TechieDrill » Java

Java Programming Tutorial – 1 – Installing the JDK

Venkatesh Viswanathan — Tue, 15 Dec 2009 17:08:22 +0000

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Tutorial on Installing the JDK.

Inner class within a for loop

karnamrajesh — Sat, 05 Dec 2009 19:03:11 +0000

class Outer

{

//int i;

public void Display()

{

System.out.println(”hi”);

for(int i=1;i<=10;i++)

{

class Inner

{

public void MethodInner()

{

System.out.println(”Hello”);

}

Inner a=new Inner();

a.MethodInner();

}

public class ForRegInnerClass

{

public static void main(String args[])

{

Outer ou=new Outer();

ou.Display();

}

A tricky for loop with inner class

class Outer

{

//int i;

public void Display()

{

System.out.println(”hi”);

for(int i=1;i<=10;i++)

{

class Inner

{

public void MethodInner()

{

System.out.println(”Hello”);

}

Inner a=new Inner();

a.MethodInner();

}

public class ForRegInnerClass

{

public static void main(String args[])

{

Outer ou=new Outer();

ou.Display();

}

Java Inner class

karnamrajesh — Sat, 05 Dec 2009 19:00:19 +0000

class Outer

{

String s=”Welcome”;

public void MethodInner()

{

Inner in=new Inner();

in.InnerMethod();

}

class Inner

{

public void InnerMethod()

{

System.out.println(”Hello!”+s);

}

class RegInnerClass

{

public static void main(String args[])

{

Outer ou=new Outer();

ou.MethodInner();

}

Classes that are defined inside another class are called inner class or nested class. Inner classes benefits in access control and naming control.

Try out the below code

class Outer

{

String s=”Welcome”;

public void MethodInner()

{

Inner in=new Inner();

in.InnerMethod();

}

class Inner

{

public void InnerMethod()

{

System.out.println(”Hello!”+s);

}

class RegInnerClass

{

public static void main(String args[])

{

Outer ou=new Outer();

ou.MethodInner();

}

Java Servlet Sample Program

karnamrajesh — Sat, 05 Dec 2009 18:42:20 +0000

import javax.servlet.*;

import java.io.*;

public class HelloServlet extends HttpServlet

{

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException

{

response.setContentType(”text/html”);

PrintWriter out = response.getWriter();

out.println(”Hello!”);

out.println(”

Hello Servlet World

”);

out.println(””);

out.close();

}

Servlets are the server side programming language.

Servlets are loaded by the container which instantiates the init() followed by the service() method. Destory() method is called to take the servlets out of service. init() and destroy() methods are called automatically and are called only once throughout the life cycle

Try out the sample simple servlet program

import javax.servlet.*;

import java.io.*;

public class HelloServlet extends HttpServlet

{

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws ServletException, IOException

{

response.setContentType(”text/html”);

PrintWriter out = response.getWriter();

out.println(”Hello!”);

out.println(”

Hello Servlet World

”);

out.println(””);

out.close();

}

Java Thread Synchronization

karnamrajesh — Sat, 05 Dec 2009 18:34:25 +0000

Consider a multi-threaded environment in which a thread access write and read of a same resource thread, java gives wrong output.

Consider a scenario in which java access a resource in which one thread does the read operation and write thread does the write operation then file will be corrupted, locking the access is necessary to overcome this.

