My Java Works

Convert Byte Array to PDF in java

2011-09-27T23:05:00.000-07:00

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
public class TestPDF {

public static void main(String[] args) throws FileNotFoundException {
Test obj = new Test();
boolean result = obj.savePDF();
System.out.println("Result of Saving PDF : "+result);
obj.getPDF();
}
public boolean savePDF() throws FileNotFoundException{
File file = new File("c:/Java/Input_File.pdf");
FileInputStream fis = new FileInputStream(file);
Connection con = null;
try{
con = DAO.getConnection();
PreparedStatement pstmt = con.prepareStatement("Insert into test (pdf) values (?)");
pstmt.setBinaryStream(1, (InputStream)fis, (int)file.length());
int flag = pstmt.executeUpdate();
con.close();
if(flag>0)
return true;
else
return false;
}catch(Exception e){
System.out.println("Exception in saving PDF : "+e);
}
return false;
}
public void getPDF(){
Connection con = null;
InputStream fetchStream = null;
try{
con = DAO.getConnection(); //Getting the Database Connection
PreparedStatement pstmt = con.prepareStatement("select pdf from test where sl=1");
ResultSet rs = pstmt.executeQuery();
if(rs.next())
{
fetchStream = rs.getBinaryStream("pdf");
}
}catch(Exception e){
System.out.println("Exception in getting PDF : "+e);
}
ByteArrayOutputStream bos = new ByteArrayOutputStream();
byte[] buf = new byte[1024];
try {
for (int readNum; (readNum = fetchStream.read(buf)) != -1;) {
bos.write(buf, 0, readNum);
System.out.println("read " + readNum + " bytes,");
}
System.out.println("Byte array is="+bos);
} catch (Exception ex) {
ex.printStackTrace();
}
byte[] bytes = bos.toByteArray();
File someFile = new File("c:/Java/Output_File.pdf");
try{
FileOutputStream fos = new FileOutputStream(someFile);
fos.write(bytes);
fos.flush();
fos.close();
}catch(Exception e){
}
}
}

MD5 Password Decryption

2011-09-26T03:03:00.001-07:00

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class AeSimpleMD5 {
private static String convertToHex(byte[] data) {
StringBuffer buf = new StringBuffer();
for (int i = 0; i < data.length; i++) {
int halfbyte = (data[i] >>> 4) & 0x0F;
int two_halfs = 0;
do {
if ((0 <= halfbyte) && (halfbyte <= 9))
buf.append((char) ('0' + halfbyte));
else
buf.append((char) ('a' + (halfbyte - 10)));
halfbyte = data[i] & 0x0F;
} while(two_halfs++ < 1);
}
return buf.toString();
}
public static String MD5(String text) throws NoSuchAlgorithmException, UnsupportedEncodingException {
MessageDigest md;
md = MessageDigest.getInstance("MD5");
byte[] md5hash = new byte[32];
md.update(text.getBytes("iso-8859-1"), 0, text.length());
md5hash = md.digest();
return convertToHex(md5hash);
}
}

**************************************

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.security.MessageDigest;
public class Ex01 {
private static String convertToHex(byte[] data) {
StringBuffer buf = new StringBuffer();
for (int i = 0; i < data.length; i++) {
int halfbyte = (data[i] >>> 6) & 0x0F;
int two_halfs = 0;
do {
if ((0 <= halfbyte) && (halfbyte <= 9))
buf.append((char) ('0' + halfbyte));
else
buf.append((char) ('a' + (halfbyte - 10)));
halfbyte = data[i] & 0x0F;
} while(two_halfs++ < 1);
}
return buf.toString();
}

public static String MD5(String text) throws NoSuchAlgorithmException, UnsupportedEncodingException {
MessageDigest md = MessageDigest.getInstance("MD5");
byte[] md5hash = new byte[32];
md.update(text.getBytes("iso-8859-1"), 0, text.length());
md5hash = md.digest();
return convertToHex(md5hash);
}

public static void main(String[] args) throws IOException {
BufferedReader userInput = new BufferedReader (new InputStreamReader(System.in));
System.out.println("Enter string:");
String rawString = userInput.readLine();
try {
System.out.println("MD5 hash of string: " + MD5(rawString));
} catch (NoSuchAlgorithmException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

