Introduction to java programming

Introduction to Java
Programming
Prepared By:
Dr.J. Shiny Duela
Assistant Professor
Dept. of CSE
Jerusalem College of Engineering

• Object is the physical as well as logical entity
• class is the logical entity only
Object in Java
• An entity that has state and behavior is known as an object
• e.g. chair, bike, marker, pen, table, car etc.
• It can be physical or logical (tangible and intangible).
• The example of intangible object is banking system.

An object has three characteristics:
• state: represents data (value) of an object.
• behavior: represents the behavior (functionality) of an object
such as deposit, withdraw etc.
• identity:
• Object identity is typically implemented via a unique ID.
• The value of the ID is not visible to the external user.
• But, it is used internally by the JVM to identify each object
uniquely.

• For Example:
• Pen is an object.
• Its name is Reynolds,
• color is white etc. known as its state.
• It is used to write, so writing is its behavior.

Object is an instance of a class.
• Class is a template or blueprint from which objects are
created.
• So object is the instance(result) of a class.
Object Definitions:
• Object is a real world entity.
• Object is a run time entity.
• Object is an entity which has state and behavior.
• Object is an instance of a class.

Class in Java
• A class is a group of objects which have common properties.
• It is a template or blueprint from which objects are created.
• A class in Java can contain:
 fields
 methods
 constructors
 blocks
 nested class and interface

Syntax to declare a class:
class <class_name>
{
field;
method;
}

Instance variable in Java
• A variable which is created inside the class but outside the
method, is known as instance variable.
• Instance variable doesn't get memory at compile time.
• It gets memory at run time when object(instance) is created.
• That is why, it is known as instance variable.

Method in Java
• In java, a method is like function i.e. used to expose behavior
of an object.
Advantage of Method
• Code Reusability
• Code Optimization

new keyword in Java
• The new keyword is used to allocate memory at run time.
• All objects get memory in Heap memory area.

Object and Class Example: main within class
• We are creating the object of the Student class by new
keyword and printing the objects value.
• Here, we are creating main() method inside the class.
• File: Student.java

class Student
{
int id; //field or data member or instance variable
String name;
public static void main(String args[]){
Student s1=new Student();//creating an object of Student
System.out.println(s1.id);//accessing member through reference v
ariable
System.out.println(s1.name);
}
}

Object and Class Example: main outside class
File: TestStudent1.java
class Student{
int id;
String name;
}
class TestStudent1{
Student s1=new Student();
System.out.println(s1.id);
System.out.println(s1.name);
}
}

3 Ways to initialize object
• There are 3 ways to initialize object in java.
• By reference variable
• By method
• By constructor

class Student{
int id;
String name;
}
class TestStudent2{
s1.id=101;
s1.name="Sonoo";
System.out.println(s1.id+" "+s1.name);//printing members with a white
space
}
}

class Student
{ int id;
String name; }
class TestStudent3{
//Initializing objects
s1.id=101;
s1.name="Sonoo";
s2.id=102;
s2.name="Amit";
System.out.println(s1.id+" "+s1.name);
System.out.println(s2.id+" "+s2.name);
} }

class Student{
int rollno;
String name;
void insertRecord(int r, String n){
rollno=r;
name=n;
}
void displayInformation()
{
System.out.println(rollno+" "+name);
}
}

class TestStudent4{
public static void main(String args[])
{
s1.insertRecord(111,"Karan");
s2.insertRecord(222,"Aryan");
s1.displayInformation();
s2.displayInformation();
}
}

class Employee{
int id;
String name;
float salary;
void insert(int i, String n, float s)
{
id=i;
name=n;
salary=s;
}
void display()
{
System.out.println(id+" "+name+" "+salary);}
}

public class TestEmployee {
public static void main(String[] args) {
Employee e1=new Employee();
e1.insert(101,"ajeet",45000);
e2.insert(102,"irfan",25000);
e3.insert(103,"nakul",55000);
e1.display();
e2.display();
e3.display();
}
}

• There are many ways to create an object in java. They are:
• By new keyword
• By newInstance() method
• By clone() method
• By deserialization
• By factory method etc.

