InstanceOfJava

Servlet example programs in eclipse

Posted by: Instanceofjava Posted date: 5:59 PM / comment : 0 Java EE , Servlets Tutorial

1.Run eclipse.File -> new -> dynamic web project

2. Give project name : example MyFirstApp and click on next

3. Please check Generate web.xml deployment descriptor to auto generate web.xml file

4.click on finish. now project structure will be created.

5.Go to webcontent and create a folder with name Jsp to place jsp files and create a index.jsp.

6. index.jsp will have an error because it is not having servlet jar. file so we need to include servler-apt.jar file.

now its fine. modify index.jsp with one text box and submit button.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org
/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Login</title>
</head>
<body>
<form action="/MyFirstApp/hello" method="post">
Name:<input type="text" name="name" value="">
<input type="submit" name="name" value="submit">
</form>
</body>
</html>

7. create a servlet

package com.instanceofjava;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Hello extends HttpServlet{
private static final long serialVersionUID = 1L;
public void doPost(HttpServletRequest req, HttpServletResponse res) throws IOException,
ServletException{
String str= (String) req.getParameter("name");
req.setAttribute("name", str);
req.getRequestDispatcher("/Jsp/welcome.jsp").forward(req, res);
}
}

8. create a welcome.jsp page.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org
/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>welcome</title>
</head>
<body>
<%
String str=null;
try{
str=(String)request.getAttribute("name");
}catch(Exception e){
}
%>
<p>Welcome: <%=str %></p>
</body>
</html>

8. go to we.xml and include our servlet

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns
/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>MyFirstApp</display-name>
<welcome-file-list>
<welcome-file>/Jsp/index.jsp</welcome-file>
</welcome-file-list>
<servlet>
<servlet-name>HelloServlet</servlet-name>
<servlet-class>com.instanceofjava.Hello</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
</web-app>

9. run the project , select tomcat 7 from apache.

10. Enter your name and click on submit.

Accessibility modifiers examples

Posted by: Instanceofjava Posted date: 8:49 PM / comment : 0 concept and program

The keywords which define accessibility permissions are called accessibility modifiers.
Java supports four accessibility modifiers to define accessibility permissions at different levels.

Accessibility modifier keywords:

1.private

2.protected

3.public

4.default(no keyword)

1.private:

The class members which have private keyword in its creation statement are called private members. Those members are only accessible within that class.
If we declare any variable or method with private accessibility modifier then those variables and methods are accessible only within that class , not accessible out side the class.

private variable are accessible within the class :

package com.instanceofjava;
public class PrivateDemo {
private String first_name;
private String last_name;
void show(){
System.out.println("First Name:="+first_name);
System.out.println("Last Name:="+last_name);
}
public static void main(String[] args) {
PrivateDemo obj= new PrivateDemo ();
obj.first_name="James";
obj.last_name="Goosling";
obj.show();
}
}

Output:

First Name:=James
Last Name:=Goosling

private variable are not accessible out side the class :

package com.instanceofjava;
public class PrivateDemo {
private String first_name;
private String last_name;
void show(){
System.out.println("First Name:="+first_name);
System.out.println("Last Name:="+last_name);
}
public static void main(String[] args) {
PrivateDemo obj= new PrivateDemo ();
obj.first_name="James";
obj.last_name="Goosling";
}
}

package instanceofjava;
class Demo {
public static void main(String[] args){
PrivateDemo obj= new PrivateDemo ();
obj.first_name="James"; // ERROR: The field PrivateDemo.first_name is not visible
}
}

private methods are accessible within the class :

package com.instanceofjava;
public class PrivateDemo {
private String first_name;
private String last_name;
private void show(){
System.out.println("First Name:="+first_name);
System.out.println("Last Name:="+last_name);
}
public static void main(String[] args) {
PrivateDemo obj= new PrivateDemo ();
obj.first_name="James";
obj.last_name="Goosling";
obj.show();
}
}

Output:

First Name:=James
Last Name:=Goosling

private variable are not accessible out side the class :

package com.instanceofjava;
public class PrivateDemo {
private String first_name;
private String last_name;
void show(){
System.out.println("First Name:="+first_name);
System.out.println("Last Name:="+last_name);
}
public static void main(String[] args) {
PrivateDemo obj= new PrivateDemo ();
obj.first_name="James";
obj.last_name="Goosling";
}
}

package instanceofjava;
class Demo {
public static void main(String[] args){
PrivateDemo obj= new PrivateDemo ();
obj.add(); // ERROR: The method add() from the type PrivateDemo is not visible
}
}

private variables and methods are accessible inside that class only. If we declare any variable or method as private , not accessible outside the class.