Factorial Value

2011-09-25T21:32:00.000-07:00

import java.math.BigInteger;

public class FactVal{
public static void main(String[] args) {
BigInteger a[]= new BigInteger[10];
a[0]=BigInteger.ONE;
a[1]=BigInteger.ONE;
for(int i=2;i<10;i++)
{
String f="";
f=f+i;
a[i]=a[i-1].multiply(BigInteger.valueOf(i));
System.out.println("The Factorial Number is ="+a[i]);
}
}
}

Matrix Substraction

2011-09-10T04:20:00.001-07:00

import java.io.*;
class matrixSub{
    int a[][] = new int[10][10];
    int b[][] = new int[10][10];
    int c[][] = new int[10][10];
    DataInputStream din = new DataInputStream(System.in);
    void getData(){
        try{
            System.out.println("Enter First Matics");
            for(int i=0;i<3;i++){
                for(int j=0;j<3;j++){
                    a[i][j] = Integer.parseInt(din.readLine());
                }
            }
            System.out.println("Enter Second Matrix");
            for(int i=0;i<3;i++){
                for(int j=0;j<3;j++){
                    b[i][j] = Integer.parseInt(din.readLine());
                }
            }
        }catch (Exception e){System.out.println("Exception araise "+e);}
    }
    void addition(){
        for(int i=0;i<3;i++){
            for(int j=0;j<3;j++){
                c[i][j] = a[i][j] - b[i][j];
            }
        }
    }
    void display(){
        System.out.println("The Sustracted Matrices are ");
        for(int i=0;i<3;i++){
            for(int j=0;j<3;j++){
                System.out.println(c[i][j]);
            }
        }
    }
}
public class MatrixSubstraction {
    public static void main(String args[]){
        matrixSub subm = new matrixSub();
        subm.getData();
        subm.addition();
        subm.display();
    }
}

Matrix Addition

2011-09-10T04:18:00.000-07:00

import java.io.*;
class matrix{
    int a[][] = new int[10][10];
    int b[][] = new int[10][10];
    int c[][] = new int[10][10];
    DataInputStream din = new DataInputStream(System.in);
    void getData(){
        try{
            System.out.println("Enter First Matics");
            for(int i=0;i<3;i++){
                for(int j=0;j<3;j++){
                    a[i][j] = Integer.parseInt(din.readLine());
                }
            }
            System.out.println("Enter Second Matrix");
            for(int i=0;i<3;i++){
                for(int j=0;j<3;j++){
                    b[i][j] = Integer.parseInt(din.readLine());
                }
            }
        }catch (Exception e){System.out.println("Exception araise "+e);}
    }
    void addition(){
        for(int i=0;i<3;i++){
            for(int j=0;j<3;j++){
                c[i][j] = a[i][j]+b[i][j];
            }
        }
    }
    void display(){
        System.out.println("The added Matrices are ");
        for(int i=0;i<3;i++){
            for(int j=0;j<3;j++){
                System.out.println(c[i][j]);
            }
        }
    }
}
public class MatrixAddition {
    public static void main(String args[]){
        matrix addm = new matrix();
        addm.getData();
        addm.addition();
        addm.display();
    }
}