• Anonymous object
• Anonymous simply means nameless.
• An object which has no reference is known as anonymous
object.
• It can be used at the time of object creation only.
• If you have to use an object only once, anonymous object is a
good approach.
• For example:

new Calculation();//anonymous object
• Calling method through reference:
Calculation c=new Calculation();
c.fact(5);
• Calling method through anonymous object
new Calculation().fact(5);

public class CommandLine
{
{
for(int i = 0; i<args.length; i++)
{
System.out.println("args[" + i + "]: " + args[i]);
} } }

$java CommandLine this is a command line 200 -100
Output
args[0]: this
args[1]: is
args[2]: a
args[3]: command
args[4]: line
args[5]: 200
args[6]: -100

Method Overloading
• When a class has two or more methods by the same name
but different parameters, it is known as method overloading.
• It is different from overriding.
• In overriding, a method has the same method name, type,
number of parameters, etc.

Introduction to java programming

Method Overriding
• If subclass (child class) has the same method as declared in
the parent class, it is known as method overriding in java.
• In other words, If subclass provides the specific
implementation of the method that has been provided by
one of its parent class, it is known as method overriding.

• Usage of Java Method Overriding
• Method overriding is used to provide specific implementation
of a method that is already provided by its super class.
• Method overriding is used for runtime polymorphism
• Rules for Java Method Overriding
• method must have same name as in the parent class
• method must have same parameter as in the parent class.
• must be IS-A relationship (inheritance).

• Understanding the problem without method overriding
class Vehicle{
void run(){System.out.println("Vehicle is running");}
}
class Bike extends Vehicle{
Bike obj = new Bike();
obj.run();
}
}

Example of method overriding
• In this example, we have defined the run method in the
subclass as defined in the parent class but it has some
specific implementation.
• The name and parameter of the method is same and there
is IS-A relationship between the classes, so there is method
overriding.

class Vehicle{
void run(){System.out.println("Vehicle is running");}
}
class Bike2 extends Vehicle{
void run(){System.out.println("Bike is running safely");}
Bike2 obj = new Bike2();
obj.run();
}

Inheritance in Java
• a mechanism in which one object acquires all the properties
and behaviors of parent object.
• Inheritance represents the IS-A relationship, also known
as parent-child relationship.
Why use inheritance in java
• For Method Overriding (so runtime polymorphism can be
achieved).
• For Code Reusability.

Syntax of Java Inheritance:
class Subclass-name extends Superclass-name
{
//methods and fields
}

• The extends keyword indicates that you are making a new
class that derives from an existing class.
• The meaning of "extends" is to increase the functionality.
• a class which is inherited is called parent or super class and
the new class is called child or subclass.

class Employee{
float salary=40000;
}
class Programmer extends Employee
{
int bonus=10000;
Programmer p=new Programmer();
System.out.println("Programmer salary is:"+p.salary);
System.out.println("Bonus of Programmer is:"+p.bonus);
}
}

Programmer salary is:40000.0
Bonus of programmer is:10000

• multiple and hybrid inheritance is supported through
interface only.
• When a class extends multiple classes i.e. known as multiple
inheritance. For Example:

Single Inheritance Example
File: TestInheritance.java
class Animal{
void eat(){System.out.println("eating...");}
}
class Dog extends Animal{
void bark(){System.out.println("barking...");}
}
class TestInheritance{
Dog d=new Dog();
d.bark();
d.eat();
}}
Output:
barking... eating...

class Animal{
}
}
class BabyDog extends Dog{
void weep(){System.out.println("weeping...");
}
}
Output:
weeping...
barking...
eating...
class TestInheritance2
{
{
BabyDog d=new BabyDog();
d.weep();
d.bark();
d.eat();
}
}

Hierarchical Inheritance Example
class Animal{
}
}
class Cat extends Animal{
void meow(){System.out.println("meowing...");}
}
Output:
meowing...
eating...

class TestInheritance3{
Cat c=new Cat();
c.meow();
c.eat();
//c.bark();//C.T.Error
}}

Why multiple inheritance is not supported in
java?
• To reduce the complexity and simplify the language, multiple
inheritance is not supported in java.
• Consider a scenario where A, B and C are three classes. The C
class inherits A and B classes. If A and B classes have same
method and you call it from child class object, there will be
ambiguity to call method of A or B class.
• Since compile time errors are better than runtime errors, java
renders compile time error if you inherit 2 classes. So whether
you have same method or different, there will be compile time
error now.

class A{
void msg(){System.out.println("Hello");}
}
class B{
void msg(){System.out.println("Welcome");}
}
class C extends A,B{//suppose if it were
Public Static void main(String args[]){
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}

Packages
• A java package is a group of similar types of classes,
interfaces and sub-packages.
• Package in java can be categorized in two form, built-in
package and user-defined package.
• There are many built-in packages such as java, lang, awt,
javax, swing, net, io, util, sql etc.
• Here, we will have the detailed learning of creating and
using user-defined packages.