Try out this example to understand better

import java.lang.*;

class MyThread extends Thread

{

static String message[] = {”I”, “LOVE”, “JAVA”, “VERY”, “MUCH”};

public MyThread(String id)

{

super(id);

}

public void run()

{

Sync.displayList(getName(),message);

}

void waiting()

{

try

{

sleep(1000);

}

catch(InterruptedException e)

{

System.out.println(”Interrupted”);

}

class Sync

{

public static void displayList(String name, String list[])

{

for(int i = 0; i < list.length ; i++)

{

MyThread thread = (MyThread)Thread.currentThread();

thread.waiting();0

System.out.println(name + list[i]);

}

class ThreadSync

{

public static void main(String args[])

{

MyThread thread1 = new MyThread(”Thread 1:”);

MyThread thread2 = new MyThread(”Thread 2:”);

thread1.start(); //starts first thread

thread2.start(); //starts second thread

}

import java.lang.*;

class MyThread extends Thread

{

static String message[] = {”I”, “LOVE”, “JAVA”, “VERY”, “MUCH”};

public MyThread(String id)

{

super(id);

}

public void run()

{

Sync.displayList(getName(),message);

}

void waiting()

{

try

{

sleep(1000);

}

catch(InterruptedException e)

{

System.out.println(”Interrupted”);

}

class Sync

{

public static void displayList(String name, String list[])

{

for(int i = 0; i < list.length ; i++)

{

MyThread thread = (MyThread)Thread.currentThread();

thread.waiting();0

System.out.println(name + list[i]);

}

class ThreadSync

{

public static void main(String args[])

{

MyThread thread1 = new MyThread(”Thread 1:”);

MyThread thread2 = new MyThread(”Thread 2:”);

thread1.start(); //starts first thread

thread2.start(); //starts second thread

}

Java SuppressWarnings

karnamrajesh — Mon, 23 Nov 2009 16:48:24 +0000

Java Annotation offers a feature to suppress warnings that occurs during compile time. Basically there are seven level of suppressions that can be added to the java class

They are as follows:

all
finally
serial
path
fallthrough
unchecked
deprecation

Find below the deprecation suppress warning example

import java.util.Date;

public class SuppressWarningsExample

{

//@SuppressWarnings(value=(”deprecation”))

public static void main(String args[])

{

Date date = new Date(2008,9,30);

System.out.println(”date = ” +date);

}

import java.util.Date;

public class SuppressWarningsExample

{

//@SuppressWarnings(value=(”deprecation”))

public static void main(String args[])

{

Date date = new Date(2008,9,30);

System.out.println(”date = ” +date);

}

Java Static ArrayCopy

karnamrajesh — Mon, 23 Nov 2009 16:40:43 +0000

Java offers a static ArrayCopy method from System class:

java.lang.System.arraycopy() method offers a convenient method to copy array elements from one array to another.

System.arraycopy(Object source_array, int source_position,

Object destination_array, int destination_position, number_of_elements)

System.arraycopy() takes five parameters.

First parameter specifies the source_array from where the elements need to be copied.

Second parameter is the position(index) value of the source array from what index array is to be copied.

Third parameter specifies the destination array to which array the values need to be copied. Fourth parameter specifies the destination array starting index.

Fifth parameter specifies the numberOfElements need to be copied from source array to destination array.

Check out your understanding with below example.

import java.io.*;

class Student

{

int b[] = new int[5];

Student(int[] a)

{

System.arraycopy(a,0,b,0,5);

}

void Display()

{

for(int i=0;i<5;i++)

{

System.out.println(b[i]);

}

class StudentClass

{

public static void main(String [] args) throws IOException

{

DataInputStream dis = new DataInputStream(System.in);

int a[] = new int[5];

System.out.print(”Enter marks : “);

for(int i=0;i<5;i++)

{

a[i]=Integer.parseInt(dis.readLine());

}

Student st = new Student(a);

st.Display();

}

import java.io.*;

class Student

{

int b[] = new int[5];

Student(int[] a)

{

System.arraycopy(a,0,b,0,5);

}

void Display()

{

for(int i=0;i<5;i++)

{

System.out.println(b[i]);

}

class StudentClass

{

public static void main(String [] args) throws IOException

{

DataInputStream dis = new DataInputStream(System.in);

int a[] = new int[5];

System.out.print(”Enter marks : “);

for(int i=0;i<5;i++)

{

a[i]=Integer.parseInt(dis.readLine());

}

Student st = new Student(a);

st.Display();

}

Java Annotations

karnamrajesh — Sun, 22 Nov 2009 08:09:27 +0000

public class OverrideAnnotation

{

public static void main(String s[])

{

OverrideExample e = new OverrideExample();

System.out.println(e.toString());

}

Annotations are kind of meta data which can be used in java classes. Typically methods, variables, classes and packages can be annotated.