2.protected

The class members which have protected keyword in its creation statements are called protected members. Those members can be accessible with in package from all classes, but from out side package only in subclass that too using subclass name or its object.
protected variables accessible inside the package anywhere. Outside package accessible only in sub classes.

Same package anywhere:

package com.instanceofjava;
public class ProtectedDemo {
protected int a;
protected int b;
protected void show(){
System.out.println("a="+a);
System.out.println("b="+b);
}
public static void main(String[] args) {
ProtectedDemo obj= new ProtectedDemo ();
obj.a=12;
obj.b=13;
obj.show();
}
}

Output:

a=12
b=13

Different package subclass:

package com.accesiblitymodifiers;
public class Sample extends ProtectedDemo {
public static void main(String[] args) {
Sample obj= new Sample();
obj.a=12;
obj.b=13;
obj.show();
}
}

Output:

a=12
b=13

3.public

If we declare any variable or method with public access specifier then those members will be accessible to everywhere.

package com.instanceofjava;
public class PublicDemo {
public int x;
public int y;
public void show(){
System.out.println("x="+x);
System.out.println("y="+y);
}
public static void main(String[] args) {
PublicDemo obj= new PublicDemo ();
obj.x=1;
obj.y=2;
obj.show();
}
}

Output:

4.default

If we declare any member with no keyword those members are called default members.
default members are accessible to package level.
Means we can access anywhere in same package but we can not access in out side the package under any condition.
So default will acts as public inside package and private out side the package.

Java data types with examples

Posted by: Instanceofjava Posted date: 4:18 PM / comment : 0 concept and program

In java mainly we have two types of data types.

Primitive data types or Fundamental data types.
Referenced data types or Derived data types.

1.Primitive Data types

1.byte

package com.instanceofjava;
public class ByteDemo {
public static void main(String[] args) {
byte a=10; // a is a variable of type byte holding the value 1;
byte b=20;// b is a variable of type byte holding the value 2;
byte c= (byte)(a+b);
System.out.println(c);
}
}

Output:

2.short

package com.instanceofjava;
public class ShortDemo {
public static void main(String[] args) {
short a=1;
short b=2;
short c= (short)(a+b);
System.out.println(c);
}
}

Output:

3.int

package com.instanceofjava;
public class IntDemo {
public static void main(String[] args) {
int a=10; // a is a variable of type integer holding the value 10;
int b=20;// b is a variable of type integer holding the value 20;
int c= a+b;
System.out.println(c);
}
}

Output:

4.long

package com.instanceofjava;
public class LongDemo {
public static void main(String[] args) {
long a=1234567890;
long b=1234567890;
long c= a+b;
System.out.println(c);
}
}

Output:

2469135780

5.float

package com.instanceofjava;
public class FloatDemo {
public static void main(String[] args) {
float a=1234567.8f;
float b=1234567.8f;
float c= a+b;
System.out.println(c);
}
}

Output:

2469135.5

7.double

package com.instanceofjava;
public class DoubleDemo {
public static void main(String[] args) {
double a=1234567910112181314151634444422233334491.1d;
double b=1234567910112181314151634444422233334491.8d;
double c= a+b;
System.out.println(c);
}
}

Output:

2.4691358202243627E39

8.char

package com.instanceofjava;
public class CharDemo {
public static void main(String[] args) {
char a='A';
char b= 'B';
System.out.println(a);
System.out.println(b);
System.out.println(a+b);
}
}

Output:

9.boolean

package com.instanceofjava;
public class BooleanDemo {
public static void main(String[] args) {
boolean a=true;
boolean b= false;
System.out.println(a);
System.out.println(b);
}
}

Output:

true
false

1.Referenced Data types

1.class

package com.instanceofjava;
public class Sample{
int a,b;
void add(){
System.out.println(a+b);
}
public static void main(String[] args) {
Sample obj= new Sample();
obj.a=10;
obj.b=12;
obj.add();
}
}

Output:

2.Array

package com.instanceofjava;
public class ArrayDemo {
public static void main(String[] args) {
int a[]={1,2,3,4,5,6};
for (int i = 0; i < a.length; i++) {
System.out.println(a[i]);
}
}
}

Output:

Constructors in java

Posted by: Instanceofjava Posted date: 5:25 PM / comment : 0 concept and program

Constructors will be executed when the object is created.
Constructors should have same name of class name.
Executed once per object.
Basically used to assign instance variables
We can overload constructors.
We can call super class constructor form sub class constructor.

package instanceofjava;
class A{
A(){
}
}
}

while creating object constructor will be executed so we can assign instance variables to some default values.

package instanceofjava;
class A{
int a,b
A(int x, int y){
a=x;
b=y;
}
public static void main(String[] args){
A a=new A(10,20);
}
}

Types of constructors:

There are two types of constructors
Default constructors
Parameterized constructor.

Default constructor:

Default constructor will not have any arguments.
If we not defined any constructor in our class. JVM automatically defines a default constructor to our class.

package instanceofjava;
class Sample{
int a,b
Sample(){
a=37;
b=46;
}
public static void main(String[] args){
Sample obj=new Sample();
System.out.println(obj.a);
System.out.println(obj.b);
}
}

Output:

Parameterized constructor

package instanceofjava;
class Sample{
int a,b
Sample(int x, int y){
a=x;
b=y;
}
public static void main(String[] args){
Sample obj=new Sample(37,46);
System.out.println(obj.a);
System.out.println(obj.b);
}
}

Output:

Constructor overloading:

package instanceofjava;
class Sample{
int a,b
Sample(){
this(1,2);
System.out.println("Default constructor");
}
Sample(int x , int y){
this(1,2,3);
a=x;
b=y;
System.out.println("Two argument constructor");
}
Sample(int a , int b,int c){
System.out.println("Three argument constructor")
}
public static void main(String[] args){
Sample obj=new Sample();
System.out.println(obj.a);
System.out.println(obj.b);
}
}

Output:

Three argument constructor
Two argument constructor
Default argument constructor
1
2

This and Super Keywords

Posted by: Instanceofjava Posted date: 3:04 PM / comment : 0 concept and program

this keyword used to store current object reference.
super keyword used to store super class reference in subclass object.

this keyword:

Predefined instance variable to hold current object reference.

It must used explicitly if non-static variable and local variable name is same.

Output:

Without this keyword if variable names are same we can not assign instance variables so this problem will solve by using this keyword

package com.instanceofjava;
public class ThisDemo {
int a, b;
ThisDemo(int a, int b){
this.a=a;
this.b=b;
}
public static void main(String[] args){
ThisDemo obj= new ThisDemo(1,2);
System.out.println(obj.a);
System.out.println(obj.b);
}
}

Output:

Used to invoke current class constructor:

package com.instanceofjava;
public class ThisDemo {
int a, b;
ThisDemo(){
System.out.println("Default constructor called");
}
ThisDemo(int a, int b){
this();
this.a=a;
this.b=b;
}
public static void main(String[] args){
ThisDemo obj= new ThisDemo(1,2);
System.out.println(obj.a);
System.out.println(obj.b);
}
}

Output:

Default constructor called
1
2

Used to call current class method:

package com.instanceofjava;
public class Sample{
int a, b;
Sample(int a, int b){
this.a=a;
this.b=b;
}
void show(){
System.out.println("Show() method called");
}
void print(){
this.show();
System.out.println(obj.a);
System.out.println(obj.b);
}
public static void main(String[] args){
Sample obj= new Sample(1,2);
obj.print()
}
}

Output:

Show() method called
1
2

super keyword:

Predefined instance variable used to hold super class object reference through sub class object.