Advantage of Java Package
1) Java package is used to categorize the classes and interfaces
so that they can be easily maintained.
2) Java package provides access protection.
3) Java package removes naming collision.

• The package keyword is used to create a package in java.
//save as Simple.java
package mypack;
public class Simple{
System.out.println("Welcome to package");
}
}
Compile:
javac -d . Simple.java
To Run:
java mypack.Simple

• The -d is a switch that tells the compiler where to put the
class file i.e. it represents destination.
• The . represents the current folder.
How to access package from another package?
• There are three ways to access the package from outside the
package.
• import package.*;
• import package.classname;
• fully qualified name.

Example of package that import the packagename.*
//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}

//save by B.java
package mypack;
import pack.*;
class B{
A obj = new A();
obj.msg();
}
}
Output:Hello

Using packagename.classname
If you import package.classname then only declared class of
this package will be accessible.
Example of package by import package.classname
//save by A.java
package pack;
public class A{
}

//save by B.java
import pack.A;
class B{
A obj = new A();
obj.msg();
}
}
Output:Hello

example of package by import fully qualified name
//save by A.java
package pack;
public class A{
}
//save by B.java
package mypack;
class B{
pack.A obj = new pack.A();//using fully qualified name
obj.msg();
}
}
Output:Hello

Abstract class in Java
• A class that is declared with abstract keyword, is known as abstract class in
java.
• It can have abstract and non-abstract methods (method with body).
• Abstraction is a process of hiding the implementation details and showing only
functionality to the user.
• Another way, it shows only important things to the user and hides the
internal details
• for example sending sms, you just type the text and send the message. You
don't know the internal processing about the message delivery.
• Abstraction lets you focus on what the object does instead of how it does it.
• Ways to achieve Abstraction
• There are two ways to achieve abstraction in java
• Abstract class (0 to 100%)
• Interface (100%)

• A class that is declared as abstract is known as abstract
class. It needs to be extended and its method implemented.
It cannot be instantiated.
• Example abstract class
abstract class A
{
}

abstract method
• A method that is declared as abstract and does not have
implementation is known as abstract method.
• Example abstract method
• abstract void printStatus();//no body and abstract

abstract class Base
{
abstract void fun();
}
class Derived extends Base
{
void fun()
{
System.out.println("Derived fun() called");
}
}
class Main
{
public static void main(String args[]) {
// Uncommenting the following line will cause compiler error as the
// line tries to create an instance of abstract class.
// Base b = new Base();
// We can have references of Base type.
Base b = new Derived();
b.fun();
}

Example of abstract class that has abstract
method
In this example,
Bike the abstract class that contains only one abstract method run.
It implementation is provided by the Honda class.
abstract class Bike
{
abstract void run();
}
class Honda4 extends Bike{
void run(){System.out.println("running safely..");}
Bike obj = new Honda4();
obj.run();
}
}
running safely..

abstract class Bank{
abstract int getRateOfInterest();
}
class SBI extends Bank{
int getRateOfInterest(){return 7;}
}
class PNB extends Bank{
int getRateOfInterest(){return 8;}
}
Rate of Interest is: 7 %
Rate of Interest is: 8 %

class TestBank
{
Bank b;
b=new SBI();
System.out.println("Rate of Interest is: "+b.getRateOfInterest()+
" %");
b=new PNB();
System.out.println("Rate of Interest is: "+b.getRateOfInterest()+
" %");
}}

An exception is an unwanted or unexpected event, which
occurs during the execution of a program i.e at run time, that
disrupts the normal flow of the program’s instructions.
• Error: An Error indicates serious problem that a reasonable application should not try to catch.
Exception: Exception indicates conditions that a reasonable application might try to catch.
• Java Exception Handling Keywords
• There are 5 keywords used in java exception handling.
• try
• catch
• finally
• throw
• throws

• Scenario where ArithmeticException occurs
• If we divide any number by zero, there occurs an
ArithmeticException.
• int a=50/0;//ArithmeticException
• Scenario where NullPointerException occurs
• If we have null value in any variable, performing any operation by
the variable occurs an NullPointerException.
• String s=null;
• System.out.println(s.length());//NullPointerException