Annotations basically provides information to the java compilers. Annotations can able to used to generate XMLs.

Try out the below sample code

public class OverrideAnnotation

{

public static void main(String s[])

{

OverrideExample e = new OverrideExample();

System.out.println(e.toString());

}

——————————————————————

class OverrideExample

{

public String field;

private String attribute;

@Override

public int hashCode()

{

return field.hashCode() + attribute.hashCode();

}

@Override

public String toString()

{

return field + ” ” + attribute;

}

Java ScriptEngineManager

karnamrajesh — Sun, 22 Nov 2009 07:39:07 +0000

ScriptEngineManager implements ScriptEngine classes. ScriptEngineManager holds key and value pair and stores the state of an object which is shared the engines created by the Manager. ScriptEngineManager uses the service provider technique to itemize implementations of ScriptEngineFactory.

Binding between the key and value pairs are traditionally referred to as the “Global Scope” that is available to all instances of ScriptEngine created by theScriptEngineManager. Value binded with the keys are made available to all the scripts.

ScriptEngineManager offers a method which returns arrays of these factories that includes few utilities on basis of mime type, languate name and file extensions.

import javax.script.*;

import java.io.*;

public class FileScript

{

public static void main(String args[])

{

ScriptEngineManager manager = new ScriptEngineManager();

ScriptEngine engine = manager.getEngineByName(”javascript”);

/*if(args.length!=1)

{

System.out.print(”Insufficient arguments”);

System.exit(0);

}*/

FileReader reader = null;

try

{

reader = new FileReader(args[0]);

}

catch(FileNotFoundException e1)

{

e1.printStackTrace();

}

try

{

engine.eval(reader);

}

catch(ScriptException e)

{

e.printStackTrace();

}

Note: Take the below code and save it as sample.js extension and run the above java code with the sample.js as run time argument for java interpreter

Eg: java FileScript sample.js

function hello()

{

println(”Hello from Function”);

}

for(i=0;i<5;i++)

{

print((i+1) + ” ” )

hello()

}

Creating Folders using simple Java

karnamrajesh — Sun, 22 Nov 2009 07:21:29 +0000

import java.io.*;

class FileEx

{

public static void main(String [] s) throws IOException

{

// File f = new File(”xyz.txt”);

// f.delete();

File f1 = new File(s[0]);

f1.mkdir();

File f2 = new File (f1,s[1]);

f2.mkdir();

File f3 = new File (f2,s[2]);

f3.mkdir();

if(f3.mkdir())

{

System.out.println(”Folder Created….”);

File ff = new File(f3,”akshatha.txt”);

ff.createNewFile();

}

else

{

System.out.println(”folder already exists…”);

}

It is quite easy to create folders using Java File concepts. Using java.io.file package we can create folders using mkdir(make directory) method. This can also handle folder already exists scenarios.

At the same time of creating folders it is also possible to create files of any extensions.

Try out the below sample code and explore things…

import java.io.*;

class FileEx

{

public static void main(String [] s) throws IOException

{

// File f = new File(”xyz.txt”);

// f.delete();

File f1 = new File(s[0]);

f1.mkdir();

File f2 = new File (f1,s[1]);

f2.mkdir();

File f3 = new File (f2,s[2]);

f3.mkdir();

if(f3.mkdir())

{

System.out.println(”Folder Created….”);

File ff = new File(f3,”sample.txt”);

ff.createNewFile();

}

else

{

System.out.println(”folder already exists…”);

}