Binary Search

2011-09-10T03:30:00.001-07:00

import java.io.*;
class Binary {
    int i=0, j=0, n=0, t=0, I=0, u=0, x=0, mid=0;
    int a[] = new int[100];
    DataInputStream s = new DataInputStream(System.in);
    void getData(){
        System.out.println("Enter the Number of elements");
        try{
            n = Integer.parseInt(s.readLine());
            System.out.println("Enter the Elements");
            for(i=0 ; i<n; i++){
                a[i] = Integer.parseInt(s.readLine());
            }
        }catch(Exception e){System.out.println("the Exception Araise ="+e);}
    }
    void arrange(){
        for(i=1 ; i<n; i++){
            for(j=1;j<n;j++){
                if(a[i]>a[j]){
                    t    = a[i];
                    a[i] = a[j];
                    a[j] = t;
                }
            }
        }
    }
    void search(){
        I = 1;
        u = n;
        System.out.println("Enter the Elements to be searched");
        try{
            x = Integer.parseInt(s.readLine());
        }catch(Exception e){System.out.println("Exception araise ="+e);}
        while(I<u){
            mid = (i+u) / 2;
            if(a[mid] == x){
                System.out.println("The Element is found on "+mid+" Location");
                System.exit(1);
            }
            else if(x > a[mid]){
                I = mid + 1;
            }
            else {u = mid-1;}
        }
        System.out.println("Element is not Found");
    }
}
public class BinarySearch{
    public static void main(String args[])throws IOException{
        Binary a = new Binary();
        a.getData();
        a.arrange();
        a.search();
    }
}

Sorting Numbers

2011-09-10T02:37:00.001-07:00

import java.io.*;
class Ascend {
    int n,i,j,t;
    String v;
    int a[] = new int[10];
    DataInputStream s = new DataInputStream(System.in);
    @SuppressWarnings("deprecation")
    void getdata(){
        System.out.println("Enter the Number of Elements =");
        try{
            v = (s.readLine());
            n = Integer.parseInt(v);
            System.out.println("Enter the elements one by one ");
            for(i=1; i<=n; i++){
                a[i] = Integer.parseInt(s.readLine());
            }
        }catch(Exception e){
            System.out.println("Excetion Raise = "+e);
        }
    }
    void arrange(){
        for(i=1; i<=n; i++){
            for(j=1; j<n; j++){
                if(a[i] > a[j]){
                    t    = a[i];
                    a[i] = a[j];
                    a[j] = t;
                }
            }
        }
    }
    void putdata(){
        System.out.println("The Sorted Elements are ");
        for(i=1;i<=n;i++){
            System.out.println(a[i]);
        }
    }
}
public class Ascending{
        public static void main(String args[]){
            Ascend a = new Ascend();
            a.getdata();
            a.arrange();
            a.putdata();
    }
}

Ping Status

2011-09-05T02:01:00.001-07:00

import java.net.*;
import java.io.*;
class pings {
public static void main (String args[])
{
   if ( args.length <= 0 ) {
                  // argument has to be passed!!
                  System.out.println("Usage java pings IPAddress(String) Port(Integer) ");
                  System.exit(1);
       }
    try
    {
      Socket localSocket = new Socket (args[0], Integer.parseInt(args[1]));
      localSocket.setSoTimeout(1000);
      System.err.println ("Server status: Connected");
      localSocket.close ();
    }
    catch (IOException e)
    {
      System.err.println ("Server status: DOWN");
      System.exit(1);
    }
}
}

Encrypted Password (Md5 Algorithm)

2011-09-05T02:00:00.001-07:00

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.security.NoSuchAlgorithmException;
import java.security.MessageDigest;
public class Ex01 {
    private static String convertToHex(byte[] data) {
           StringBuffer buf = new StringBuffer();
           for (int i = 0; i < data.length; i++) {
               int halfbyte = (data[i] >>> 6) & 0x0F;
               int two_halfs = 0;
               do {
                   if ((0 <= halfbyte) && (halfbyte <= 9))
                       buf.append((char) ('0' + halfbyte));
                   else
                       buf.append((char) ('a' + (halfbyte - 10)));
                   halfbyte = data[i] & 0x0F;
               } while(two_halfs++ < 1);
           }
           return buf.toString();
       }