• Scenario where NumberFormatException occurs
• The wrong formatting of any value, may occur
NumberFormatException. Suppose I have a string variable
that have characters, converting this variable into digit will
occur NumberFormatException.
• String s="abc";
• int i=Integer.parseInt(s);//NumberFormatException

• Scenario where ArrayIndexOutOfBoundsException occurs
• If you are inserting any value in the wrong index, it would
result ArrayIndexOutOfBoundsException as shown below:
• int a[]=new int[5];
• a[10]=50; //ArrayIndexOutOfBoundsException

public class TestMultipleCatchBlock{
try{
int a[]=new int[5];
a[5]=30/0;
}
catch(ArithmeticException e)
{System.out.println("task1 is completed");}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("task 2 completed");}
catch(Exception e)
{System.out.println("common task completed");}
System.out.println("rest of the code...");
}
}
task1 is completed
rest of the code...

• At a time only one Exception is occured and at a time only
one catch block is executed.
• All catch blocks must be ordered from most specific to most
general i.e. catch for ArithmeticException must come before
catch for Exception .

class TestMultipleCatchBlock1{
try{
int a[]=new int[5];
a[5]=30/0;
}
catch(Exception e)
{System.out.println("common task completed");}
catch(ArithmeticException e)
{System.out.println("task1 is completed");}
catch(ArrayIndexOutOfBoundsException e)
{System.out.println("task 2 completed");}
}
}
Output:
Compile-time error

Java Nested try block
• The try block within a try block is known as nested try block in java.
Why use nested try block
• Sometimes a situation may arise where a part of a block may cause
one error and the entire block itself may cause another error. In such
cases, exception handlers have to be nested.

class Excep6{
try{
try{
System.out.println("going to divide");
int b =39/0;
}catch(ArithmeticException e){System.out.println(e);}
try{
int a[]=new int[5];
a[5]=4;
}catch(ArrayIndexOutOfBoundsException e){System.out.println(e);}
System.out.println("other statement);
}catch(Exception e){System.out.println("handeled");}
System.out.println("normal flow..");
}
}

• Why use java finally
• Finally block in java can be used to put "cleanup" code such
as closing a file, closing connection etc.
• For each try block there can be zero or more catch blocks,
but only one finally block.
• The finally block will not be executed if program exits(either
by calling System.exit() or by causing a fatal error that
causes the process to abort).

class TestFinallyBlock{
try{
int data=25/5;
System.out.println(data);
}
catch(NullPointerException e){System.out.println(e);}
finally{System.out.println("finally block is always executed");}
}
}

• Java throw keyword
• The Java throw keyword is used to explicitly throw an exception.
• The throw keyword in Java is used to explicitly throw an
exception from a method or any block of code.
• The throw keyword is mainly used to throw custom exceptions.
Syntax:
• throw Instance
• Example: throw new ArithmeticException("/ by zero");

public class TestThrow1{
static void validate(int age){
if(age<18)
throw new ArithmeticException("not valid");
else
System.out.println("welcome to vote");
}
{
validate(13);
}
}
Exception in thread main
java.lang.ArithmeticException:not valid

Java throws keyword
• The Java throws keyword is used to declare an exception. It gives an
information to the programmer that there may occur an exception so it
is better for the programmer to provide the exception handling code so
that normal flow can be maintained.
• throws is a keyword in Java which is used in the signature of method to
indicate that this method might throw one of the listed type exceptions.
• The caller to these methods has to handle the exception using a try-
catch block.
• Syntax:
• type method_name(parameters) throws exception_list
• exception_list is a comma separated list of all the exceptions which a
method might throw.

import java.io.IOException;
class Testthrows1{
void m()throws IOException{
throw new IOException("device error");//checked exception
}
void n()throws IOException{
m();
}
void p(){
try{
n();
}catch(Exception e){System.out.println("exception handled");}
}
Testthrows1 obj=new Testthrows1();
obj.p();
System.out.println("normal flow...");
}
}

• Multithreading in java is a process of executing multiple
threads simultaneously.
• Thread is basically a lightweight sub-process, a smallest unit
of processing. Multiprocessing and multithreading, both are
used to achieve multitasking.
• But we use multithreading than multiprocessing because
threads share a common memory area. They don't allocate
separate memory area so saves memory, and context-
switching between the threads takes less time than process.
• Java Multithreading is mostly used in games, animation etc

• Advantages of Java Multithreading
• 1) It doesn't block the user because threads are independent
and you can perform multiple operations at same time.
• 2) You can perform many operations together so it saves
time.
• 3) Threads are independent so it doesn't affect other threads
if exception occur in a single thread.