Super keyword interview questions java

Used to call super class constructor:

package com.instanceofjava;
public class A{
int a, b;
A(int a, int b){
a=a;
b=b;
System.out.println("super class constructor called");
}
}

package com.instanceofjava;
public class B extends A{
int a, b;
A(int a, int b){
super(a,b);
a=a;
b=b;
}
public static void main(String[] args){
B obj= new B(1,2);
}
}
}

Output:

super class constructor called
1
2

Used to call super class methods from subclass:

package com.instanceofjava;
public class A{
int a, b;
A(int a, int b){
a=a;
b=b;
System.out.println("super class constructor called");
}
void add(){
System.out.println("super class method called");
}
}

package com.instanceofjava;
public class B extends A{
int a, b;
A(int a, int b){
super(a,b);
a=a;
b=b;
}
void print(){
super.add();
System.out.println(this.a);
System.out.println(this.b);
}
public static void main(String[] args){
B obj= new B(1,2);
obj.print();
}
}
}

Output:

super class method called
1
2

Abstraction

Posted by: Instanceofjava Posted date: 1:42 PM / comment : 2 concept and program

Abstraction is fundamental principle of modeling. A system model is created at different levels, starting at the higher levels and adding more levels with more details as more is understood about the system. When complete, the model can be viewed at several levels.
So abstraction is about,

Looking only at the information that is relevant at that time
Hiding details so as not to confuse the bigger picture

Abstraction is a process of developing a class by hiding or removing non essential details relevant to user. here non essential details means method body and logic
Basically Abstraction provides a contract between a service provider and clients.
we can achieve this by using abstract class

package Abstraction;
public abstract class AbstractDemo {
abstract void add(); // abstract method
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

Abstract class can not be instantiated directly.
Means we can not create object for abstract class directly

package Abstraction;
public abstract class AbstractDemo {
abstract void add(); // abstract method
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
public static void main(String[] args){
AbstractDemo obj= new AbstractDemo();
// ERROR: Cannot instantiate the type AbstractDemo
}
}

Can we declare abstract methods as static?

package Abstraction;
public abstract class AbstractDemo {
static abstract void add(); // ERROR: illegal combination of modifiers
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

No, we are not allowed to declare method as static . It leads to compile time error.

Can we declare abstract methods as final?

package Abstraction;
public abstract class AbstractDemo {
final abstract void add(); // ERROR: illegal combination of modifiers
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

No, we are not allowed to declare method as static . It leads to compile time error.
Because it should be inherited to sub class.

Can we declare abstract methods as private?

package Abstraction;
public abstract class AbstractDemo {
final abstract void add(); // ERROR: illegal combination of modifiers
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

No, we are not allowed to declare method as static . It leads to compile time error.
Because it should be inherited to sub class.

Legal modifiers allowed in combination with abstract

1.native

package Abstraction;
public abstract class AbstractDemo {
native abstract void add(); // ERROR: illegal combination of modifiers
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

2.protected

package Abstraction;
public abstract class AbstractDemo {
protected abstract void add();
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

3.public

package Abstraction;
public abstract class AbstractDemo {
public abstract void add();
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

4.default

package Abstraction;
public abstract class AbstractDemo {
abstract void add();
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

How it works:

package Abstraction;
public abstract class AbstractDemo {
abstract void add();
void show(){ // normal method
System.out.println("this is concrete method present in abstract class");
}
}

package Abstraction;
public abstract class Example extends AbstractDemo {
abstract void add(){
System.out.println("this is implemented method in sub class");
}
}

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Servlet example programs in eclipse

Accessibility modifiers examples

Accessibility modifier keywords:

1.private

2.protected

3.public

4.default(no keyword)

1.private:

private variable are accessible within the class :

Output:

private variable are not accessible out side the class :

private methods are accessible within the class :

Output:

private variable are not accessible out side the class :

2.protected

Same package anywhere:

Output:

Different package subclass:

Output:

3.public

Output:

4.default

Java data types with examples

1.Primitive Data types

1.byte

Output:

2.short

Output:

3.int

Output:

4.long

Output:

5.float

Output:

7.double

Output:

8.char

Output:

9.boolean

Output:

1.Referenced Data types

1.class

Output:

2.Array

Output:

Constructors in java

Types of constructors:

Default constructor:

Output:

Parameterized constructor

Output:

Constructor overloading:

Output:

This and Super Keywords

this keyword:

It must used explicitly if non-static variable and local variable name is same.

Top 10 Java interview questions and programs on this keyword

Output:

Output:

Used to invoke current class constructor:

Output:

Used to call current class method:

Output:

super keyword:

Used to call super class constructor:

Output:

Used to call super class methods from subclass:

Output:

Abstraction

Can we declare abstract methods as static?

Can we declare abstract methods as final?

Can we declare abstract methods as private?

Legal modifiers allowed in combination with abstract

1.native

2.protected

3.public

4.default

How it works: