Overview Java CHAPTERS 2,3,4,7,11,12

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Overview Java CHAPTERS 2,3,4,7,11,12

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Title: Overview Java CHAPTERS 2,3,4,7,11,12

1
Overview JavaCHAPTERS 2,3,4,7,11,12

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

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

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 ...

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")

33
Syntax 2.7 Constructor Implementation

accessSpecifier class ClassName ...
accessSpecifier ClassName(parameterTypeparameterN
ame ...) 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

35
File Greeter.java

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

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

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

/ 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

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

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

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

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

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 /

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

60
Chapter 4

Applet

61
Chapter Goals

To be able to write simple applets
To display graphical shapes such as lines and
ellipses
To use colors
To display text in multiple fonts
To select appropriate units for drawing
To develop test cases that validate the
correctness of your programs

62
Console Application
63
Graphical Application
64
Applets

Graphical Java programs
Run inside web browser
Platform-neutral
Easy deployment--loads when needed
Secure

65
Web Browsers Accessing a Web Server
66
A Web Browser
67
Brief Introduction to HTML

Text and tagsJava is an ltigtobject-orientedlt/igt
programming language
Browser renders the tagsJava is an
object-oriented programming language
Bulleted list (like this one) is defined by
tagsltulgtltligt. . .lt/ligtltligt. . .lt/ligtltligt. .
.lt/ligtlt/ulgt
Use ltgt for ltgt symbols

68
Images, Links and Applets

Image tag has attributes source, size, alternate
textltimg src"hamster.jpeg" width"640"
height"480"alt"A photo of Harry, the horrible
hamster" /gt
Link tag has attribute for link, body for linked
textlta href"http//java.sun.comgtJavalt/agt is an
. . .
Applets need class for applet code and
sizeltapplet code"HamsterApplet.class"width"64
0" height"480"gt

69
Viewing an Applet

Make one or more Java source files to implement
your applet
One of the source files must define the applet
class
Compile the source files into class files
Make an HTML file with the applet tag that
references the applet class
Run appletviewer myapplet.html
Or load the HTML file into a Java 2 compliant
browser

70
The RectangleApplet in the Applet Viewer
71
The RectangleApplet in a Browser
72
Applet Class Outline

class MyApplet extends Applet
public void paint(Graphics g)Graphics2D g2
(Graphics2D)g// add drawing operations. . .

73
File RectangleApplet.java

1import java.applet.Applet
2import java.awt.Graphics
3import java.awt.Graphics2D
4import java.awt.Rectangle
5
6/
7 An applet that draws two rectangles.
8/
9public class RectangleApplet extends Applet
10
Continue

11 public void paint(Graphics g)
12
13 // recover Graphics2D
14
15 Graphics2D g2 (Graphics2D)g
16
17 // construct a rectangle and draw it
18
19 Rectangle cerealBox new Rectangle(5, 10,
20, 30)
20 g2.draw(cerealBox)
21
22 // move rectangle 15 units sideways and 25 uni
ts down
23 Continue

24 cerealBox.translate(15, 25)
25
26 // draw moved rectangle
27
28 g2.draw(cerealBox)
29
30

76
Graphical Shapes

Shape classes Ellipse2D.Double, Line2D.Double,
etc.
We won't use the .Float classes
These classes are inner classes--doesn't matter
to us except for the import statementimport
java.awt.geom.Ellipse2D // no .Double
Must construct and draw the shape
Ellipse2D.Double easterEgg new
Ellipse2D.Double(5, 10, 15, 20)g2.draw(easterEgg
)

77
Specifying an Ellipse
78
Lines and Points

Line2D.Double segment new Line2D.Double(x1, x2,
y1, y2)
More object-oriented to use Point2D.Double for
the end pointsPoint2D.Double from new
Point2D.Double(x1, y1)Point2D.Double to new
Point2D.Double(x2, y2)Line2D.Double segment
new Line2D.Double(from, to)
Draw thick linesg2.setStroke(new
BasicStroke(4.0F)) // 4 pixels

79
Colors

Specify red, green, blue between 0.0F and
1.0FColor magenta new Color(1.0F, 0.0F, 1.0F)
Standard colorsColor.blackColor.yellowColor.pin
k. . .
Set color in graphics contextg2.setColor(Color.p
ink)
Then draw or fill shapesg2.fill(easterEgg)

80
Text and Fonts