• Multitasking
• Multitasking is a process of executing multiple tasks
simultaneously. We use multitasking to utilize the CPU.
Multitasking can be achieved by two ways:
• Process-based Multitasking(Multiprocessing)
• Thread-based Multitasking(Multithreading)

Life cycle of a Thread (Thread States)
• The life cycle of the thread in java is controlled by JVM. The
java thread states are as follows:
• New
• Runnable
• Running
• Non-Runnable (Blocked)
• Terminated

• 1) New
• The thread is in new state if you create an instance of Thread
class but before the invocation of start() method.
• 2) Runnable
• The thread is in runnable state after invocation of start() method,
but the thread scheduler has not selected it to be the running
thread.
• 3) Running
• The thread is in running state if the thread scheduler has selected
it.
• 4) Non-Runnable (Blocked)
• This is the state when the thread is still alive, but is currently not
eligible to run.
• 5) Terminated
• A thread is in terminated or dead state when its run() method
exits.

• How to create thread
• There are two ways to create a thread:
• By extending Thread class
• By implementing Runnable interface.

Thread class:
• Thread class provide constructors and methods to create and
perform operations on a thread.
• Thread class extends Object class and implements Runnable
interface.
• Commonly used Constructors of Thread class:
• Thread()
• Thread(String name)
• Thread(Runnable r)
• Thread(Runnable r,String name)

• Commonly used methods of Thread class:
• public void run(): is used to perform action for a thread.
• public void start(): starts the execution of the thread.JVM calls the run() method on
the thread.
• public void sleep(long miliseconds): Causes the currently executing thread to sleep
(temporarily cease execution) for the specified number of milliseconds.
• public void join(): waits for a thread to die.
• public void join(long miliseconds): waits for a thread to die for the specified
miliseconds.
• public int getPriority(): returns the priority of the thread.
• public int setPriority(int priority): changes the priority of the thread.

• public String getName(): returns the name of the thread.
• public void setName(String name): changes the name of the
thread.
• public Thread currentThread(): returns the reference of currently
executing thread.
• public int getId(): returns the id of the thread.
• public Thread.State getState(): returns the state of the thread.
• public boolean isAlive(): tests if the thread is alive.
• public void yield(): causes the currently executing thread object to
temporarily pause and allow other threads to execute.
• public void suspend(): is used to suspend the thread(depricated).
• public void resume(): is used to resume the suspended
thread(depricated).

• public void stop(): is used to stop the thread(depricated).
• public boolean isDaemon(): tests if the thread is a daemon thread.
• public void setDaemon(boolean b): marks the thread as daemon or
user thread.
• public void interrupt(): interrupts the thread.
• public boolean isInterrupted(): tests if the thread has been
interrupted.
• public static boolean interrupted(): tests if the current thread has
been interrupted.

• Runnable interface:
• The Runnable interface should be implemented by any class
whose instances are intended to be executed by a thread.
Runnable interface have only one method named run().
• public void run(): is used to perform action for a thread.

• Starting a thread:
• start() method of Thread class is used to start a newly
created thread. It performs following tasks:A new thread
starts(with new callstack).
• The thread moves from New state to the Runnable state.
• When the thread gets a chance to execute, its target run()
method will run.

Java Thread Example by extending Thread
class
class Multi extends Thread{
public void run(){
System.out.println("thread is running...");
}
Multi t1=new Multi();
t1.start();
}
}

Java Thread Example by implementing
Runnable interface
class Multi3 implements Runnable{
public void run(){
System.out.println("thread is running...");
}
Multi3 m1=new Multi3();
Thread t1 =new Thread(m1);
t1.start();
}
}

class TestSleepMethod1 extends Thread{
public void run(){
for(int i=1;i<5;i++){
try{Thread.sleep(500);}catch(InterruptedException e){System.out.println(e)
;}
System.out.println(i);
}
}
TestSleepMethod1 t1=new TestSleepMethod1();
TestSleepMethod1 t2=new TestSleepMethod1();
t1.start();
t2.start();
}
}
1
1
2
2
3
3
4
4

Can we start a thread twice
• No. After starting a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown.
In such case, thread will run once but for second time, it will throw exception.
public class TestThreadTwice1 extends Thread{
public void run(){
System.out.println("running...");
}
TestThreadTwice1 t1=new TestThreadTwice1();
t1.start();
t1.start();
}
}
running
Exception in thread "main" java.lang.IllegalThreadStateException

