

42
Summary

Thus class declarations look like
// import declarations are listed here
public class SomeClassName
// instance and class variables are usually
listed first
// methods are listed second
// constructors
// set methods
// get methods
// other methods
// end class someClassName
where all of the above is in a text file named
SomeClassName.java

43
Summary

Instance variable declarations look like
private lttypegt someVariableName
Class variable declarations look like
private static lttypegt someVariableName
Constants look like
public static final lttypegt SOMECONSTANTNAME
ltvaluegt
where lttypegt is a built-in or reference type

44
Summary

Method declarations look like
public static void main ( String args )
// local variables and statements to
perform application
// end method main
public void someMethodName ( ltparameter listgt
)
// local variables and statements go here
// end method someMethodName
public ltreturn typegt someMethodName(
ltparameter listgt )
// local variables and statements go here
return someValue
// end method someMethodName
where ltreturn typegt is built-in or reference type
and ltparameter listgt is a comment separated list
of zero or more parameters of the form lttypegt
variableName