        public static String MD5(String text) throws NoSuchAlgorithmException, UnsupportedEncodingException {
           MessageDigest md = MessageDigest.getInstance("MD5");
           byte[] md5hash = new byte[32];
           md.update(text.getBytes("iso-8859-1"), 0, text.length());
           md5hash = md.digest();
           return convertToHex(md5hash);
       }





    public static void main(String[] args) throws IOException {
        BufferedReader userInput = new BufferedReader (new InputStreamReader(System.in));
        System.out.println("Enter string:");
        String rawString = userInput.readLine();
        try {
            System.out.println("MD5 hash of string: " + MD5(rawString));
        } catch (NoSuchAlgorithmException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Resize the image from database and insert it...

2011-09-05T01:48:00.001-07:00

import java.sql.*;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.FileOutputStream;
import java.io.File;
import javax.imageio.ImageIO;

public class ImageReSizeTnsec{
    public static void main(String args[]){
        if ( args.length < 2 )
         {
             System.out.println("Usage: java ImageReWorkNew <dtname> ");
             System.exit(1);
         }
        String dcode = args[0];
        String tname = args[1];
        if (!( args[1].equals("detail") || args[1].equals("sup")) ){
            System.out.println("Usage: java ImageReWorkNew <tname> should be detail or sup ");
            System.exit(1);
        }
        String connectionURL = "jdbc:postgresql://localhost:5432/test";
        String imageQuery    = "select uidno from "+dcode+".e_"+tname;
        try{
            Class.forName("org.postgresql.Driver").newInstance();
            Connection con = DriverManager.getConnection(connectionURL,"uname","pwd");
            Statement st1=con.createStatement();
            ResultSet rs1 = st1.executeQuery(imageQuery);
            while (rs1.next())
            {
                long uidNo   = rs1.getLong(1);
                String ImageQuery2="select photo from "+dcode+".e_"+tname+" where uidno="+uidNo;
                Statement st2=con.createStatement();
                ResultSet rs2 = st2.executeQuery(ImageQuery2);
                while(rs2.next()){
                        InputStream imageStream = rs2.getBinaryStream("photo");
                        String imageName = "/tmp/"+dcode+tname+uidNo+"image.jpg";
                        if(rs2.getBinaryStream("photo") == null || rs2.getBinaryStream("photo").equals("") || rs2.getBinaryStream("photo").equals(" "))
                        {
                            String uidInsertQuery = "insert into "+dcode+"."+tname+"_photo(uidno) values (?)";
                            PreparedStatement pstUid = con.prepareStatement(uidInsertQuery);
                            pstUid.setLong(1, uidNo);
                            pstUid.executeUpdate();
                            pstUid.close();
                        }else{
                            FileOutputStream f = new FileOutputStream(imageName);
                            byte buff[] = new byte[1024];
                            int l;
                            while ((l = imageStream.read(buff)) > 0)
                            {
                                f.write(buff, 0, l);
                            }
                            f.close();
                            File fl = new File("/tmp/"+dcode+tname+"image.jpg");
                            System.out.println("File Created and length is="+fl.length());
                            if(fl.length() > 0)
                            {
                                String osExe = "convert /tmp/"+dcode+tname+uidNo+"image.jpg -colors 8 -resize 100x100 /tmp/"+dcode+tname+uidNo+"Image2.jpg";
                                Process p = Runtime.getRuntime().exec(osExe);
                                Thread.sleep(100);
                                BufferedImage originalImage = ImageIO.read(new File("/tmp/"+dcode+tname+uidNo+"Image2.jpg"));
                                ByteArrayOutputStream baos = new ByteArrayOutputStream();
                                ImageIO.write(originalImage, "jpg", baos );
                                byte[] imageInByte = baos.toByteArray();
                                String imgInsertQuery = "insert into "+dcode+".e_"+tname+"_photo(uidno, photo) values (?,?)";
                                PreparedStatement pst = con.prepareStatement(imgInsertQuery);
                                pst.setLong(1, uidNo);
                                pst.setBytes(2, imageInByte);
                                pst.executeUpdate();
                                pst.close();
                                baos.close();
                            }else{
                                String uidInsertQuery = "insert into "+dcode+".e_"+tname+"_photo(uidno) values (?)";
                                PreparedStatement pstUid = con.prepareStatement(uidInsertQuery);
                                pstUid.setLong(1, uidNo);
                                pstUid.executeUpdate();
                                System.out.println("Uid Only inserted in file");
                                pstUid.close();
                            }
                            fl.deleteOnExit();
                }
                }
                rs2.close();
            }
            rs1.close();
            st1.close();
            con.close();
      }
      catch (Exception e){
         e.printStackTrace();
      }
   }
}

Image Write into Folder

2011-09-05T01:47:00.000-07:00

import java.sql.*;
import java.io.InputStream;
import java.io.FileOutputStream;
import java.io.File;

public class WriteImages {
    public static void main(String args[]){
        String dcode = "chn";
        String tname = "sup";
        String connectionURL = "jdbc:postgresql://localhost:5432/db";
        String imageQuery    = "select uidno, photo from "+dcode+".e_"+tname+" where photo is not null limit 10";
        String imageQueryphoto = "select uidno, photo from "+dcode+".e_"+tname+"_photo where photo is not null limit 10";
        try{
            Class.forName("org.postgresql.Driver").newInstance();
            Connection con = DriverManager.getConnection(connectionURL,"uname","pwd");
            Statement st1=con.createStatement();
            ResultSet rs1 = st1.executeQuery(imageQuery);
            while (rs1.next())
            {
                long uidNo   = rs1.getLong(1);
                InputStream imageStream = rs1.getBinaryStream(2);
                String imageName = "/tmp/"+uidNo+"image.jpg";
                FileOutputStream f = new FileOutputStream(imageName);
               byte buff[] = new byte[1024];
                    int l;
                     while ((l = imageStream.read(buff)) > 0)
                       {
                           f.write(buff, 0, l);
                       }
                       f.close();
                       File fl = new File("/tmp/"+uidNo+"image.jpg");
                       System.out.println("File 1 Created and length is="+fl.length());
            }
            Statement st2=con.createStatement();
            ResultSet rs2 = st2.executeQuery(imageQueryphoto);
            while(rs2.next()){
                long uidNo2   = rs2.getLong(1);
                InputStream imageStream2 = rs2.getBinaryStream(2);
                String imageName = "/tmp/"+uidNo2+"image2.jpg";

                FileOutputStream f2 = new FileOutputStream(imageName);
                    byte buff2[] = new byte[1024];
                         int l2;
                          while ((l2 = imageStream2.read(buff2)) > 0)
                            {
                                f2.write(buff2, 0, l2);
                            }
                            f2.close();
                            File fl2 = new File("/tmp/"+uidNo2+"image2.jpg");
                            System.out.println("File 2 Created and length is="+fl2.length());
                }
            rs2.close();
            st2.close();
            rs1.close();
            st1.close();
            con.close();
      }
      catch (Exception e){
         e.printStackTrace();
      }
   }
}

points to remember about Java

2011-09-05T00:44:00.001-07:00

Hi all

points to remember about Java
1) Java is a programming language as well as a Platform.
2) Machine code is the sequences of binary 1 & 0's that the processor understands.

JVM

1) Editor – To type your program into , a notepad could be used for this
2) Compiler – To convert your high language program into native machine code
3) Linker – To combine different program files reference in your main program together.
4) Loader – To load the files from your secondary storage device like Hard Disk , Flash Drive , CD into RAM for execution. The loading is automatically done when your execute your code.
5) Execution – Actual execution of the code which is handled by your OS & processor.

why is Java both interpreted and complied language ?
Programming languages are classifies as
Higher Level Language Ex. C++ , Java
Middle Level Languages Ex. C
Low Level Language Ex Assembly
& finally the lowest level as the Machine Language.

A compiler is a program which converts a program from one level of language to another. Example conversion of C++ program into machine code.
The java compiler is a convert’s high level java code into bytecode (which is also a type of machine code).
A interpreter is a program which converts a program at one level to another programming language at the same level. Example conversion of Java program into C++
In Java , the Just In Time Code generator converts the bytecode into the native machine code which are at the same programming levels.
Hence java is both compiled as well as interpreted language.

why is Java slow ?
The two main reasons behind the slowness of Java are

1) Dynamic Linking = Unlike C, linking is done at run-time , every time the program is run in Java.
2) Run-time Interpreter = The conversion of byte code into native machine code is done at run-time in Java which furthers slows down the speed.

Unstructured Programming Languages: The most primitive of all programming langauges having sequentially flow of control. Code is repeated through out the program

Structured Programming Languages: Has non-sequentially flow of control. Use of functions allows for re-use of code.

Object Oriented Programming: Combines Data & Action Together.

Java Abstraction:

Abstraction in the process of selecting important data sets for an Object in your software , and leaving out the insignificant ones.
Once you have modeled your object using Abstraction , the same set of data could be used in different applications.

Java Encapsulation:

With Encapsualtion you can hide (restrict access) to critical data members in your code , which improves security
Encapsualtion combines data and actions together (just like a capsule).

Inheritance & Ploymorphism :

Inheritance: In simple words , Inheritance is way to define new a class , using classes which have already been defined.

Ploymorphism: Is the ability of methods to behave differently , based on the object calling it.

Integer data types
byte (1 byte)
short (2 bytes)
int (4 bytes)
long (8 bytes)

What is the difference between a Class & Object ?
A class is a blueprint or prototype that defines the variables and the methods (functions) common to all objects of a certain kind.

An object is a specimen of a class. Software objects are often used to model real-world objects you find in everyday life.

Command Line Arguments
During program execution, information passed following a programs name in the command line is called Command Line Arguments.

1)Command Line Arguments can be used to specify configuration information while launching your application.
2)There is no restriction on the number of command line arguments.You can specify any number of arguments
3)Information is passed as Strings.
4)They are captured into the String argument of your main method

class Demo{
public static void main(String b[]){
System.out.println("Argument one = "+b[0]);
System.out.println("Argument two = "+b[1]);
}
}

Run the code as javac Demo java programming

"this" keyword
1) "this" is a reference to the current object, whose method is being called upon.
2) You can use "this" keyword to avoid naming conflicts in the method/constructor of your instance/object.

public class ThisDemo {
int a;
int b;

public void setData(int a ,int b){
a = a; // this.a = a;
b = b; // this.b = b;
}
public void showData(){
System.out.println("Value of A ="+a);
System.out.println("Value of B ="+b);
}
public static void main(String args[]){
ThisDemo obj = new ThisDemo();
obj.setData(2,3);
obj.showData();
}
}
Run the above program Result will come 0 , 0
remove the a =a; & b=b; then add this.a = a; & this.b = b;
Run the above program Result will come 2, 3

Garbage Collection
In the Java programming language, dynamic allocation of objects is achieved using the new operator. An object once created uses some memory and the memory remains allocated till there are references for the use of the object. When there are no references for an object, it is assumed to be no longer needed and the memory occupied by the object can be reclaimed.

There is no explicit need to destroy an object as java handles the de-allocation automatically. The technique that accomplishes this is known as Garbage Collection.
Programs that do not de-allocate memory can eventually crash when there is no memory left in the system to allocate. These programs are said to have memory leaks

In Java,Garbage collection happens automatically during the lifetime of a java program, eliminating the need to de-allocate memory and avoiding memory leaks.

In C language, it is the programmer’s responsibility to de-allocate memory allocated dynamically using free() function.

Note : All objects are created in Heap Section of memory. More on this in a later tutorial.

1) If you want to make your object eligible for Garbage Collection , assign its reference variable to null.
2) Primitive types are not objects. They cannot be assigned null.

Stack and Heap
The JVM divided the memory into following sections. This division of memory is required for its effective management.

1)Heap - The Heap section contains Objects (may also contain reference variables).
2)Stack - The Stack section of memory contains methods, local variables and reference variables.
3)Code - The code section contains your bytecode.
4)Static - The Static section contains Static data/methods.

Note:
When a method is called , a frame is created on the top of stack.
Once a method has completed execution , flow of control returns to the calling method and its corresponding stack frame is flushed.
Local variables are created in the stack
Instance variables are created in the heap & are part of the object they belong to.
Reference variables are created in the stack.

difference between instance variables and local variables

Instance variables are declared inside a class but not inside a method
class Student{
int num; // num is instance variable
public void showData{}

Local variables are declared inside a method including method arguments.
public void sum(int a){
int x = int a + 3;
// a , x are local variables
}

Inheritance
The parent class is termed super class and the inherited class is the sub class.
The keyword extends is used by the sub class to inherit the features of super class.
Inheritance is important since it leads to reusability of code

class Doctor
// Instance Variables and Methods for the Doctor Class
}
class Surgeon extends Doctor{
// Inherits instance variables & methods of the doctor class
//may have variables and methods of its own.
}

method overriding
Redefining a super class method in a sub class is called method overriding

Rules for Method Overriding
The method signature i.e. method name, parameter list and return type have to match exactly.
The overridden method can widen the accessibility but not narrow it, i.e. if it is private in the base class, the child class can make it public but not vice versa.

Doctor doctorObj = new Doctor()
doctorObj.treatPatient()
// treatPatient method in class Doctor will be executed
Surgeon surgeonObj = new Surgeon();
surgeonObj.treatPatient()
// treatPatient method in class Surgeon will be executed

dynamic polymorphism.
A reference variable of the super class can refer to a sub class object
Doctor obj = new Surgeon();

Consider the statement
obj.treatPatient();
Here the reference variable "obj" is of the parent class , but the object it is poiting to is of the child class (as show in diagram).
obj.treatPatient() will execute treatPatient() method of the sub-class - Surgeon
If a base class reference is used to call a method, the method to be invoked is decided by the JVM, depending on the object the reference is pointing to
For example, even though obj is a reference to Doctor, it calls the method of Surgeon, as it points to a Surgeon object
This is decided during run-time and hence termed dynamic or run-time polymorphism

super
What if the treatPatient method in the Surgeon class wants to do the functionality defined in Doctor class and then perform its own specific functionality?
In this case keyword super can be used to access methods of the parent class from the child class.
The treatPatient method in the Surgeon class could be written as:

treatPatient(){
super.treatPatient();
//add code specific to Surgeon
}

What is the difference method overloading & method overriding ?
Method overloading:
Method overloading is in the same class , where more than one method have the same name but different signatures
Ex
void sum (int a , int b);
void sum (int a , int b, int c);
void sum (float a, double b);

Method overriding :
Method overriding is when one of the methods in the super class is redefined in the sub-class. In this case the signature of the method remains the same.
Ex
class X{
public int sum(){
// some code
}
}
class Y extends X{
public int sum(){
//overridden method
//signature is same
}
}

What is the difference between static & Dynamic polymorphism ?
Static Polymorphism : It relates to method overloading .Errors ,if any, are resolved at compile time. Since the code is not executed during execution , the name static.
Ex:
void sum (int a , int b);
void sum (float a, double b);
int sum (int a, int b); //compiler gives error.

Dynamic Polymorphism: It relates to method overriding.
In case a reference variable is calling an overridden method, the method to be invoked is determined by the object ,your reference variable is pointing to. This is can be only determined at run -time when code in under execution , hence the name dynamic.