Interface in Java
• An interface in java is a blueprint of a class. It has static constants
and abstract methods.
• The interface in java is a mechanism to achieve abstraction. There
can be only abstract methods in the java interface not method
body. It is used to achieve abstraction and multiple inheritance in
Java.
• Java Interface also represents IS-A relationship.
• It cannot be instantiated just like abstract class.
•

• Why use Java interface?
• There are mainly three reasons to use interface. They are
given below.
• It is used to achieve abstraction.
• By interface, we can support the functionality of multiple
inheritance.
• It can be used to achieve loose coupling.

• The java compiler adds public and abstract keywords before the
interface method. More, it adds public, static and final
keywords before data members.
interface <interface_name>
{
// declare constant fields
// declare methods that abstract
// by default.
}

• Understanding relationship between classes and interfaces
• As shown in the figure given below, a class extends another
class, an interface extends another interface but a class
implements an interface.

Java Interface Example
In this example, Printable interface has only one method, its
implementation is provided in the A class.
interface printable{
void print();
}
class A6 implements printable{
public void print(){System.out.println("Hello");}
A6 obj = new A6();
obj.print();
}
}

Drawable.java
//Interface declaration: by first user
interface Drawable{
void draw();
}

class Rectangle implements Drawable{
public void draw()
{System.out.println("drawing rectangle");}
}
class Circle implements Drawable{
public void draw(){System.out.println("drawing circle");}
}
class TestInterface1{
Drawable d=new Circle();
d.draw();
}}

Nested Interfaces
interface NewsPaper
{
news();
}
interface Magazine extends NewsPaper
{
colorful();
}

interface Moveable
{
int AVG-SPEED = 40;
void move();
}
class Vehicle implements Moveable
{
public void move()
{
System .out. println ("Average speed is"+AVG-SPEED");
}
public static void main (String[] args)
{
Vehicle vc = new Vehicle();
vc.move();
} }

Java Applet
• Applet is a special type of program that is embedded in the
webpage to generate the dynamic content. It runs inside the
browser and works at client side.
Advantage of Applet
• There are many advantages of applet. They are as follows:
• It works at client side so less response time.
• Secured
• It can be executed by browsers running under many
platforms, including Linux, Windows, Mac Os etc.

Drawback of Applet
• Plugin is required at client browser to execute applet.
Lifecycle of Java Applet
• Applet is initialized.
• Applet is started.
• Applet is painted.
• Applet is stopped.
• Applet is destroyed.
•

java.applet.Applet class
• For creating any applet java.applet.Applet class must be inherited.
It provides 4 life cycle methods of applet.
• public void init(): is used to initialized the Applet. It is invoked only
once.
• public void start(): is invoked after the init() method or browser is
maximized. It is used to start the Applet.
• public void stop(): is used to stop the Applet. It is invoked when
Applet is stop or browser is minimized.
• public void destroy(): is used to destroy the Applet. It is invoked
only once.

java.awt.Component class
• The Component class provides 1 life cycle method of applet.
• public void paint(Graphics g): is used to paint the Applet. It
provides Graphics class object that can be used for drawing oval,
rectangle, arc etc.
How to run an Applet?
• There are two ways to run an applet
• By html file.
• By appletViewer tool (for testing purpose).
•

Simple example of Applet by html file:
To execute the applet by html file, create an applet and compile it.
After that create an html file and place the applet code in html
file. Now click the html file.
//First.java
import java.applet.Applet;
import java.awt.Graphics;
public class First extends Applet{
public void paint(Graphics g){
g.drawString("welcome",150,150);
}
}

myapplet.html
<html>
<body>
<applet code="First.class" width="300" height="300">
</applet>
</body>
</html>

Simple example of Applet by appletviewer tool:
To execute the applet by appletviewer tool, create an applet that contains applet tag in
comment and compile it. After that run it by: appletviewer First.java. Now Html file
is not required but it is for testing purpose only.
//First.java
import java.awt.Graphics;
public class First extends Applet{
g.drawString("welcome to applet",150,150);
}
}
/*
<applet code="First.class" width="300" height="300">
</applet>
*/

• To execute the applet by appletviewer tool, write in
command prompt:
c:>javac First.java
c:>appletviewer First.java

 Displaying Graphics in Applet
 java.awt.Graphics class provides many methods for graphics
programming.
 Commonly used methods of Graphics class:
• public abstract void drawString(String str, int x, int y): is used to draw
the specified string.
• public void drawRect(int x, int y, int width, int height): draws a
rectangle with the specified width and height.
• public abstract void fillRect(int x, int y, int width, int height): is used to
fill rectangle with the default color and specified width and height.
• public abstract void drawOval(int x, int y, int width, int height): is used
to draw oval with the specified width and height.
• public abstract void fillOval(int x, int y, int width, int height): is used to
fill oval with the default color and specified width and height.
• public abstract void drawLine(int x1, int y1, int x2, int y2): is used to
draw line between the points(x1, y1) and (x2, y2).

• public abstract boolean drawImage(Image img, int x, int y,
ImageObserver observer): is used draw the specified image.
• public abstract void drawArc(int x, int y, int width, int height, int
startAngle, int arcAngle): is used draw a circular or elliptical arc.
• public abstract void fillArc(int x, int y, int width, int height, int
startAngle, int arcAngle): is used to fill a circular or elliptical arc.
• public abstract void setColor(Color c): is used to set the graphics
current color to the specified color.
• public abstract void setFont(Font font): is used to set the graphics
current font to the specified font.

import java.awt.*;
public class GraphicsDemo extends Applet{
g.setColor(Color.red);
g.drawString("Welcome",50, 50);
g.drawLine(20,30,20,300);
g.drawRect(70,100,30,30);
g.fillRect(170,100,30,30);
g.drawOval(70,200,30,30);
g.setColor(Color.pink);
g.fillOval(170,200,30,30);
g.drawArc(90,150,30,30,30,270);
g.fillArc(270,150,30,30,0,180);
}
}

myapplet.html
<html>
<body>
<applet code="GraphicsDemo.class" width="300" height="300">
</applet>
</body>
</html>

Streams and IO
• Java I/O (Input and Output) is used to process the input and produce the output.
• Java uses the concept of stream to make I/O operation fast.
• The java.io package contains all the classes required for input and output operations.

Stream
• A stream is a sequence of data.
• In Java a stream is composed of bytes.
• In java, 3 streams are created for us automatically. All these streams are attached with
console.
• 1) System.out: standard output stream
• 2) System.in: standard input stream
• 3) System.err: standard error stream

Syntax
• code to print output and error message to the console.
System.out.println("simple message");
System.err.println("error message");
• code to get input from console.
int i=System.in.read();//returns ASCII code of 1st character
System.out.println((char)i);//will print the character

OutputStream vs InputStream
• OutputStream
• Java application uses an output stream to write data to a destination, it may be a file,
an array, peripheral device or socket.
• InputStream
• Java application uses an input stream to read data from a source, it may be a file, an
array, peripheral device or socket.

OutputStream class
• OutputStream class is an abstract class.
• It is the super class of all classes representing an output stream of bytes.
• An output stream accepts output bytes and sends them to some sink.
• Useful methods of OutputStream:
Method Description
1) public void write(int)throws IOException is used to write a byte to the current output stream.
2)public void write(byte[])throws IOException is used to write an array of byte to the current output stream.
3) public void flush()throws IOException flushes the current output stream.
4) public void close()throws IOException is used to close the current output stream.

InputStream class
• InputStream class is an abstract class.
• It is the super class of all classes representing an input stream of bytes.
• Useful methods of InputStream:
Method Description
1) public abstract int read()throws IOException reads the next byte of data from the input stream. It returns
-1 at the end of file.
2) public int available()throws IOException returns an estimate of the number of bytes that can be read
from the current input stream.
3) public void close()throws IOException is used to close the current input stream.

Java FileOutputStream Class
• Java FileOutputStream is an output stream used for writing data to a file.
• If you have to write primitive values into a file, use FileOutputStream class.
• You can write byte-oriented as well as character-oriented data through FileOutputStream class.
• But, for character-oriented data, it is preferred to use FileWriter than FileOutputStream.
• FileOutputStream class declaration:
public class FileOutputStream extends OutputStream

FileOutputStream class methods:
Method Description
protected void finalize() It is used to clean up the connection with the file output
stream.
void write(byte[] ary) It is used to write ary.length bytes from the byte array to
the file output stream.
void write(byte[] ary, int off, int len) It is used to write len bytes from the byte array starting at
offset off to the file output stream.
void write(int b) It is used to write the specified byte to the file output
stream.
FileChannel getChannel() It is used to return the file channel object associated with
the file output stream.
FileDescriptor getFD() It is used to return the file descriptor associated with the
stream.
void close() It is used to closes the file output stream.

