Core java complete notes - Contact at +91-814-614-5674

Core Java Complete Tutorial
Prepared by:
LOKESH KAKKAR
eMail : kakkar.lokesh@gmail.com
LinkedIn : https://www.linkedin.com/in/lokeshkakkar
Mobile : +91 814 614 5674
1

Core Java Tutorial
Sr. No. Topic Name
01 OOPS Concepts
02 Java Evolution
03 Class Object basic
04 Class Object Constructor overloading
05 Inheritance
06 Array and String
07 Final Abstract class and interfaces
08 Exceptions
09 Streams
10 GUI Applications
11 Applet Programming
12 Network Programming and Java Sockets
13 Java Treads
2

Introduction to Object Oriented
Design
3

Overview
 Understand Classes and Objects.
 Understand some of the key
concepts/features in the Object Oriented
paradigm.
 Benefits of Object Oriented Design
paradigm.
4

OOP: model, map, reuse, extend
 Model the real world
problem to user‘s
perceive;
 Use similar metaphor in
computational env.
 Construct reusable
components;
 Create new components
from existing ones.
5

Examples of Objects
Figure 1.9: Examples of objects
CAR
VDU
BOY GIRL
TREEBOOK
CLOCK
TRIANGLE
6

Classes: Objects with the same
attributes and behavior
Person Objects
Vehicle Objects
Polygon Objects
Abstract Person Class
Attributes:
Operations:
Name, Age, Sex
Speak(), Listen(), Walk()
Into
Abstract Vehicle Class
Attributes:
Operations:
Name, Model, Color
Start(), Stop(), Accelerate()
Into
Abstract
Polygon Class
Attributes:
Operations: Draw(), Erase(), Move()
Vertices, Border,
Color, FillColorInto
Figure 1.12: Objects and classes 7

Object Oriented Paradigm: Features
OOP
Paradigm
Encapsulation
Multiple Inheritance
Genericity
Delegation
Persistence
Polymorphism
Single Inheritance
Data Abstraction
8

Java‘s OO Features
OOP
Paradigm
Encapsulation
Genericity
Delegation
Persistence
Polymorphism
Single Inheritance
Data Abstraction
Java
9

Encapsulation
 It associates the code
and the data it
manipulates into a
single unit; and
keeps them safe from
external interference
and misuse.OOP
Paradigm
Encapsulation
Genericity
Delegation
Persistence
Polymorphism
Single Inheritance
Data Abstraction
Data
Functions
10

Data Abstraction
 The technique of
creating new data types
that are well suited to an
application.
 It allows the creation of
user defined data types,
having the properties of
built data types and a set
of permitted operators.
 In Java, partial support.
 In C++, fully supported
(e.g., operator
overloading).
OOP
Paradigm
Encapsulation
Genericity
Delegation
Persistence
Polymorphism
Single Inheritance
Data Abstraction
11

Abstract Data Type (ADT)
 A structure that contains both data
and the actions to be performed on
that data.
 Class is an implementation of an
Abstract Data Type.
12

Class- Example
class Account {
private String accountName;
private double accountBalance;
public withdraw();
public deposit();
public determineBalance();
} // Class Account
13

Class
 Class is a set of attributes and operations
that are performed on the attributes.
Account
accountName
accountBalance
withdraw()
deposit()
determineBalance()
Student
name
age
studentId
getName()
getId()
Circle
centre
radius
area()
circumference()
14

Objects
 An Object Oriented system is a
collection of interacting Objects.
 Object is an instance of a class.
15

Classes/Objects
Student
:John
:Jill
John and Jill are
objects of class
Student
Circle
:circleA
:circleB
circleA and circleB
are
objects of class
Circle
16

Class
 A class represents a template for several
objects that have common properties.
 A class defines all the properties common
to the object - attributes and methods.
 A class is sometimes called the object‘s
type.
17

Object
 Objects have state and classes don‘t.
John is an object (instance) of class Student.
name = ―John‖, age = 20, studentId = 1236
Jill is an object (instance) of class Student.
name = ―Jill‖, age = 22, studentId = 2345
circleA is an object (instance) of class Circle.
centre = (20,10), radius = 25
circleB is an object (instance) of class Circle.
centre = (0,0), radius = 10
18

Encapsulation
 All information (attributes and methods) in an
object oriented system are stored within the
object/class.
 Information can be manipulated through
operations performed on the object/class –
interface to the class. Implementation is hidden
from the user.
 Object support Information Hiding – Some
attributes and methods can be hidden from the
user.
19

Encapsulation - Example
class Account {
private double accountBalance;
public withdraw();
public deposit();
} // Class Account
Deposit
Withdraw
Determine Balance
Account
balance
message
message
message
20

Data Abstraction
 The technique of creating new data types
that are well suited to an application.
 It allows the creation of user defined data
types, having the properties of built in
data types and more.
21

Abstraction - Example
class Account {
private double
accountBalance;
public withdraw();
public deposit();
} // Class Account
Creates a data
type Account
Account acctX;
22

Inheritance
 New data types (classes) can be defined
as extensions to previously defined types.
 Parent Class (Super Class) – Child Class
(Sub Class)
 Subclass inherits
properties from the
parent class.
Parent
Child
Inherited
capability
23

Inheritance - Example
 Example
 Define Person to be a class
 A Person has attributes, such as age, height, gender
 Assign values to attributes when describing object
 Define student to be a subclass of Person
 A student has all attributes of Person, plus attributes of
his/her own ( student no, course_enrolled)
 A student has all attributes of Person, plus attributes of
his/her own (student no, course_enrolled)
 A student inherits all attributes of Person
 Define lecturer to be a subclass of Person
 Lecturer has all attributes of Person, plus attributes of
his/her own ( staff_id, subjectID1, subjectID2)
24

 Circle Class can be a subclass (inherited
from ) of a parent class - Shape
Shape
Circle Rectangle
25

 Inheritance can also have multiple levels.
Shape
Circle Rectangle
GraphicCircle
26

Uses of Inheritance - Reuse
 If multiple classes have common
attributes/methods, these methods can be
moved to a common class - parent class.
 This allows reuse since the implementation is
not repeated.
Example : Rectangle and Circle method have a
common method move(), which requires changing
the centre coordinate.
27

move(newCentre){
centre = newCentre;
}
Circle
centre
radius
area()
circumference()
move(newCentre)
Rectangle
centre
height
width
area()
circumference()
move(newCentre)
move(newCentre){
centre = newCentre;
}
28

Shape
centre
area()
circumference()
move(newCentre)
Rectangle
height
width
area()
circumference()
Circle
radius
area()
circumference()
move(newCentre){
centre = newCentre
}
29

Uses of Inheritance - Specialization
 Specialized behavior can be added to the
child class.
 In this case the behaviour will be
implemented in the child class.
 E.g. The implementation of area() method in
the Circle class is different from the
Rectangle class.
 area() method in the child classes
override the method in parent classes().
30

area(){
return height*width;
}
Circle
centre
radius
area()
circumference()
move(newCentre)
Rectangle
centre
height
width
area()
circumference()
move(newCentre)
area(){
return pi*r^2;
}
31

Shape
centre
area()
circumference()
move(newCentre)
Rectangle
height
width
area()
circumference()
Circle
radius
area()
circumference()
area(); - Not implemented
And left for the child classes
To implement
area(){
return pi*r^2;
}
area(){
return height*width;
}
32

Uses of Inheritance – Common Interface
 All the operations that are supported for
Rectangle and Circle are the same.
 Some methods have common implementation
and others don‘t.
 move() operation is common to classes and can be
implemented in parent.
 circumference(), area() operations are significantly
different and have to be implemented in the
respective classes.
 The Shape class provides a common interface
where all 3 operations move(), circumference()
and area().
33

Uses of Inheritance - Extension
 Extend functionality of a class.
 Child class adds new operations to the
parent class but does not change the
inherited behavior.
 E.g. Rectangle class might have a special
operation that may not be meaningful to the
Circle class - rotate90degrees()
34

Uses of Inheritance - Extension
Shape
centre
area()
circumference()
move(newCentre)
Rectangle
height
width
area()
circumference()
rotate90degrees()
Circle
radius
area()
circumference()
35

Uses of Inheritance – Multiple Inheritance
 Inherit properties from more than one
class.
 This is called Multiple Inheritance.
Shape
Circle
Graphics
36

Uses of Multiple Inheritance
 This is required when a class has to
inherit behavior from multiple classes.
 In the example Circle class can inherit
move() operation from the Shape class
and the paint() operation from the
Graphics class.
 Multiple inheritance is not supported in
JAVA but is supported in C++.
37

Uses of Inheritance – Multiple Inheritance
Shape
centre
area()
circumference()
move(newCentre)
Circle
radius
area()
circumference()
GraphicCircle
color
paint()
38

Polymorphism
 Polymorphic which means ―many forms‖ has
Greek roots.
 Poly – many
 Morphos - forms.
 In OO paradigm polymorphism has many
forms.
 Allow a single object, method, operator
associated with different meaning depending
on the type of data passed to it.
39

Polymorphism
 An object of type Circle or Rectangle can be
assigned to a Shape object. The behavior of the
object will depend on the object passed.
circleA = new Circle(); Create a new circle object
Shape shape = circleA;
shape.area(); area() method for circle class will be executed
rectangleA = new Rectangle(); Create a new rectangle object
shape= rectangle;
shape.area() area() method for rectangle will be executed.
40

Polymorphism – Method Overloading
 Multiple methods can be defined with the
same name, different input arguments.
Method 1 - initialize(int a)
Method 2 - initialize(int a, int b)
 Appropriate method will be called based
on the input arguments.
initialize(2) Method 1 will be called.
initialize(2,4) Method 2 will be called.
41

Polymorphism – Operator Overloading
 Allows regular operators such as +, -, *, /
to have different meanings based on the
type.
 E.g. + operator for Circle can re-defined
Circle c = c + 2;
 Not supported in JAVA. C++ supports it.
42

Persistence
 The phenomenon where the object
outlives the program execution.
 Databases support this feature.
 In Java, this can be supported if users
explicitly build object persistency using
IO streams.
43

Why OOP?
 Greater Reliability
 Break complex software projects into
small, self-contained, and modular objects
 Maintainability
 Modular objects make locating bugs
easier, with less impact on the overall
project
 Greater Productivity through Reuse!
 Faster Design and Modelling
44

Benefits of OOP..
 Inheritance: Elimination of Redundant
Code and extend the use of existing
classes.
 Build programs from existing working
modules, rather than having to start from
scratch.  save development time and
get higher productivity.
 Encapsulation: Helps in building secure
programs.
45

Benefits of OOP..
 Multiple objects to coexist without any
interference.
 Easy to map objects in problem
domain to those objects in the
program.
 It is easy to partition the work in a
project based on objects.
 The Data-Centered Design enables us
in capturing more details of model in
an implementable form.
46

Benefits of OOP..
 Object Oriented Systems can be easily
upgraded from small to large systems.
 Message-Passing technique for
communication between objects make
the interface descriptions with external
systems much simpler.
 Software complexity can be easily
managed.
47

Summary
 Object Oriented Design, Analysis, and Programming
is a Powerful paradigm
 Enables Easy Mapping of Real world Objects to
Objects in the Program
 This is enabled by OO features:
 Encapsulation
 Data Abstraction
 Inheritance
 Polymorphism
 Persistence
 Standard OO Design (UML) and Programming
Languages (C++/Java) are readily accessible.
48

Contents
 Java Introduction
 Java Features
 How Java Differs from other OO
languages
 Java and the World Wide Web
 Java Environment
 Build your first Java Program
 Summary and Reference
50

Java - An Introduction
 Java - The new programming language
developed by Sun Microsystems in 1991.
 Originally called Oak by James Gosling,
one of the inventors of the Java
Language.
 Java -The name that survived a patent
search
 Java Authors: James , Arthur Van , and
others
 Java is really ―C++ -- ++ ―
51

Java Introduction
 Originally created for consumer
electronics (TV, VCR, Freeze, Washing
Machine, Mobile Phone).
 Java - CPU Independent language
 Internet and Web was just emerging,
so Sun turned it into a language of
Internet Programming.
 It allows you to publish a webpage
with Java code in it.
52

Java Milestones
Year Development
1990 Sun decided to developed special software that could be
used for electronic devices. A project called Green Project
created and head by James Gosling.
1991 Explored possibility of using C++, with some updates
announced a new language named ―Oak‖
1992 The team demonstrated the application of their new
language to control a list of home appliances using a
hand held device.
53

Java Milestones
Year Development
1994 The team developed a new Web browsed called ―Hot
Java‖ to locate and run Applets. HotJava gained instance
success.
1995 Oak was renamed to Java, as it did not survive ―legal‖
registration. Many companies such as Netscape and
Microsoft announced their support for Java
1996 Java established itself it self as both 1. ―the language for
Internet programming‖ 2. a general purpose OO
language.
54

Sun white paper defines Java as:
 Simple and Powerful
 Safe
 Object Oriented
 Robust
 Architecture Neutral and Portable
 Interpreted and High Performance
 Threaded
 Dynamic
55

Java Attributes
 Familiar, Simple, Small
 Compiled and Interpreted
 Platform-Independent and Portable
 Object-Oriented
 Robust and Secure
 Distributed
 Multithreaded and Interactive
 High Performance
 Dynamic and Extensible
56

Java is Compiled and Interpreted
Text Editor Compiler Interpreter
Programmer
Source Code
.java file
Byte
Code
.class
file
Hardware and
Operating System
Notepad,
emacs,vi
java
c
java
appletviewer
netscape
57

Compiled Languages
Text Editor Compiler linker
Programmer
Source Code
.c file
Object
Code
.o file
Notepad,
emacs,vi
gcc
Executabl
e Code
a.out
file
58

Total Platform Independence
JAVA COMPILER
(translator)
JAVA BYTE CODE
JAVA INTERPRETER
Windows 95 Macintosh Solaris Windows NT
(same for all platforms)
(one for each different system)
59

Architecture Neutral & Portable
 Java Compiler - Java source code (file
with extension .java) to bytecode (file
with extension .class)
 Bytecode - an intermediate form, closer
to machine representation
 A interpreter (virtual machine) on any
target platform interprets the bytecode.
60

Architecture Neutral & Portable
 Porting the java system to any new
platform involves writing an interpreter.
 The interpreter will figure out what the
equivalent machine dependent code to
run
61

Rich Class Environment
 Core Classes
language
Utilities
Input/Output
Low-Level Networking
Abstract Graphical User Interface
 Internet Classes
TCP/IP Networking
WWW and HTML
Distributed Programs
62

How Does Java Compares to
C++ and Other OO Languages
63

Overlap of C, C++, and Java
C
C++
Java
64

Java better than C++ ?
 No Typedefs, Defines, or Preprocessor
 No Global Variables
 No Goto statements
 No Pointers
 No Unsafe Structures
 No Multiple Inheritance
 No Operator Overloading
 No Automatic Coercions
 No Fragile Data Types
65

Object Oriented Languages -A Comparison
Feature C++ Objective
C
Ada Java
Encapsulation Yes Yes Yes Yes
Inheritance Yes Yes No Yes
Multiple Inherit. Yes Yes No No
Polymorphism Yes Yes Yes Yes
Binding (Early or Late) Both Both Early Late
Concurrency Poor Poor Difficult Yes
Garbage Collection No Yes No Yes
Genericity Yes No Yes Limited
Class Libraries Yes Yes Limited Yes
66

Java Integrates
Power of Compiled Languages
and
Flexibility of Interpreted
Languages
67

Java Applications
 We can develop two types of Java
programs:
 Stand-alone applications
 Web applications (applets)
68

Applications v/s Applets
 Different ways to run a Java
executable are:
Application- A stand-alone program that can
be invoked from command line . A program
that has a “main” method
Applet- A program embedded in a web page
, to be run when the page is browsed . A
program that contains no “main” method
69

Applets v/s Applications
 Different ways to run a Java executable
are
Application- A stand-alone program that can be
invoked from command line . A program that has a
“main” method
Applet- A program embedded in a web page , to be
run when the page is browsed . A program that
contains no “main” method
 Application –Executed by the Java
interpreter.
 Applet- Java enabled web browser.
70

Java and World Wide Web
Turning the Web into an
Interactive and Application
Delivery Platform
71

What is World Wide Web ?
 Web is an open-ended information
retrieval system designed to be used in
the Internet wide distributed system.
 It contains Web pages (created using
HTML) that provide both information and
controls.
 Unlike a menu driven system--where we
are guided through a particular direction
using a decision tree, the web system is
open ended and we can navigate to a new
document in any direction.
72

Web Structure of Information
Search/Navigation
73

Execution of Applets
Hello
Hello Java
<app=
“Hello”>
4
APPLET
Development
“hello.java”
AT
SUN.COM
The Internet
hello.class
AT SUN’S
WEB
SERVER
2 31 5
Create
Applet
tag in
HTML
document
Accessing
from
Unimelb.edu.au
The browser
creates
a new
window and
a new thread
and
then runs the
code
74

Significance of downloading Applets
 Interactive WWW
 Flashy animation instead of static web pages
 Applets react to users input and dynamically
change
 Display of dynamic data
 WWW with Java - more than a document
publishing medium
 http://www.javasoft.com/applets/alpha/applets/St
ockDemo/standalone.html
75

Power of Java and the Web
 Deliver applications, not just
information
 Eliminate porting
 Eliminate end-user installation
 Slash software distribution costs
 Reach millions of customers - instantly
76

Java Development Kit
 javac - The Java Compiler
 java - The Java Interpreter
 jdb- The Java Debugger
 appletviewer -Tool to run the applets
 javap - to print the Java bytecodes
 javaprof - Java profiler
 javadoc - documentation generator
 javah - creates C header files
77

Java Development Kit
 javac - The Java Compiler
 java - The Java Interpreter
 jdb- The Java Debugger
 appletviewer -Tool to run the applets
 javap - to print the Java bytecodes
 javaprof - Java profiler
 javadoc - documentation generator
 javah - creates C header files
79

Process of Building and Running Java
Programs
Text Editor
Java Source
Code
javac
Java Class File
java
Outout
javadoc
javah
jdb
HTML Files
Header Files
80

Let us Try Out
Building your first Java Program
81

Hello Internet
// hello.java: Hello Internet program
class HelloInternet
{
public static void main(String args[])
{
System.out.println(―Hello Internet‖);
}
}
82

Program Processing
 Compilation
# javac hello.java
results in HelloInternet.class
 Execution
# java HelloInternet
Hello Internet
#
83

Simple Java Applet
//HelloWorld.java
import java.applet.Applet;
import java.awt.*;
public class HelloWorld extends Applet {
public void paint(Graphics g) {
g.drawString (“Hello World !”,25, 25);
}
}
84

Calling an Applet
<HTML>
<TITLE>HELLO WORLD APPLET</TITLE>
<HEAD>THE HELLO WORLD APPLET</HEAD>
<APPLET CODE=―HelloWorld.class‖ width=500
height=500>
</APPLET>
</HTML>
85

Applet Execution
Using AppletViewer
Using Browser
86

Summary
 Java has emerged as a general
purpose OO language.
 It supports both stand alone and
Internet Applications.
 Makes the Web Interactive and
medium for application delivery.
 Provides an excellent set of Tools for
Application Development.
 Java is ubiquitous!
87

Classes and Objects in Java
Basics of Classes in Java
88

Contents
 Introduce to classes and objects in Java.
 Understand how some of the OO
concepts learnt so far are supported in
Java.
 Understand important features in Java
classes.
89

Introduction
 Java is a true OO language and therefore the
underlying structure of all Java programs is classes.
 Anything we wish to represent in Java must be
encapsulated in a class that defines the ―state‖ and
―behaviour‖ of the basic program components known
as objects.
 Classes create objects and objects use methods to
communicate between them. They provide a
convenient method for packaging a group of logically
related data items and functions that work on them.
 A class essentially serves as a template for an object
and behaves like a basic data type ―int‖. It is therefore
important to understand how the fields and methods
are defined in a class and how they are used to build a
Java program that incorporates the basic OO concepts
such as encapsulation, inheritance, and polymorphism.90

Classes
 A class is a collection of fields (data) and
methods (procedure or function) that
operate on that data.
Circle
centre
radius
circumference()
area()
91

Classes
 A class is a collection of fields (data) and methods
(procedure or function) that operate on that data.
 The basic syntax for a class definition:
 Bare bone class – no fields, no methods
public class Circle {
// my circle class
}
class ClassName [extends
SuperClassName]
{
[fields declaration]
[methods declaration]
}
92

Adding Fields: Class Circle with fields
 Add fields
 The fields (data) are also called the
instance varaibles.
public double x, y; // centre coordinate
public double r; // radius of the circle
}
93

Adding Methods
 A class with only data fields has no life.
Objects created by such a class cannot
respond to any messages.
 Methods are declared inside the body of
the class but immediately after the
declaration of data fields.
 The general form of a method declaration
is:
type MethodName (parameter-list)
{
Method-body;
}
94

Adding Methods to Class Circle
public double x, y; // centre of the circle
public double r; // radius of circle
//Methods to return circumference and area
public double circumference() {
return 2*3.14*r;
}
public double area() {
return 3.14 * r * r;
}
}
Method Body
95

Data Abstraction
 Declare the Circle class, have created a
new data type – Data Abstraction
 Can define variables (objects) of that
type:
Circle aCircle;
Circle bCircle;
96

Class of Circle cont.
 aCircle, bCircle simply refers to a Circle
object, not an object itself.
aCircle
Points to nothing (Null Reference)
bCircle
Points to nothing (Null Reference)
null null
97

Creating objects of a class
 Objects are created dynamically using the
new keyword.
 aCircle and bCircle refer to Circle objects
bCircle = new Circle() ;aCircle = new Circle() ;
98

aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;
99

aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;
P
aCircle
Q
bCircle
Before Assignment
P
aCircle
Q
bCircle
Before Assignment
100

Automatic garbage collection
 The object does not have a
reference and cannot be used in
future.
 The object becomes a candidate for
automatic garbage collection.
 Java automatically collects garbage
periodically and releases the memory
used to be used in the future.
Q
101

Accessing Object/Circle Data
 Similar to C syntax for accessing data
defined in a structure.
Circle aCircle = new Circle();
aCircle.x = 2.0 // initialize center and radius
aCircle.y = 2.0
aCircle.r = 1.0
ObjectName.VariableName
ObjectName.MethodName(parameter-list)
102

Executing Methods in Object/Circle
 Using Object Methods:
double area;
aCircle.r = 1.0;
area = aCircle.area();
sent ‘message’ to aCircle
103

Using Circle Class
// Circle.java: Contains both Circle class and its user class
//Add Circle class code here
class MyMain
{
{
Circle aCircle; // creating reference
aCircle = new Circle(); // creating object
aCircle.x = 10; // assigning value to data field
aCircle.y = 20;
aCircle.r = 5;
double area = aCircle.area(); // invoking method
double circumf = aCircle.circumference();
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf);
}
}
[raj@mundroo]%: java MyMain
Radius=5.0 Area=78.5
Radius=5.0 Circumference =31.400000000000002
104

Summary
 Classes, objects, and methods are the basic
components used in Java programming.
 We have discussed:
 How to define a class
 How to create objects
 How to add data fields and methods to classes
 How to access data fields and methods to classes
105

Classes and Objects in Java
Constructors, Overloading, Static
Members
106

Refer to the Earlier Circle Program
class MyMain
{
{
aCircle.x = 10; // assigning value to data field
aCircle.y = 20;
aCircle.r = 5;
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf);
}
}
107

Better way of Initialising or Access
Data Members x, y, r
 When there too many items to update/access
and also to develop a readable code, generally
it is done by defining specific method for each
purpose.
 To initialise/Update a value:
 aCircle.setX( 10 )
 To access a value:
 aCircle.getX()
 These methods are informally called as
Accessors or Setters/Getters Methods.
108

Accessors – ―Getters/Setters‖
public double x,y,r;
public double getX() { return x;}
public double getY() { return y;}
public double getR() { return r;}
public double setX(double x_in) { x = x_in;}
public double serY(double y_in) { y = y_in;}
public double setR(double r_in) { r = r_in;}
}
109

How does this code looks ? More
readable ?
class MyMain
{
{
aCircle.setX(10);
aCircle.setY(20);
aCircle.setR(5);
System.out.println("Radius="+aCircle.getR()+" Area="+area);
System.out.println("Radius="+aCircle.getR()+" Circumference ="+circumf);
}
}
110

Object Initialisation
 When objects are created, the initial value of data fields is
unknown unless its users explicitly do so. For example,
 ObjectName.DataField1 = 0; // OR
 ObjectName.SetDataField1(0);
 In many cases, it makes sense if this initialisation can be
carried out by default without the users explicitly initialising
them.
 For example, if you create an object of the class called
―Counter‖, it is natural to assume that the counter record-
keeping field is initialised to zero unless otherwise specified
differently.
class Counter
{
int CounterIndex;
…
}
Counter counter1 = new Counter();
 What is the value of ―counter1.CounterIndex‖ ?
 In Java, this can be achieved though a mechanism called
constructors.
111

What is a Constructor?
 Constructor is a special method that gets invoked
―automatically‖ at the time of object creation.
 Constructor is normally used for initializing objects with
default values unless different values are supplied.
 Constructor has the same name as the class name.
 Constructor cannot return values.
 A class can have more than one constructor as long as
they have different signature (i.e., different input
arguments syntax).
112

Defining a Constructor
 Like any other method
 Invoking:
 There is NO explicit invocation statement needed: When
the object creation statement is executed, the constructor
method will be executed automatically.
public class ClassName {
// Data Fields…
// Constructor
public ClassName()
{
// Method Body Statements initialising Data Fields
}
//Methods to manipulate data fields
}
113

Defining a Constructor: Example
public class Counter {
int CounterIndex;
// Constructor
public Counter()
{
CounterIndex = 0;
}
//Methods to update or access counter
public void increase()
{
CounterIndex = CounterIndex + 1;
}
public void decrease()
{
CounterIndex = CounterIndex - 1;
}
int getCounterIndex()
{
return CounterIndex;
}
}
114

Trace counter value at each
statement and What is the output ?
class MyClass {
{
counter1.increase();
int a = counter1.getCounterIndex();
int b = counter1.getCounterIndex();
if ( a > b )
else
counter1.decrease();
System.out.println(counter1.getCounterIndex());
}
}
115

A Counter with User Supplied Initial
Value ?
 This can be done by adding another constructor method to
the class.
public class Counter {
int CounterIndex;
// Constructor 1
public Counter()
{
CounterIndex = 0;
}
public Counter(int InitValue )
{
CounterIndex = InitValue;
}
}
// A New User Class: Utilising both constructors
Counter counter2 = new Counter (10);
116

Adding a Multiple-Parameters
Constructor to our Circle Class
// Constructor
public Circle(double centreX, double centreY,
double radius)
{
x = centreX;
y = centreY;
r = radius;
}
public double circumference() { return 2*3.14*r; }
public double area() { return 3.14 * r * r; }
}
117

Constructors initialise Objects
 Recall the following OLD Code Segment:
aCircle.x = 10.0; // initialize center and radius
aCircle.y = 20.0
aCircle.r = 5.0;
aCircle = new Circle() ;
At creation time the center and
radius are not defined.
These values are explicitly set later.
118

Constructors initialise Objects
 With defined constructor
Circle aCircle = new Circle(10.0, 20.0, 5.0);
aCircle = new Circle(10.0, 20.0, 5.0) ;
aCircle is created with center (10, 20)
and radius 5
119

Multiple Constructors
 Sometimes want to initialize in a
number of different ways, depending
on circumstance.
 This can be supported by having
multiple constructors having different
input arguments.
120

Multiple Constructors
public double x,y,r; //instance variables
// Constructors
public Circle(double centreX, double cenreY, double radius) {
x = centreX; y = centreY; r = radius;
}
public Circle(double radius) { x=0; y=0; r = radius; }
public Circle() { x=0; y=0; r=1.0; }
public double circumference() { return 2*3.14*r; }
public double area() { return 3.14 * r * r; }
}
121

Initializing with constructors
public class TestCircles {
public static void main(String args[]){
Circle circleA = new Circle( 10.0, 12.0, 20.0);
Circle circleB = new Circle(10.0);
Circle circleC = new Circle();
}
}
circleA = new Circle(10, 12, 20) circleB = new Circle(10)
Centre = (0,0)
Radius=10
circleC = new Circle()
Centre = (0,0)
Radius = 1
Centre = (10,12)
Radius = 20
122

Method Overloading
 Constructors all have the same name.
 Methods are distinguished by their signature:
 name
 number of arguments
 type of arguments
 position of arguments
 That means, a class can also have multiple
usual methods with the same name.
 Not to confuse with method overriding (coming
up), method overloading:
123

Polymorphism
 Allows a single method or operator
associated with different meaning depending
on the type of data passed to it. It can be
realised through:
 Method Overloading
 Operator Overloading (Supported in C++, but
not in Java)
 Defining the same method with different
argument types (method overloading) -
polymorphism.
 The method body can have different logic
depending on the date type of arguments.
124

Scenario
 A Program needs to find a maximum of two
numbers or Strings. Write a separate function
for each operation.
 In C:
 int max_int(int a, int b)
 int max_string(char *s1, char *s2)
 max_int (10, 5) or max_string (―melbourne‖, ―sydney‖)
 In Java:
 int max(int a, int b)
 int max(String s1, String s2)
 max(10, 5) or max(―melbourne‖, ―sydney‖)
 Which is better ? Readability and intuitive wise ?
125

A Program with Method Overloading// Compare.java: a class comparing different items
class Compare {
static int max(int a, int b)
{
if( a > b)
return a;
else
return b;
}
static String max(String a, String b)
{
if( a.compareTo (b) > 0)
return a;
else
return b;
}
{
String s1 = "Melbourne";
String s2 = "Sydney";
String s3 = "Adelaide";
int a = 10;
int b = 20;
System.out.println(max(a, b)); // which number is big
System.out.println(max(s1, s2)); // which city is big
System.out.println(max(s1, s3)); // which city is big
}
}
126

The New this keyword
 this keyword can be used to refer to the object itself.
It is generally used for accessing class members
(from its own methods) when they have the same
name as those passed as arguments.
// Constructor
public Circle (double x, double y, double r) {
this.x = x;
this.y = y;
this.r = r;
}
}
127

Static Members
 Java supports definition of global methods and
variables that can be accessed without creating
objects of a class. Such members are called Static
members.
 Define a variable by marking with the static
methods.
 This feature is useful when we want to create a
variable common to all instances of a class.
 One of the most common example is to have a
variable that could keep a count of how many
objects of a class have been created.
 Note: Java creates only one copy for a static
variable which can be used even if the class is
never instantiated.
128

Static Variables
 Define using static:
 Access with the class name (ClassName.StatVarName):
// class variable, one for the Circle class, how many circles
public static int numCircles;
//instance variables,one for each instance of a Circle
// Constructors...
}
nCircles = Circle.numCircles;
129

Static Variables - Example
 Using static variables:
// class variable, one for the Circle class, how many circles
private static int numCircles = 0;
private double x,y,r;
// Constructors...
Circle (double x, double y, double r){
this.x = x;
this.y = y;
this.r = r;
numCircles++;
}
}
130

Class Variables - Example
 Using static variables:
public class CountCircles {
public static void main(String args[]){
Circle circleA = new Circle( 10, 12, 20); // numCircles = 1
Circle circleB = new Circle( 5, 3, 10); // numCircles = 2
}
}
circleA = new Circle(10, 12, 20) circleB = new Circle(5, 3, 10)
numCircles
131

Instance Vs Static Variables
 Instance variables : One copy per object.
Every object has its own instance
variable.
 E.g. x, y, r (centre and radius in the circle)
 Static variables : One copy per class.
 E.g. numCircles (total number of circle
objects created)
132

Static Methods
 A class can have methods that are defined
as static (e.g., main method).
 Static methods can be accessed without
using objects. Also, there is NO need to
create objects.
 They are prefixed with keyword ―static‖
 Static methods are generally used to group
related library functions that don‘t depend
on data members of its class. For example,
Math library functions.
133

Comparator class with Static methods
// Comparator.java: A class with static data items comparision methods
class Comparator {
public static int max(int a, int b)
{
if( a > b)
return a;
else
return b;
}
public static String max(String a, String b)
{
if( a.compareTo (b) > 0)
return a;
else
return b;
}
}
class MyClass {
{
String s1 = "Melbourne";
String s2 = "Sydney";
String s3 = "Adelaide";
int a = 10;
int b = 20;
System.out.println(Comparator.max(a, b)); // which number is big
System.out.println(Comparator.max(s1, s2)); // which city is big
System.out.println(Comparator.max(s1, s3)); // which city is big
}
}
Directly accessed using ClassName (NO Objects)
134

Static methods restrictions
 They can only call other static
methods.
 They can only access static data.
 They cannot refer to ―this‖ or ―super‖
(more later) in anyway.
135

Summary
 Constructors allow seamless initialization of
objects.
 Classes can have multiple methods with the
same name [Overloading]
 Classes can have static members, which serve
as global members of all objects of a class.
 Keywords: constructors, polymorphism,
method overloading, this, static variables, static
methods.
136

Inheritance
Classes and Subclasses
Or Extending a Class
(only for reusability)
137

Inheritance: Introduction
 Reusability--building new components by
utilising existing components- is yet another
important aspect of OO paradigm.
 It is always good/―productive‖ if we are able to
reuse something that is already exists rather
than creating the same all over again.
 This is achieve by creating new classes, reusing
the properties of existing classes.
138

 This mechanism of
deriving a new class
from existing/old class is
called ―inheritance‖.
 The old class is known as
―base‖ class, ―super‖
class or ―parent‖ class‖;
and the new class is
known as ―sub‖ class,
―derived‖ class, or ―child‖
class.
Parent
Child
Inherited
capability
139

 The inheritance allows subclasses to inherit
all properties (variables and methods) of
their parent classes. The different forms of
inheritance are:
 Single inheritance (only one super class)
 Multiple inheritance (several super classes)
 Hierarchical inheritance (one super class, many
sub classes)
 Multi-Level inheritance (derived from a derived
class)
 Hybrid inheritance (more than two types)
 Multi-path inheritance (inheritance of some
properties from two sources).
140

Forms of Inheritance
A
B
(a) Single Inheritance
A
C
(b) Multiple Inheritance
B A
C
(c) Hierarchical Inheritance
B D
A
C
(a) Multi-Level Inheritance
B
B
D
(b) Hybrid Inheritance
c
A
B
D
(b) Multipath Inheritance
c
A
141

Defining a Sub class
 A subclass/child class is defined as follows:
 The keyword ―extends‖ signifies that the properties
of super class are extended to the subclass. That
means, subclass contains its own members as well
of those of the super class. This kind of situation
occurs when we want to enhance properties of
existing class without actually modifying it.
class SubClassName extends SuperClassName
{
fields declaration;
methods declaration;
}
142

Subclasses and Inheritance
 Circle class captures basic properties
 For drawing application, need a circle to
draw itself on the screen, GraphicCircle...
 This can be realised either by updating the
circle class itself (which is not a good
Software Engineering method) or creating a
new class that builds on the existing class
and add additional properties.
143

Without Inheritance
 Not very elegant
public class GraphicCircle {
public Circle c; // keep a copy of a circle
public double area() { return c.area(); }
public double circumference (){ return c.circumference(); }
// new instance variables, methods for this class
public Color outline, fill;
public void draw(DrawWindow dw) { /* drawing code here */ }
}
144

Subclasses and Inheritance
 Circle class captures basic properties
 For drawing application need a circle to
draw itself on the screen, GraphicCircle
 Java/OOP allows for Circle class code to be
implicitly (re)used in defining a GraphicCircle
 GraphicCircle becomes a subclass of Circle,
extending its capabilities
145

Circle
x,y,r : double
area ( ) : double
circumference(): double
GraphicCircle
outline, fill : Color
draw (DrawWindow ) : void
Superclass
base class,
Or parent
class
Subclass,
Derived
class, or
Child class
Subclassing Circle
146

Subclassing
 Subclasses created by the keyword
extends:
 Each GraphicCircle object is also a
Circle!
public class GraphicCircle extends Circle {
// automatically inherit all the variables and methods
// of Circle, so only need to put in the ‘new stuff’
Color outline, fill;
public void draw(DrawWindow dw) {
dw.drawCircle(x,y,r,outline,fill);
}
}
147

Final Classes
 Declaring class with final modifier
prevents it being extended or subclassed.
 Allows compiler to optimize the invoking
of methods of the class
final class Cirlce{
…………
}
148

Subclasses & Constructors
 Default constructor automatically calls
constructor of the base class:
GraphicCircle drawableCircle = new GraphicCircle();
default constructor
for Circle class is
called
149

Subclasses & Constructors
 Defined constructor can invoke base class
constructor with super:
public GraphicCircle(double x, double y, double r,
Color outline, Color fill) {
super(x, y, r);
this.outline = outline;
this fill = fill
}
150

Shadowed Variables
 Subclasses defining variables with the
same name as those in the
superclass, shadow them:
151

Shadowed Variables - Example
public float r = 100;
}
public class GraphicCircle extends Circle {
public float r = 10; // New variable, resolution in dots per inch
}
public class CircleTest {
public static void main(String[] args){
GraphicCircle gc = new GraphicCircle();
Circle c = gc;
System.out.println(― GraphicCircleRadius= ― + gc.r); // 10
System.out.println (― Circle Radius = ― + c.r); // 100
}
}
152

Overriding Methods
 Derived/sub classes defining methods
with same name, return type and
arguments as those in the parent/super
class, override their parents methods:
153

Overriding Methods
class A {
int j = 1;
int f( ) { return j; }
}
class B extends A {
int j = 2;
int f( ) {
return j; }
}
154

Overriding Methods
class override_test {
public static void main(String args[]) {
B b = new B();
System.out.println(b.j); // refers to B.j prints 2
System.out.println(b.f()); // refers to B.f prints 2
A a = (A) b;
System.out.println(a.j); // now refers to a.j prints 1
System.out.println(a.f()); // overridden method still refers to B.f() prints 2 !
}
}
Object Type Casting
[raj@mundroo] inheritance [1:167] java override_test
2
2
1
2
155

Using All in One: Person and Student
Person
name: String
sex: char
age: int
Display ( ) : void
Student
RollNo: int
Branch: String
Display() : void
Superclass
class
Subclass
class.
156

Person class: Parent class
// Student.java: Student inheriting properties of person class
class person
{
private String name;
protected char sex; // note protected
public int age;
person()
{
name = null;
sex = 'U'; // unknown
age = 0;
}
person(String name, char sex, int age)
{
this.name = name;
this.sex = sex;
this.age = age;
}
String getName()
{
return name;
}
void Display()
{
System.out.println("Name = "+name);
System.out.println("Sex = "+sex);
System.out.println("Age = "+age);
}
}
157

Student class: Derived class
class student extends person
{
private int RollNo;
String branch;
student(String name, char sex, int age, int RollNo, String branch)
{
super(name, sex, age); // calls parent class's constructor with 3 arguments
this.RollNo = RollNo;
this.branch = branch;
}
void Display() // Method Overriding
{
System.out.println("Roll No = "+RollNo);
System.out.println("Name = "+getName());
System.out.println("Sex = "+sex);
System.out.println("Age = "+age);
System.out.println("Branch = "+branch);
}
void TestMethod() // test what is valid to access
{
// name = "Mark"; Error: name is private
sex = 'M';
RollNo = 20;
}
}
What happens if super class constructor is not explicitly invoked ?
(default constructor will be invoked).
158

Driver Class
class MyTest
{
public static void main(String args[] )
{
student s1 = new student("Rama", 'M', 21, 1, "Computer Science");
student s2 = new student("Sita", 'F', 19, 2, "Software Engineering");
System.out.println("Student 1 Details...");
s1.Display();
System.out.println("Student 2 Details...");
s2.Display();
person p1 = new person("Rao", 'M', 45);
System.out.println("Person Details...");
p1.Display();
}
}
Can we create Object of person class ?
159

Output
[raj@mundroo] inheritance [1:154] java MyTest
Student 1 Details...
Roll No = 1
Name = Rama
Sex = M
Age = 21
Branch = Computer Science
Student 2 Details...
Roll No = 2
Name = Sita
Sex = F
Age = 19
Branch = Software Engineering
Person Details...
Name = Rao
Sex = M
Age = 45
[raj@mundroo] inheritance [1:155]
160

Summary
 Inheritance promotes reusability by supporting the
creation of new classes from existing classes.
 Various forms of inheritance can be realised in
Java.
 Child class constructor can be directed to invoke
selected constructor from parent using super
keyword.
 Variables and Methods from parent classes can be
overridden by redefining them in derived classes.
 New Keywords: extends, super, final
161

Arrays - Introduction
 An array is a group of contiguous or related data items that
share a common name.
 Used when programs have to handle large amount of data
 Each value is stored at a specific position
 Position is called a index or superscript. Base index = 0
 The ability to use a single name to represent a collection of
items and refer to an item by specifying the item number
enables us to develop concise and efficient programs. For
example, a loop with index as the control variable can be used
to read the entire array, perform calculations, and print out
the results.
163

Arrays - Introduction
69
61
70
89
23
10
9
0
1
2
3
4
5
6
index
values
164

Declaration of Arrays
 Like any other variables, arrays must declared and created before they can
be used. Creation of arrays involve three steps:
 Declare the array
 Create storage area in primary memory.
 Put values into the array (i.e., Memory location)
 Declaration of Arrays:
 Form 1:
Type arrayname[]
 Form 2:
 Type [] arrayname;
 Examples:
int[] students;
int students[];
 Note: we don’t specify the size of arrays in the declaration.
165

Creation of Arrays
 After declaring arrays, we need to
allocate memory for storage array items.
 In Java, this is carried out by using
―new‖ operator, as follows:
 Arrayname = new type[size];
 Examples:
 students = new int[7];
166

Initialisation of Arrays
 Once arrays are created, they need to be initialised
with some values before access their content. A
general form of initialisation is:
 Arrayname [index/subscript] = value;
 Example:
 students[0] = 50;
 students[1] = 40;
 Like C, Java creates arrays starting with subscript 0
and ends with value one less than the size specified.
 Unlike C, Java protects arrays from overruns and
under runs. Trying to access an array beyond its
boundaries will generate an error message.
167

Arrays – Length
 Arrays are fixed length
 Length is specified at create time
 In java, all arrays store the allocated
size in a variable named ―length‖.
 We can access the length of arrays as
arrayName.length:
e.g. int x = students.length; // x = 7
 Accessed using the index
e.g. int x = students [1]; // x = 40
168

Arrays – Example
// StudentArray.java: store integers in arrays and access
public class StudentArray{
public static void main(String[] args) {
int[] students;
students = new int[7];
System.out.println("Array Length = " + students.length);
for ( int i=0; i < students.length; i++)
students[i] = 2*i;
System.out.println("Values Stored in Array:");
System.out.println(students[i]);
}
}
:w
169

Arrays – Initializing at Declaration
 Arrays can also be initialised like standard variables
at the time of their declaration.
 Type arrayname[] = {list of values};
 Example:
int[] students = {55, 69, 70, 30, 80};
 Creates and initializes the array of integers of
length 5.
 In this case it is not necessary to use the new
operator.
170

Arrays – Example
// StudentArray.java: store integers in arrays and access
public class StudentArray{
public static void main(String[] args) {
int[] students = {55, 69, 70, 30, 80};
System.out.println("Array Length = " + students.length);
System.out.println("Values Stored in Array:");
System.out.println(students[i]);
}
}
171

Two Dimensional Arrays
 Two dimensional
arrays allows us to
store data that are
recorded in table. For
example:
 Table contains 12
items, we can think
of this as a matrix
consisting of 4 rows
and 3 columns.
Item1 Item2 Item3
Salesgirl #1 10 15 30
Sold
Person
172

2D arrays manipulations
 Declaration:
 int myArray [][];
 Creation:
 myArray = new int[4][3]; // OR
 int myArray [][] = new int[4][3];
 Initialisation:
 Single Value;
 myArray[0][0] = 10;
 Multiple values:
 int tableA[2][3] = {{10, 15, 30}, {14, 30, 33}};
 int tableA[][] = {{10, 15, 30}, {14, 30, 33}};
173

Variable Size Arrays
 Java treats multidimensional arrays
as ―arrays of arrays‖. It is possible
to declare a 2D arrays as follows:
 int a[][] = new int [3][];
 a[0]= new int [3];
 a[1]= new int [2];
 a[2]= new int [4];
174

Try: Write a program to Add to Matrix
 Define 2 dimensional matrix variables:
 Say: int a[][], b[][];
 Define their size to be 2x3
 Initialise like some values
 Create a matrix c to storage sum value
 c[0][0] = a[0][0] + b[0][0]
 Print the contents of result matrix.
175

Arrays of Objects
 Arrays can be used to store objects
Circle[] circleArray;
circleArray = new Circle[25];
 The above statement creates an array that can
store references to 25 Circle objects.
 Circle objects are not created.
176

Arrays of Objects
 Create the Circle objects and stores
them in the array.
 //declare an array for Circle
Circle circleArray[] = new Circle[25];
int r = 0;
// create circle objects and store in array
for (r=0; r <25; r++)
circleArray[r] = new Circle(r);
177

Introduction
 String manipulation is the most common operation performed in Java
programs. The easiest way to represent a String (a sequence of
characters) is by using an array of characters.
 Example:
 char place[] = new char[4];
 place[0] = ‗J‘;
 place[1] = ‗a‘;
 place[2] = ‗v‘;
 place[3] = ‗a‘;
 Although character arrays have the advantage of being able to query
their length, they themselves are too primitive and don‘t support a range
of common string operations. For example, copying a string, searching
for specific pattern etc.
 Recognising the importance and common usage of String manipulation in
large software projects, Java supports String as one of the fundamental
data type at the language level. Strings related book keeping operations
(e.g., end of string) are handled automatically.
179

String Operations in Java
 Following are some useful classes
that Java provides for String
operations.
 String Class
 StringBuffer Class
 StringTokenizer Class
180

String Class
 String class provides many operations
for manipulating strings.
 Constructors
 Utility
 Comparisons
 Conversions
 String objects are read-only
(immutable)
181

Strings Basics
 Declaration and Creation:
 String stringName;
 stringName = new String (―string value‖);
 Example:
 String city;
 city = new String (―Bangalore‖);
 Length of string can be accessed by invoking length()
method defined in String class:
 int len = city.length();
182

String operations and Arrays
 Java Strings can be concatenated using the + operator.
 String city = ―New‖ + ―York‖;
 String city1 = ―Delhi‖;
 String city2 = ―New ―+city1;
 Strings Arrays
 String city[] = new String[5];
 city[0] = new String(―Melbourne‖);
 city[1] = new String(―Sydney‖);
 …
 String megacities[] = {―Brisbane‖, ―Sydney‖, ―Melbourne‖,
―Adelaide‖, ―Perth‖};
183

String class - Constructors
public String() Constructs an empty String.
Public String(String value) Constructs a new string
copying the specified string.
184

String – Some useful operations
public int length() Returns the length of the
string.
public charAt(int index) Returns the character at the
specified location (index)
public int compareTo( String
anotherString)
public int
compareToIgnoreCase( String
anotherString)
Compare the Strings.
185

String – Some useful operations
public String replace(char
oldChar, char newChar)
Returns a new string with all
instances of the oldChar
replaced with newChar.
public trim() Trims leading and trailing
white spaces.
public String toLowerCase()
public String toUpperCase()
Changes as specified.
186

String Class - example
// StringDemo.java: some operations on strings
class StringDemo {
public static void main(String[] args)
{
String s = new String("Have a nice Day");
// String Length = 15
System.out.println("String Length = " + s.length() );
// Modified String = Have a Good Day
System.out.println("Modified String = " + s.replace('n', 'N'));
// Converted to Uppercse = HAVE A NICE DAY"
System.out.println("Converted to Uppercase = " + s.toUpperCase());
// Converted to Lowercase = have a nice day"
System.out.println("Converted to Lowercase = " + s.toLowerCase());
}
}
187

StringDemo Output
 [raj@mundroo] Arrays [1:130] java StringDemo
String Length = 15
Modified String = Have a Nice Day
Converted to Uppercase = HAVE A NICE DAY
Converted to Lowercase = have a nice day
 [raj@mundroo] Arrays [1:131]
188

Summary
 Arrays allows grouping of sequence of related
items.
 Java supports powerful features for declaring,
creating, and manipulating arrays in efficient
ways.
 Each items of arrays of arrays can have same
or variable size.
 Java provides enhanced support for
manipulating strings and manipulating them
appears similar to manipulating standard data
type variables.
189

Final and Abstract Classes
190

Restricting Inheritance
Parent
Child
Inherited
capability
191

Final Members: A way for Preventing
Overriding of Members in Subclasses
 All methods and variables can be overridden by
default in subclasses.
 This can be prevented by declaring them as final
using the keyword ―final‖ as a modifier. For
example:
 final int marks = 100;
 final void display();
 This ensures that functionality defined in this
method cannot be altered any. Similarly, the
value of a final variable cannot be altered.
192

Final Classes: A way for Preventing
Classes being extended
 We can prevent an inheritance of classes by other
classes by declaring them as final classes.
 This is achieved in Java by using the keyword final
as follows:
final class Marks
{ // members
}
final class Student extends Person
{ // members
}
 Any attempt to inherit these classes will cause an
error.
193

Abstract Classes
 When we define a class to be ―final‖, it cannot be
extended. In certain situation, we want to
properties of classes to be always extended and
used. Such classes are called Abstract Classes.
 An Abstract class is a conceptual class.
 An Abstract class cannot be instantiated – objects
cannot be created.
 Abstract classes provides a common root for a
group of classes, nicely tied together in a package:
194

Abstract Class Syntax
abstract class ClassName
{
...
…
abstract Type MethodName1();
…
…
Type Method2()
{
// method body
}
}
 When a class contains one or more abstract methods, it should be declared
as abstract class.
 The abstract methods of an abstract class must be defined in its subclass.
 We cannot declare abstract constructors or abstract static methods.
195

Abstract Class -Example
 Shape is a abstract class.
Shape
Circle Rectangle
196

The Shape Abstract Class
 Is the following statement valid?
 Shape s = new Shape();
 No. It is illegal because the Shape class is an abstract class,
which cannot be instantiated to create its objects.
public abstract class Shape {
public abstract double area();
public void move() { // non-abstract method
// implementation
}
}
197

Abstract Classes
public Circle extends Shape {
protected double r;
protected static final double PI =3.1415926535;
public Circle() { r = 1.0; )
public double area() { return PI * r * r; }
…
}
public Rectangle extends Shape {
protected double w, h;
public Rectangle() { w = 0.0; h=0.0; }
public double area() { return w * h; }
}
198

Abstract Classes Properties
 A class with one or more abstract methods is
automatically abstract and it cannot be instantiated.
 A class declared abstract, even with no abstract
methods can not be instantiated.
 A subclass of an abstract class can be instantiated if
it overrides all abstract methods by implementation
them.
 A subclass that does not implement all of the
superclass abstract methods is itself abstract; and it
cannot be instantiated.
199

Summary
 If you do not want (properties of) your class to be extended
or inherited by other classes, define it as a final class.
 Java supports this is through the keyword ―final‖.
 This is applied to classes.
 You can also apply the final to only methods if you do not
want anyone to override them.
 If you want your class (properties/methods) to be extended
by all those who want to use, then define it as an abstract
class or define one or more of its methods as abstract
methods.
 Java supports this is through the keyword ―abstract‖.
 This is applied to methods only.
 Subclasses should implement abstract methods;
otherwise, they cannot be instantiated.
200

Interfaces
Design Abstraction and a way for
loosing realizing Multiple
Inheritance
201

Interfaces
 Interface is a conceptual entity similar to
a Abstract class.
 Can contain only constants (final
variables) and abstract method (no
implementation) - Different from Abstract
classes.
 Use when a number of classes share a
common interface.
 Each class should implement the interface.
202

Interfaces: An informal way of
realising multiple inheritance
 An interface is basically a kind of class—it contains
methods and variables, but they have to be only
abstract classes and final fields/variables.
 Therefore, it is the responsibility of the class that
implements an interface to supply the code for
methods.
 A class can implement any number of interfaces,
but cannot extend more than one class at a time.
 Therefore, interfaces are considered as an informal
way of realising multiple inheritance in Java.
203

Interface - Example
speak()
Politician Priest
<<Interface>>
Speaker
speak() speak()
Lecturer
speak()
204

Interfaces Definition
 Syntax (appears like abstract class):
 Example:
interface InterfaceName {
// Constant/Final Variable Declaration
// Methods Declaration – only method body
}
interface Speaker {
public void speak( );
}
205

Implementing Interfaces
 Interfaces are used like super-classes
who properties are inherited by
classes. This is achieved by creating a
class that implements the given
interface as follows:
class ClassName implements InterfaceName [, InterfaceName2, …]
{
// Body of Class
}
206

Implementing Interfaces Example
class Politician implements Speaker {
public void speak(){
System.out.println(―Talk politics‖);
}
}
class Priest implements Speaker {
System.out.println(―Religious Talks‖);
}
}
class Lecturer implements Speaker {
System.out.println(―Talks Object Oriented Design and Programming!‖);
}
}
207

Extending Interfaces
 Like classes, interfaces can also be
extended. The new sub-interface will inherit
all the members of the superinterface in the
manner similar to classes. This is achieved
by using the keyword extends as follows:
interface InterfaceName2 extends InterfaceName1
{
// Body of InterfaceName2
}
208

Inheritance and Interface
Implementation
 A general form of interface implementation:
 This shows a class can extended another class while
implementing one or more interfaces. It appears like a
multiple inheritance (if we consider interfaces as special
kind of classes with certain restrictions or special
features).
class ClassName extends SuperClass implements InterfaceName [,
InterfaceName2, …]
{
// Body of Class
}
209

Student Assessment Example
 Consider a university where students who participate in
the national games or Olympics are given some grace
marks. Therefore, the final marks awarded =
Exam_Marks + Sports_Grace_Marks. A class diagram
representing this scenario is as follow:
Student Sports
Exam
Results
extends
extends
implements
210

Software Implementation
class Student
{
// student no and access methods
}
interface Sport
{
// sports grace marks (say 5 marks) and abstract methods
}
class Exam extends Student
{
// example marks (test1 and test 2 marks) and access methods
}
class Results extends Exam implements Sport
{
// implementation of abstract methods of Sport interface
// other methods to compute total marks = test1+test2+sports_grace_marks;
// other display or final results access methods
}
211

Summary
 Data and methods may be hidden or encapsulated
within a class by specifying the private or
protected visibility modifiers.
 An abstract method has no method body. An
abstract class contains abstract methods.
 An interface is a collection of abstract methods
and constants. A class implements an interface by
declaring it in its implements clause, and providing
a method body for each abstract method.
212

Exceptions:
Way of Handling Errors
213

Introduction
 Rarely does a program runs successfully at its
very first attempt.
 It is common to make mistakes while
developing as well as typing a program.
 Such mistakes are categorised as:
 syntax errors - compilation errors.
 semantic errors– leads to programs producing
unexpected outputs.
 runtime errors – most often lead to abnormal
termination of programs or even cause the system
to crash.
214

Common Runtime Errors
 Dividing a number by zero.
 Accessing an element that is out of bounds of an array.
 Trying to store incompatible data elements.
 Using negative value as array size.
 Trying to convert from string data to a specific data
value (e.g., converting string ―abc‖ to integer value).
 File errors:
 opening a file in ―read mode‖ that does not exist or no read
permission
 Opening a file in ―write/update mode‖ which has ―read only‖
permission.
 Corrupting memory: - common with pointers
 Any more ….
215

Without Error Handling – Example 1
class NoErrorHandling{
int a,b;
a = 7;
b = 0;
System.out.println(“Result is “ + a/b);
System.out.println(“Program reached this line”);
}
}
Program does not reach here
No compilation errors. While running it reports an error and stops without
executing further statements:
java.lang.ArithmeticException: / by zero at Error2.main(Error2.java:10)
216

Traditional way of Error Handling -
Example 2
class WithErrorHandling{
int a,b;
a = 7; b = 0;
if (b != 0){
System.out.println(“Result is “ + a/b);
}
else{
System.out.println(“ B is zero);
}
System.out.println(“Program is complete”);
}
}
Program reaches here
217

Error Handling
 Any program can find itself in unusual
circumstances – Error Conditions.
 A ―good‖ program should be able to
handle these conditions gracefully.
 Java provides a mechanism to handle
these error condition - exceptions
218

Exceptions
 An exception is a condition that is caused by
a runtime error in the program.
 Provide a mechanism to signal errors directly
without using flags.
 Allow errors to be handled in one central part
of the code without cluttering code.
219

Exceptions and their Handling
 When the JVM encounters an error such as
divide by zero, it creates an exception object
and throws it – as a notification that an error
has occurred.
 If the exception object is not caught and
handled properly, the interpreter will display an
error and terminate the program.
 If we want the program to continue with
execution of the remaining code, then we
should try to catch the exception object thrown
by the error condition and then take
appropriate corrective actions. This task is
known as exception handling.
220

Common Java Exceptions
 ArithmeticException
 ArrayIndexOutOfBoundException
 ArrayStoreException
 FileNotFoundException
 IOException – general I/O failure
 NullPointerException – referencing a null object
 OutOfMemoryException
 SecurityException – when applet tries to perform an
action not allowed by the browser‘s security setting.
 StackOverflowException
 StringIndexOutOfBoundException
221

Exceptions in Java
 A method can signal an error condition by
throwing an exception – throws
 The calling method can transfer control to
a exception handler by catching an
exception - try, catch
 Clean up can be done by - finally
222

Exception Handling Mechanism
try Block
Statements that causes
an exception
catch Block
Statements that
handle the exception
Throws
exception
Object
223

Syntax of Exception Handling Code
…
…
try {
// statements
}
catch( Exception-Type e)
{
// statements to process exception
}
..
..
224

With Exception Handling - Example 3
class WithExceptionHandling{
int a,b; float r;
a = 7; b = 0;
try{
r = a/b;
System.out.println(“Result is “ + r);
}
catch(ArithmeticException e){
}
System.out.println(“Program reached this line”);
}
}
Program Reaches here
225

Finding a Sum of Integer Values
Passed as Command Line Parameters
// ComLineSum.java: adding command line parameters
class ComLineSum
{
{
int InvalidCount = 0;
int number, sum = 0;
for( int i = 0; i < args.length; i++)
{
try {
number = Integer.parseInt(args[i]);
}
catch(NumberFormatException e)
{
InvalidCount++;
System.out.println("Invalid Number: "+args[i]);
continue;//skip the remaining part of loop
}
sum += number;
}
System.out.println("Number of Invalid Arguments = "+InvalidCount);
System.out.println("Number of Valid Arguments = "+(args.length-InvalidCount));
System.out.println("Sum of Valid Arguments = "+sum);
}
}
226

Sample Runs
[raj@mundroo] java ComLineSum 1 2
Number of Invalid Arguments = 0
Number of Valid Arguments = 2
Sum of Valid Arguments = 3
[raj@mundroo] java ComLineSum 1 2 abc
Invalid Number: abc
Number of Invalid Arguments = 1
Number of Valid Arguments = 2
Sum of Valid Arguments = 3
227

Multiple Catch Statements
 If a try block is likely to raise more than one type of exceptions,
then multiple catch blocks can be defined as follows:
…
…
try {
// statements
}
catch( Exception-Type1 e)
{
// statements to process exception 1
}
..
..
catch( Exception-TypeN e)
{
// statements to process exception N
}
…
228

finally block
 Java supports definition of another block called finally that be used to handle any
exception that is not caught by any of the previous statements. It may be added
immediately after the try block or after the last catch block:
…
try {
// statements
}
catch( Exception-Type1 e)
{
// statements to process exception 1
}
..
..
finally {
….
}
 When a finally is defined, it is executed regardless of whether or not an exception is
thrown. Therefore, it is also used to perform certain house keeping operations such as
closing files and releasing system resources.
229

class WithExceptionCatchThrow{
int a,b; float r; a = 7; b = 0;
try{
r = a/b;
}
throw e;
}
}
}
Program Does Not
reach here
when exception occurs
230

class WithExceptionCatchThrowFinally{
int a,b; float r; a = 7; b = 0;
try{
r = a/b;
}
throw e;
}
finally{
}
}
}
Program reaches here
231

Summary
 A good programs does not produce unexpected
results.
 It is always a good practice to check for potential
problem spots in programs and guard against
program failures.
 Exceptions are mainly used to deal with runtime
errors.
 Exceptions also aid in debugging programs.
 Exception handling mechanisms can effectively
used to locate the type and place of errors.
232

Summary
 Try block, code that could have exceptions /
errors
 Catch block(s), specify code to handle various
types of exceptions. First block to have
appropriate type of exception is invoked.
 If no ‗local‘ catch found, exception propagates up
the method call stack, all the way to main()
 Any execution of try, normal completion, or catch
then transfers control on to finally block
233

Streams and Input/Output Files
234

Introduction
 So far we have used variables and arrays for storing data
inside the programs. This approach poses the following
limitations:
 The data is lost when variable goes out of scope or when
the program terminates. That is data is stored in
temporary/mail memory is released when program
terminates.
 It is difficult to handle large volumes of data.
 We can overcome this problem by storing data on secondary
storage devices such as floppy or hard disks.
 The data is stored in these devices using the concept of Files
and such data is often called persistent data.
235

File Processing
 Storing and manipulating data using files is
known as file processing.
 Reading/Writing of data in a file can be
performed at the level of bytes, characters,
or fields depending on application
requirements.
 Java also provides capabilities to read and
write class objects directly. The process of
reading and writing objects is called object
serialisation.
236

C Input/Output Revision
FILE* fp;
fp = fopen(―In.file‖, ―rw‖);
fscanf(fp, ……);
frpintf(fp, …..);
fread(………, fp);
fwrite(……….., fp);
237

I/O and Data Movement
 The flow of data into a
program (input) may come
from different devices such
as keyboard, mouse,
memory, disk, network, or
another program.
 The flow of data out of a
program (output) may go to
the screen, printer, memory,
disk, network, another
program.
 Both input and output share
a certain common property
such as unidirectional
movement of data – a
sequence of bytes and
characters and support to
the sequential access to the
data.
238

Streams
 Java Uses the concept of
Streams to represent the
ordered sequence of data,
a common characteristic
shared by all I/O devices.
 Streams presents a
uniform, easy to use,
object oriented interface
between the program and
I/O devices.
 A stream in Java is a path
along which data flows (like
a river or pipe along which
water flows).
239

Stream Types
 The concepts of sending
data from one stream to
another (like a pipe
feeding into another
pipe) has made streams
powerful tool for file
processing.
 Connecting streams can
also act as filters.
 Streams are classified
into two basic types:
 Input Steam
 Output Stream
Source Program
Input Stream
reads
SourceProgram
Output Stream
writes
240

Java Stream Classes
 Input/Output related classes are defined in
java.io package.
 Input/Output in Java is defined in terms of
streams.
 A stream is a sequence of data, of no particular
length.
 Java classes can be categorised into two groups
based on the data type one which they
operate:
 Byte streams
 Character Streams
241

Streams
Byte Streams Character streams
Operated on 8 bit (1
byte) data.
Operates on 16-bit
(2 byte) unicode
characters.
Input
streams/Output
streams
Readers/ Writers
242

Classification of Java Stream Classes
Byte Stream
classes
Character Stream
classes
243

Byte Input Streams
InputStream
ObjectInputStream
SequenceInputStream
ByteArrayInputStream
PipedInputStream
FilterInputStream
PushbackInputStream
DataInputStream
BufferedInputStream
244

Byte Input Streams - operations
public abstract int read() Reads a byte and returns as
a integer 0-255
public int read(byte[] buf,
int offset, int count)
Reads and stores the bytes
in buf starting at offset.
Count is the maximum read.
public int read(byte[] buf) Same as previous offset=0
and length=buf.length()
245

Byte Input Stream - example
 Count total number of bytes in the file
import java.io.*;
class CountBytes {
public static void main(String[] args)
throws FileNotFoundException, IOException
{
FileInputStream in;
in = new FileInputStream(―InFile.txt‖);
int total = 0;
while (in.read() != -1)
total++;
System.out.println(total + ― bytes‖);
}
}
246

What happens if the file did not exist
 JVM throws exception and terminates the program since there is
no exception handler defined.
[raj@mundroo] Streams [1:165] java CountBytes
Exception in thread "main" java.io.FileNotFoundException: FileIn.txt
(No such file or directory)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:64)
at CountBytes.main(CountBytes.java:12)
247

Byte Output Streams
OutputStream
ObjectOutputStream
SequenceOutputStream
ByteArrayOutputStream
PipedOutputStream
FilterOutputStream
PrintStream
DataOutputStream
BufferedOutputStream
248

Byte Output Streams - operations
public abstract void
write(int b)
Write b as bytes.
public void write(byte[] buf,
int offset, int count)
Write count bytes starting
from offset in buf.
public void write(byte[]
buf)
Same as previous offset=0
and count = buf.length()
249

Byte Output Stream - example
 Read from standard in and write to standard out
import java.io.*;
class ReadWrite {
public static void main(string[] args)
throws IOException
{
int b;
while (( b = System.in.read()) != -1)
{
System.out.write(b);
}
}
250

Summary
 Streams provide uniform interface
for managing I/O operations in Java
irrespective of device types.
 Java supports classes for handling
Input Steams and Output steams via
java.io package.
 Exceptions supports handling of
errors and their propagation during
file operations.
251

Graphical User Interface (GUI)
Applications
Abstract Windowing Toolkit (AWT)
Events Handling
Applets
252

Introduction
 Java began as a language to be integrated with
browsers.
 But it as evolved as a powerful language for
developing stand-alone graphical applications
and also server-side applications.
 Today, Java has large and powerful libraries to
deal with 2D and 3D graphics and imaging, as
well as the ability to build complex client-side
interactive systems.
 Our focus: Simple GUI apps and Applets and
Graphics. More on graphics in your 3rd year
subject on ―Interactive Computing‖.
253

AWT - Abstract Windowing
Toolkit
 Single Windowing Interface on Multiple
Platforms
 Supports functions common to all window
systems
 Uses Underlying Native Window system
 AWT provides
 GUI widgets
 Event Handling
 Containers for widgets
 Layout managers
 Graphic operations
254

AWT - Abstract Window Toolkit
 Portable GUI - preserves native look
and feel
 Standard GUI Components (buttons…)
 Containers - Panels, Frames, Dialogs
 Graphics class for custom drawing
 Layouts responsible for actual
positioning of components:
 BorderLayout, GridLayout, FlowLayout,
Null layout
255

Adding Components via Layouts
256

Building Graphical User Interfaces
 import java.awt.*;
 Assemble the GUI
 use GUI components,
 basic components (e.g., Button, TextField)
 containers (Frame, Panel)
 set the positioning of the components
 use Layout Managers
 Attach events
257

A sample GUI program
import java.awt.*;
public class MyGui
{
{
Frame f = new Frame ("My Frame");
Button b = new Button("OK");
TextField tf = new TextField("Programming in Java", 20);
f.setLayout(new FlowLayout());
f.add(b);
f.add(tf);
f.setSize(300, 300);
f.setVisible(true);
}
}
258

Events
 Each GUI component (e.g., a Button) that wishes to respond
to an event type (e.g., click), must register an event handler,
called a Listener.
 The listener is an object of a "Listener" interface.
 A Listener class can be created by subclassing (through
"implements") one of Listener interfaces (all listener
inrefaces are in the java.awt.event package = > must import
java.awt.event.*; )
 The registration of the listener is done by a call to a method
such as addActionListener(<Listener Object>). Each GUI
component class has one or more such add…() methods,
where applicable.
260

Events
b.addActionListener( );
method to add a listener listener object
Button
f.addWindowListener( );
Frame
261

Listener Interfaces in java.awt.event.*
 [1] ActionListener
 [2] ItemListener
 [3] MouseMotionListener
 [4] MouseListener
 [5] KeyListener
 [6] FocusListener
 [7] AdjustmentListener
 [8] ComponentListener
 [9] WindowListener
 [10] ContainerListener
 [11] TextListener
262

Listener Interfaces
 Each listener interface has methods that need to be
implemented for handling different kinds of events.
 For example 1, the ActionListener interface has a
method actionPerformed() button component is
operated.
 For example2, the MouseMotionListener interface has
two methods:
 1) mouseDragged(MouseEvent) - Invoked when a mouse
button is pressed on a component and then dragged.
 2) mouseMoved(MouseEvent) - Invoked when the mouse
button has been moved on a component (with no buttons
down).
263

Implementing the ActionListener Interface and
attaching an event handler to a button
import java.awt.*;
import java.awt.event.*;
public class MyGui1
{
{
MyGuiAction ga = new MyGuiAction(f);
}
}
class MyGuiAction implements ActionListener
{
static int count = 0;
Button b;
TextField tf;
MyGuiAction(Frame f)
{
b = new Button("OK");
b.addActionListener(this);
tf = new TextField("Hello Java", 20);
f.setLayout(new FlowLayout());
f.add(b);
f.add(tf);
f.setVisible(true);
}
public void actionPerformed( ActionEvent e)
{
if(e.getSource() == b)
{
count++;
System.out.println("Button is Pressed");
tf.setText("Hello Java Click "+count);
}
}
}
264

Output and Clicks on ―OK‖ Button
Exec started 1st click on OK button 2nd click on OK button
265

BorderLayout Example
import java.awt.*;
public class MyGui2
{
{
f.setLayout(new BorderLayout());
// Add text field to top
f.add("North",new TextField());
// Create the panel with buttons at the bottom...
Panel p = new Panel(); // FlowLayout
p.add(new Button("OK"));
p.add(new Button("Cancel"));
f.add("South",p);
f.add("Center", new TextField("Center region"));
f.setVisible(true);
}
}
266

Applets Programming
Enabling Application Delivery Via
the Web
268

Introduction
 Applets are small Java programs that are
embedded in Web pages.
 They can be transported over the Internet
from one computer (web server) to
another (client computers).
 They transform web into rich media and
support the delivery of applications via the
Internet.
269

Applet: Making Web Interactive and
Application Delivery Media
Hello
Hello Java
<app=
“Hello”>
4
APPLET
Development
“hello.java”
AT
SUN.COM
The Internet
hello.class
AT SUN’S
WEB
SERVER
2 31 5
Create
Applet
tag in
HTML
document
Accessing
from
Your Organisation
The browser
creates
a new
window and
a new
thread and
then runs the
code
270

How Applets Differ from Applications
 Although both the Applets and stand-alone applications are Java
programs, there are certain restrictions are imposed on Applets
due to security concerns:
 Applets don‘t use the main() method, but when they are load,
automatically call certain methods (init, start, paint, stop,
destroy).
 They are embedded inside a web page and executed in
browsers.
 They cannot read from or write to the files on local computer.
 They cannot communicate with other servers on the network.
 They cannot run any programs from the local computer.
 They are restricted from using libraries from other languages.
 The above restrictions ensures that an Applet cannot do any
damage to the local system.
271

Building Applet Code: An Example
//HelloWorldApplet.java
import java.awt.*;
public class HelloWorldApplet extends Applet {
g.drawString ("Hello World of Java!",25, 25);
}
}
272

Embedding Applet in Web Page
<HTML>
<HEAD>
<TITLE>
Hello World Applet
</TITLE>
</HEAD>
<body>
<h1>Hi, This is My First Java Applet on the Web!</h1>
<APPLET CODE="HelloWorldApplet.class" width=500 height=400>
</APPLET>
</body>
</HTML>
273

Accessing Web page (runs Applet)
274

Applet Life Cycle
 Every applet inherits a set of default behaviours from the
Applet class. As a result, when an applet is loaded, it
undergoes a series of changes in its state. The applet
states include:
 Initialisation – invokes init()
 Running – invokes start()
 Display – invokes paint()
 Idle – invokes stop()
 Dead/Destroyed State – invokes destroy()
275

Applet States
 Initialisation – invokes init() – only once
 Invoked when applet is first loaded.
 Running – invokes start() – more than once
 For the first time, it is called automatically by the system after init()
method execution.
 It is also invoked when applet moves from idle/stop() state to active
state. For example, when we return back to the Web page after
temporary visiting other pages.
 Display – invokes paint() - more than once
 It happens immediately after the applet enters into the running
state. It is responsible for displaying output.
 Idle – invokes stop() - more than once
 It is invoked when the applet is stopped from running. For example,
it occurs when we leave a web page.
 Dead/Destroyed State – invokes destroy() - only once
 This occurs automatically by invoking destroy() method when we
quite the browser.
276

Applet Life Cycle Diagram
Born
Running Idle
Dead
Begin
init()
start()
paint()
stop()
start()
destroy()
End
277

Passing Parameters to Applet
<HTML>
<HEAD>
<TITLE>
Hello World Applet
</TITLE>
</HEAD>
<body>
<h1>Hi, This is My First Communicating Applet on the Web!</h1>
<APPLET
CODE="HelloAppletMsg.class" width=500 height=400>
<PARAM NAME="Greetings" VALUE="Hello Friend, How are you?">
</APPLET>
</body>
</HTML>
278

Applet Program Accepting Parameters
//HelloAppletMsg.java
import java.awt.*;
public class HelloAppletMsg extends Applet {
String msg;
public void init()
{
msg = getParameter("Greetings");
if( msg == null)
msg = "Hello";
}
g.drawString (msg,10, 100);
}
} This is name of parameter specified in PARAM tag;
This method returns the value of paramter.
279

What happen if we don‘t pass
parameter? See HelloAppletMsg1.html
<HTML>
<HEAD>
<TITLE>
Hello World Applet
</TITLE>
</HEAD>
<body>
<h1>Hi, This is My First Communicating Applet on the Web!</h1>
<APPLET
CODE="HelloAppletMsg.class" width=500 height=400>
</APPLET>
</body>
</HTML>
281

getParameter() returns null. Some
default value may be used.
282

Displaying Numeric Values
//SumNums.java
import java.awt.*;
public class SumNums extends Applet {
int num1 = 10;
int num2 = 20;
int sum = num1 + num2;
String str = "Sum: "+String.valueOf(sum);
g.drawString (str,100, 125);
}
}
283

SunNums.html
<HTML>
<HEAD>
<TITLE>
Hello World Applet
</TITLE>
</HEAD>
<body>
<h1>Sum of Numbers</h1>
<APPLET CODE="SumNums.class" width=500 height=400>
</APPLET>
</body>
</HTML>
284

Interactive Applets
 Applets work in a graphical environment.
Therefore, applets treats inputs as text strings.
 We need to create an area on the screen in
which use can type and edit input items.
 We can do this using TextField class of the applet
package.
 When data is entered, an event is generated.
This can be used to refresh the applet output
based on input values.
286

Interactive Applet Program..(cont)
//SumNumsInteractive..java
import java.awt.*;
public class SumNumsInteractive extends Applet
{
TextField text1, text2;
public void init()
{
text1 = new TextField(10);
text2 = new TextField(10);
text1.setText("0");
text2.setText("0");
add(text1);
add(text2);
}
public boolean action(Event ev, Object obj)
{
repaint();
return true;
}
 next code is here 
public void paint(Graphics g)
{
int num1 = 0;
int num2 = 0;
int sum;
String s1, s2, s3;
g.drawString("Input a number in
each box ", 10, 50);
try
{
s1 =
text1.getText();
num1 =
Integer.parseInt(s1);
s2 =
text2.getText();
num2 =
Integer.parseInt(s2);
}
catch(Exception e1)
{}
sum = num1 + num2;
String str = "THE SUM IS:
"+String.valueOf(sum);
g.drawString (str,100, 125);
}
}
287

Interactive Applet Execution
288

Summary
 Applets are designed to operate in
Internet and Web environment. They
enable the delivery of applications via the
Web.
 This is demonstrate by things that we
learned in this lecture such as:
 How do applets differ from applications?
 Life cycles of applets
 How to design applets?
 How to execute applets?
 How to provide interactive inputs?
289

Network Programming and
Java Sockets
290

Agenda
 Introduction
 Elements of Client Server Computing
 Networking Basics
 Understanding Ports and Sockets
 Java Sockets
 Implementing a Server
 Implementing a Client
 Sample Examples
 Conclusions
291

Introduction
 Internet and WWW have emerged as
global ubiquitous media for communication
and changing the way we conduct science,
engineering, and commerce.
 They also changing the way we learn, live,
enjoy, communicate, interact, engage, etc.
It appears like the modern life activities
are getting completely centered around
the Internet.
292

Internet Applications Serving Local
and Remote Users
Internet
Server
PC
client
Local Area Network
PDA
293

Internet & Web as a delivery Vehicle
294

Increased demand for Internet
applications
 To take advantage of opportunities presented by
the Internet, businesses are continuously seeking
new and innovative ways and means for offering
their services via the Internet.
 This created a huge demand for software
designers with skills to create new Internet-
enabled applications or migrate existing/legacy
applications on the Internet platform.
 Object-oriented Java technologies—Sockets,
threads, RMI, clustering, Web services-- have
emerged as leading solutions for creating
portable, efficient, and maintainable large and
complex Internet applications.
295

Elements of C-S Computing
a client, a server, and network
Network
Client
Server
Client machine
Server machine
296

Networking Basics
 Applications Layer
 Standard apps
 HTTP
 FTP
 Telnet
 User apps
 Transport Layer
 TCP
 UDP
 Programming Interface:
 Sockets
 Network Layer
 IP
 Link Layer
 Device drivers
 TCP/IP Stack
Application
(http,ftp,telnet,…)
Transport
(TCP, UDP,..)
Network
(IP,..)
Link
(device driver,..)
297

Networking Basics
 TCP (Transport Control
Protocol) is a connection-
oriented protocol that
provides a reliable flow
of data between two
computers.
 Example applications:
 HTTP
 FTP
 Telnet
 TCP/IP Stack
Application
Transport
(TCP, UDP,..)
Network
(IP,..)
Link
(device driver,..)
298

Networking Basics
 UDP (User Datagram
Protocol) is a protocol
that sends independent
packets of data, called
datagrams, from one
computer to another with
no guarantees about
arrival.
 Example applications:
 Clock server
 Ping
 TCP/IP Stack
Application
Transport
(TCP, UDP,..)
Network
(IP,..)
Link
(device driver,..)
299

Understanding Ports
 The TCP and UDP
protocols use ports to
map incoming data to a
particular process
running on a computer.
server
P
o
r
t
Client
TCP
TCP or UDP
port port port port
app app app app
port# dataData
Packet
300

Understanding Ports
 Port is represented by a positive (16-bit)
integer value
 Some ports have been reserved to
support common/well known services:
 ftp 21/tcp
 telnet 23/tcp
 smtp 25/tcp
 login 513/tcp
 User level process/services generally use
port number value >= 1024
301

Sockets
 Sockets provide an interface for programming
networks at the transport layer.
 Network communication using Sockets is very
much similar to performing file I/O
 In fact, socket handle is treated like file handle.
 The streams used in file I/O operation are also
applicable to socket-based I/O
 Socket-based communication is programming
language independent.
 That means, a socket program written in Java
language can also communicate to a program written
in Java or non-Java socket program.
302

Socket Communication
 A server (program) runs on a specific
computer and has a socket that is
bound to a specific port. The server
waits and listens to the socket for a
client to make a connection request.
server
Client
Connection request
port
303

Socket Communication
 If everything goes well, the server accepts the
connection. Upon acceptance, the server gets a new
socket bounds to a different port. It needs a new
socket (consequently a different port number) so that it
can continue to listen to the original socket for
connection requests while serving the connected client.
server
Client
Connection
port
port
port 304

Sockets and Java Socket Classes
 A socket is an endpoint of a two-way
communication link between two
programs running on the network.
 A socket is bound to a port number so
that the TCP layer can identify the
application that data destined to be sent.
 Java‘s .net package provides two classes:
 Socket – for implementing a client
 ServerSocket – for implementing a server
305

Java Sockets
Socket(“128.250.25.158”, 1234)
It can be host_name like “mandroo.cs.mu.oz.au”
ServerSocket(1234)
Output/write stream
Input/read stream
Server
Client
306

Implementing a Server
1. Open the Server Socket:
ServerSocket server;
DataOutputStream os;
DataInputStream is;
server = new ServerSocket( PORT );
2. Wait for the Client Request:
Socket client = server.accept();
3. Create I/O streams for communicating to the client
is = new DataInputStream( client.getInputStream() );
os = new DataOutputStream( client.getOutputStream() );
4. Perform communication with client
Receive from client: String line = is.readLine();
Send to client: os.writeBytes("Hellon");
5. Close sockets: client.close();
For multithreaded server:
while(true) {
i. wait for client requests (step 2 above)
ii. create a thread with ―client‖ socket as parameter (the thread creates streams (as in
step (3) and does communication as stated in (4). Remove thread once service is
provided.
}
307

Implementing a Client
1. Create a Socket Object:
client = new Socket( server, port_id );
2. Create I/O streams for communicating with the server.
is = new DataInputStream(client.getInputStream() );
os = new DataOutputStream( client.getOutputStream() );
3. Perform I/O or communication with the server:
 Receive data from the server:
String line = is.readLine();
 Send data to the server:
os.writeBytes("Hellon");
4. Close the socket when done:
client.close();
308

A simple server (simplified code)
// SimpleServer.java: a simple server program
import java.net.*;
import java.io.*;
public class SimpleServer {
public static void main(String args[]) throws IOException {
// Register service on port 1234
ServerSocket s = new ServerSocket(1234);
Socket s1=s.accept(); // Wait and accept a connection
// Get a communication stream associated with the socket
OutputStream s1out = s1.getOutputStream();
DataOutputStream dos = new DataOutputStream (s1out);
// Send a string!
dos.writeUTF("Hi there");
// Close the connection, but not the server socket
dos.close();
s1out.close();
s1.close();
}
}
309

A simple client (simplified code)
// SimpleClient.java: a simple client program
import java.net.*;
import java.io.*;
public class SimpleClient {
// Open your connection to a server, at port 1234
Socket s1 = new Socket("mundroo.cs.mu.oz.au",1234);
// Get an input file handle from the socket and read the input
InputStream s1In = s1.getInputStream();
DataInputStream dis = new DataInputStream(s1In);
String st = new String (dis.readUTF());
System.out.println(st);
// When done, just close the connection and exit
dis.close();
s1In.close();
s1.close();
}
}
310

Run
 Run Server on mundroo.cs.mu.oz.au
 [raj@mundroo] java SimpleServer &
 Run Client on any machine (including mundroo):
 [raj@mundroo] java SimpleClient
Hi there
 If you run client when server is not up:
 [raj@mundroo] sockets [1:147] java SimpleClient
Exception in thread "main" java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:320)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:133)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:120)
at java.net.Socket.<init>(Socket.java:273)
at java.net.Socket.<init>(Socket.java:100)
at SimpleClient.main(SimpleClient.java:6)
311

Socket Exceptions
try {
Socket client = new Socket(host, port);
handleConnection(client);
}
catch(UnknownHostException uhe) {
System.out.println("Unknown host: " + host);
uhe.printStackTrace();
}
catch(IOException ioe) {
System.out.println("IOException: " + ioe);
ioe.printStackTrace();
}
312

ServerSocket & Exceptions
 public ServerSocket(int port) throws IOException
 Creates a server socket on a specified port.
 A port of 0 creates a socket on any free port. You can use
getLocalPort() to identify the (assigned) port on which this
socket is listening.
 The maximum queue length for incoming connection
indications (a request to connect) is set to 50. If a connection
indication arrives when the queue is full, the connection is
refused.
 Throws:
 IOException - if an I/O error occurs when opening the socket.
 SecurityException - if a security manager exists and its
checkListen method doesn't allow the operation.
313

Server in Loop: Always up
// SimpleServerLoop.java: a simple server program that runs forever in a single thead
import java.net.*;
import java.io.*;
public class SimpleServerLoop {
// Register service on port 1234
ServerSocket s = new ServerSocket(1234);
while(true)
{
Socket s1=s.accept(); // Wait and accept a connection
// Get a communication stream associated with the socket
OutputStream s1out = s1.getOutputStream();
DataOutputStream dos = new DataOutputStream (s1out);
// Send a string!
dos.writeUTF("Hi there");
// Close the connection, but not the server socket
dos.close();
s1out.close();
s1.close();
}
}
}
314

Multithreaded Server: For Serving
Multiple Clients Concurrently
Server
Threads
Server ProcessClient 1 Process
Client 2 Process
Internet
315

Conclusion
 Programming client/server applications in
Java is fun and challenging.
 Programming socket programming in Java is
much easier than doing it in other languages
such as C.
 Keywords:
 Clients, servers, TCP/IP, port number,
sockets, Java sockets
316

Multithreaded Programming
in Java
317

Agenda
 Introduction
 Thread Applications
 Defining Threads
 Java Threads and States
 Examples
318

A single threaded program
class ABC
{
….
public void main(..)
{
…
..
}
}
begin
body
end
319

A Multithreaded Program
Main Thread
Thread A Thread B Thread C
start start
start
Threads may switch or exchange data/results
320

Web/Internet Applications:
Serving Many Users Simultaneously
Internet
Server
PC
client
Local Area Network
PDA
321

Multithreaded Server: For Serving
Multiple Clients Concurrently
Server
Threads
Server ProcessClient 1 Process
Client 2 Process
 Internet
322

Modern Applications need Threads (ex1):
Editing and Printing documents in background.
Printing Thread
Editing Thread
323

Multithreaded/Parallel File Copy
reader()
{
- - - - - - - - -
-
lock(buff[i]);
read(src,buff[i]);
unlock(buff[i]);
- - - - - - - - -
-
}
writer()
{
- - - - - - - - - -
lock(buff[i]);
write(src,buff[i]);
unlock(buff[i]);
- - - - - - - - - -
}
buff[0]
buff[1]
Cooperative Parallel Synchronized
Threads
324

Levels of Parallelism
Code-Granularity
Code Item
Large grain
(task level)
Program
Medium grain
(control level)
Function (thread)
Fine grain
(data level)
Loop (Compiler)
Very fine grain
(multiple issue)
With hardware
Task i-l Task i Task i+1
func1 ( )
{
....
....
}
func2 ( )
{
....
....
}
func3 ( )
{
....
....
}
a ( 0 ) =..
b ( 0 ) =..
a ( 1 )=..
b ( 1 )=..
a ( 2 )=..
b ( 2 )=..
+ x Load
Sockets/
PVM/MPI
Threads
Compilers
CPU
325

Single and Multithreaded
Processes
Single-threaded Process
Single instruction stream Multiple instruction stream
Multiplethreaded Process
Threads of
Execution
Common
Address Space
threads are light-weight processes within a process
326

Multithreading - Multiprocessors
Process Parallelism
P1
P2
P3
time
No of execution process more the number of CPUs
CPU
CPU
CPU
327

Multithreading on Uni-processor
 Concurrency Vs Parallelism
 Process Concurrency
Number of Simultaneous execution units > number of CPUs
P1
P2
P3
time
CPU
328

What are Threads?
 A piece of code that run in concurrent with other
threads.
 Each thread is a statically ordered sequence of
instructions.
 Threads are being extensively used express
concurrency on both single and multiprocessors
machines.
 Programming a task having multiple threads of
control – Multithreading or Multithreaded
Programming.
329

Java Threads
 Java has built in thread support for
Multithreading
 Synchronization
 Thread Scheduling
 Inter-Thread Communication:
 currentThread start setPriority
 yield run getPriority
 sleep stop suspend
 resume
 Java Garbage Collector is a low-priority thread
330

Threading Mechanisms...
 Create a class that extends the Thread class
 Create a class that implements the Runnable
interface
331

1st method: Extending Thread
class
 Threads are implemented as objects that contains a method called
run()
class MyThread extends Thread
{
public void run()
{
// thread body of execution
}
}
 Create a thread:
MyThread thr1 = new MyThread();
 Start Execution of threads:
thr1.start();
332

An example
class MyThread extends Thread { // the thread
public void run() {
System.out.println(" this thread is running ... ");
}
} // end class MyThread
class ThreadEx1 { // a program that utilizes the thread
public static void main(String [] args ) {
MyThread t = new MyThread();
// due to extending the Thread class (above)
// I can call start(), and this will call
// run(). start() is a method in class Thread.
t.start();
} // end main()
} // end class ThreadEx1
333

2nd method: Threads by
implementing Runnable interface
class MyThread implements Runnable
{
.....
public void run()
{
// thread body of execution
}
}
 Creating Object:
MyThread myObject = new MyThread();
 Creating Thread Object:
Thread thr1 = new Thread( myObject );
 Start Execution:
thr1.start();
334

An example
class MyThread implements Runnable {
public void run() {
System.out.println(" this thread is running ... ");
}
} // end class MyThread
class ThreadEx2 {
public static void main(String [] args ) {
Thread t = new Thread(new MyThread());
// due to implementing the Runnable interface
// I can call start(), and this will call run().
t.start();
} // end main()
} // end class ThreadEx2
335

Life Cycle of Thread
new
runnable non-runnable
dead
wait()
sleep()
suspend()
blocked
notify()
slept
resume()
unblocked
start()
stop()
336

A Program with Three Java Threads
 Write a program that creates 3 threads
337

Three threads example
 class A extends Thread
 {
 public void run()
 {
 for(int i=1;i<=5;i++)
 {
 System.out.println("t From ThreadA: i= "+i);
 }
 System.out.println("Exit from A");
 }
 }
 class B extends Thread
 {
 {
 for(int j=1;j<=5;j++)
 {
 System.out.println("t From ThreadB: j= "+j);
 }
 System.out.println("Exit from B");
 }
 } 338

 class C extends Thread
 {
 {
 for(int k=1;k<=5;k++)
 {
 System.out.println("t From ThreadC: k= "+k);
 }
 System.out.println("Exit from C");
 }
 }
 class ThreadTest
 {
 public static void main(String args[])
 {
 new A().start();
 new B().start();
 new C().start();
 }
 }
339

Run 1
 [raj@mundroo] threads [1:76] java ThreadTest
From ThreadA: i= 1
From ThreadA: i= 2
From ThreadA: i= 3
From ThreadA: i= 4
From ThreadA: i= 5
Exit from A
From ThreadC: k= 1
From ThreadC: k= 2
From ThreadC: k= 3
From ThreadC: k= 4
From ThreadC: k= 5
Exit from C
From ThreadB: j= 1
From ThreadB: j= 2
From ThreadB: j= 3
From ThreadB: j= 4
From ThreadB: j= 5
Exit from B
340

Run2
 [raj@mundroo] threads [1:77] java ThreadTest
From ThreadA: i= 1
From ThreadA: i= 2
From ThreadA: i= 3
From ThreadA: i= 4
From ThreadA: i= 5
From ThreadC: k= 1
From ThreadC: k= 2
From ThreadC: k= 3
From ThreadC: k= 4
From ThreadC: k= 5
Exit from C
From ThreadB: j= 1
From ThreadB: j= 2
From ThreadB: j= 3
From ThreadB: j= 4
From ThreadB: j= 5
Exit from B
Exit from A
341

Process Parallelism
 int add (int a, int b, int & result)
 // function stuff
 int sub(int a, int b, int & result)
 // function stuff Processor
pthread t1, t2;
pthread-create(&t1, add, a,b, & r1);
pthread-create(&t2, sub, c,d, & r2);
pthread-par (2, t1, t2);
MISD and MIMD Processing
a
b
r1
c
d
r2
add
sub
Data
IS1
IS2
Processor
342

Data Parallelism
 sort( int *array, int count)
 //......
 //...... do
“
“
dn/2
dn2/+1
“
“
dn
Sort
Data
IS
pthread-t, thread1, thread2;
“
“
pthread-create(& thread1, sort, array, N/2);
pthread-create(& thread2, sort, array, N/2);
pthread-par(2, thread1, thread2);
SIMD Processing
Sort
Processor
Processor
343

Accessing Shared Resources
 Applications Access to Shared
Resources need to be coordinated.
 Printer (two person jobs cannot be
printed at the same time)
 Simultaneous operations on your bank
account
344

Online Bank: Serving Many Customers
and Operations
Internet Bank
Server
PC client
Local Area Network
PDA
Bank
Database
345

Thread Priority
 In Java, each thread is assigned priority,
which affects the order in which it is
scheduled for running. The threads so far
had same default priority (ORM_PRIORITY)
and they are served using FCFS policy.
 Java allows users to change priority:
 ThreadName.setPriority(intNumber)
 MIN_PRIORITY = 1
 NORM_PRIORITY=5
 MAX_PRIORITY=10
346

Thread Priority Example
class A extends Thread
{
public void run()
{
System.out.println("Thread A started");
for(int i=1;i<=4;i++)
{
System.out.println("t From ThreadA: i= "+i);
}
System.out.println("Exit from A");
}
}
class B extends Thread
{
public void run()
{
System.out.println("Thread B started");
for(int j=1;j<=4;j++)
{
System.out.println("t From ThreadB: j= "+j);
}
System.out.println("Exit from B");
}
}
347

Thread Priority Example
class C extends Thread
{
public void run()
{
System.out.println("Thread C started");
for(int k=1;k<=4;k++)
{
System.out.println("t From ThreadC: k= "+k);
}
System.out.println("Exit from C");
}
}
class ThreadPriority
{
{
A threadA=new A();
B threadB=new B();
C threadC=new C();
threadC.setPriority(Thread.MAX_PRIORITY);
threadB.setPriority(threadA.getPriority()+1);
threadA.setPriority(Thread.MIN_PRIORITY);
System.out.println("Started Thread A");
threadA.start();
System.out.println("Started Thread B");
threadB.start();
System.out.println("Started Thread C");
threadC.start();
System.out.println("End of main thread");
}
}
348

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