Java FileOutputStream Example 1: write byte
import java.io.FileOutputStream;
public class FileOutputStreamExample {
try{
FileOutputStream fout=new FileOutputStream("D:testout.txt");
fout.write(65);
fout.close();
System.out.println("success...");
}catch(Exception e){System.out.println(e);}
}
}
Output:
Success...
testout.txt
A

Java FileOutputStream example 2: write string
import java.io.FileOutputStream;
public class FileOutputStreamExample {
try{
String s="Welcome to the world of Java";
byte b[]=s.getBytes();//converting string into byte array
fout.write(b);
fout.close();
System.out.println("success...");
}
}
Output:
Success...
testout.txt
Welcome to the world of Java

Java FileInputStream Class
• Java FileInputStream class obtains input bytes from a file.
• It is used for reading byte-oriented data (streams of raw bytes) such as image data, audio, video etc.
• You can also read character-stream data.
• But, for reading streams of characters, it is recommended to use FileReader class.
• Java FileInputStream class declaration
public class FileInputStream extends InputStream

Java FileInputStream class methods
Method Description
int available() It is used to return the estimated number of bytes that can be
read from the input stream.
int read() It is used to read the byte of data from the input stream.
int read(byte[] b) It is used to read up to b.length bytes of data from the input
stream.
int read(byte[] b, int off, int len) It is used to read up to len bytes of data from the input stream.
long skip(long x) It is used to skip over and discards x bytes of data from the
input stream.
FileChannel getChannel() It is used to return the unique FileChannel object associated
with the file input stream.
FileDescriptor getFD() It is used to return the FileDescriptor object.
protected void finalize() It is used to ensure that the close method is call when there is
no more reference to the file input stream.
void close() It is used to closes the stream.

Java FileInputStream example 1: read single character
import java.io.FileInputStream;
public class DataStreamExample {
try{
FileInputStream fin=new FileInputStream("D:testout.txt");
int i=fin.read();
System.out.print((char)i);
fin.close();
}
}
Before running the code, a text file named as "testout.txt" is
required to be created.
In this file, we are having following content:
Welcome to javatpoint.
After executing the above program, you will get a single
character from the file which is 87 (in byte form).
To see the text, you need to convert it into character.
Output:
W

Java FileInputStream example 2: read all characters
import java.io.FileInputStream;
public class DataStreamExample {
try{
FileInputStream fin=new FileInputStream("D:testout.txt");
int i=0;
while((i=fin.read())!=-1){
System.out.print((char)i);
}
fin.close();
}
}
Output:
Welcome to javaTpoint

Java BufferedOutputStream Class
• Java BufferedOutputStream class is used for buffering an output stream.
• It internally uses buffer to store data.
• It adds more efficiency than to write data directly into a stream.
• So, it makes the performance fast.
• For adding the buffer in an OutputStream, use the BufferedOutputStream class.
• syntax for adding the buffer in an OutputStream:
OutputStream os= new BufferedOutputStream(new FileOutputStream("D:IO Packagetestout.txt"));
• Java BufferedOutputStream class declaration
public class BufferedOutputStream extends FilterOutputStream

Java BufferedOutputStream class constructors
Constructor Description
BufferedOutputStream(OutputStream os) It creates the new buffered output stream which is used for
writing the data to the specified output stream.
BufferedOutputStream(OutputStream os, int size) It creates the new buffered output stream which is used for
writing the data to the specified output stream with a specified
buffer size.
Java BufferedOutputStream class methods
Method Description
void write(int b) It writes the specified byte to the buffered output stream.
void write(byte[] b, int off, int len) It write the bytes from the specified byte-input stream into a
specified byte array, starting with the given offset
void flush() It flushes the buffered output stream.

Example of BufferedOutputStream class:
• In this example, we are writing the textual information in the BufferedOutputStream object which is connected to the FileOutputStream object.
• The flush() flushes the data of one stream and send it into another.
• It is required if you have connected the one stream with another.
import java.io.*;
public class BufferedOutputStreamExample{
public static void main(String args[])throws Exception{
BufferedOutputStream bout=new BufferedOutputStream(fout);
String s="Welcome to javaTpoint.";
byte b[]=s.getBytes();
bout.write(b);
bout.flush();
bout.close();
fout.close();
System.out.println("success");
}

Introduction to java programming

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