Java Today: Servlets

Java Today: Servlets:                 Basic Information about Servlets in Java: 1. Servlets are server side components that provide a powerful mechanism...

Interview Questions - Servlets and latest version

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Some brief information about Java:

1. Java is a general-purpose, concurrent, class-based, object-oriented computer programming language that is specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" meaning that code that runs on one platform does not need to be recompiled to run on another.

2.  Java applications are typically compiled to bytecode (class file) that can run on any JVM regardless of computer architecture. Java is, as of 2012, one of the most popular programming languages in use, particularly for client-server web applications, with a reported 15 million users. Java was originally developed by James Gosling at Sun Microsystems (which has since merged into Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C++, but it has fewer low-level facilities than either of them.

Some questions about Servlets:

Technical Interviewers ask mostly from this topic just to check your basic skills.

Q: What is the difference between HttpServlet and GenericServlet? A: A GenericServlet has a service() method aimed to handle requests.  HttpServlet extends GenericServlet and adds support for doGet(),  doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(),  doDelete(), doTrace() methods (HTTP).  Both these classes are abstract. Q: What is the difference between ServletContext and ServletConfig? A: ServletContext:  Defines a set of methods that a servlet uses to  communicate with its servlet container, for example, to get the  MIME type of a file, dispatch requests, or write to a log file. The ServletContext object is contained within the ServletConfig object,  which the Web server provides the servlet when the servlet is initialized  ServletConfig:  The object created after a servlet is instantiated and  its default constructor is read. It is created to pass initialization information  to the servlet.

sleep() method in Java...

                               sleep() method in Java...







1. see which exception it throws

2. see how it works..







Thread.sleep causes the current thread to suspend execution for a specified period. This is an efficient means of making processor time available to the other threads of an application or other applications that might be running on a computer system. The sleep method can also be used for pacing, as shown in the example that follows, and waiting for another thread with duties that are understood to have time requirements, as with the SimpleThreads example in a later section.

Two overloaded versions of sleep are provided: one that specifies the sleep time to the millisecond and one that specifies the sleep time to the nanosecond. However, these sleep times are not guaranteed to be precise, because they are limited by the facilities provided by the underlying OS. Also, the sleep period can be terminated by interrupts, as we'll see in a later section. In any case, you cannot assume that invoking sleep will suspend the thread for precisely the time period specified.

The SleepMessages example uses sleep to print messages at four-second intervals:

public class SleepMessages {
    public static void main(String args[])
        throws InterruptedException {
        String importantInfo[] = {
            "Mares eat oats",
            "Does eat oats",
            "Little lambs eat ivy",
            "A kid will eat ivy too"
        };

        for (int i = 0;
             i < importantInfo.length;
             i++) {
            //Pause for 4 seconds
            Thread.sleep(4000);
            //Print a message
            System.out.println(importantInfo[i]);
        }
    }
}

Notice that main declares that it throws InterruptedException. This is an exception that sleep throws when another thread interrupts the current thread while sleep is active. Since this application has not defined another thread to cause the interrupt, it doesn't bother to catch InterruptedException.

Exceptions in java with an example

Overview

Java has a powerful concept for exception and error handling. An exception is an error that occurs at runtime. It is either generated by the Java Virtual Machine (VM) in response to an unexpected condition or it is generated by your code as a result of executing a throw statement.

Understanding Exceptions

Exceptions generated from runtime are called unchecked exceptions, since it is not possible for the compiler to determine that your code will handle the exception. Exception classes that descend from RuntimeException and Error classes are unchecked exceptions. Examples for RuntimeException are illegal cast operation, inappropriate use of a null pointer, referencing an out of bounds array element. Errorexception classes signal critical problems that typically cannot be handled by your application. Examples are out of memory error, stack overflow, failure of the Java VM.

Thrown exceptions are referred to as checked exceptions. The compiler will confirm at compile time that the method includes code that might throw an exception. Moreover the compiler requires the code that calls such a method to include this call within atry block, and provide an appropriate catch block to catch the exception.

java.lang.Object
   |
   +--java.lang.Throwable
         |
         +--java.lang.Exception
         |     |
         |     +--java.lang.ClassNotFoundException
         |     |
         |     +--java.io.IOException
         |     |     |
         |     |     +--java.io.FileNotFoundException
         |     |
         |     +--java.lang.RuntimeException
         |           |
         |           +--java.lang.NullPointerException
         |           |
         |           +--java.lang.IndexOutOfBoundsException
         |                 |
         |                 +--java.lang.ArrayIndexOutOfBoundsException
         |
         +--java.lang.Error
               |
               +--java.lang.VirtualMachineError
                     |
                     +--java.lang.OutOfMemoryError

When an (either checked or unchecked) exception is thrown, execution will attempt to immediately branch to the first catch block whose associated exception class matches the class or a superclass of the thrown exception. If the exception does not occur within a try block or the thrown exception is not caught in a matching catchblock, execution of the method immediately terminates and control returns to the invoker of the method, where this process is repeated. The result is that the exception chain is escalated until a matching catch block is found. If not, the thread containing the thrown exception is terminated.

Example

The following Demo1 class demonstrates the behaviour of exceptions and applications.

import java.io.*;

class Demo1 {

  public static FileInputStream f1(String fileName)
    throws FileNotFoundException
  {
    FileInputStream fis = new FileInputStream(fileName);
    System.out.println("f1: File input stream created");
    return fis;
  }

  public static FileInputStream f2(String fileName)
  {
    FileInputStream fis = null;
    try
    {
      fis = new FileInputStream(fileName);
    }
    catch (FileNotFoundException ex)
    {
      System.out.println("f2: Oops, FileNotFoundException caught");
    }
    finally
    {
      System.out.println("f2: finally block");
    }
    System.out.println("f2: Returning from f2");
    return fis;
  }

  public static void main(String args[])
  {
    FileInputStream fis1 = null;
    FileInputStream fis2 = null;
    String fileName = "foo.bar";
    // String fileName = null;

    System.out.println(  "main: Starting " + Demo1.class.getName()
                       + " with file name = " + fileName);

    // get file input stream 1
    try {
      fis1 = f1(fileName);
    }
    catch (FileNotFoundException ex)
    {
      System.out.println("main: Oops, FileNotFoundException caught");
    }
    catch (Exception ex)
    {
      System.out.println("main: Oops, genreal exception caught");
    }

    // get file input stream 2
    fis2 = f2(fileName);

    System.out.println("main: " + Demo1.class.getName() + " ended");
  }
}

Compile and run the Demo1 class will generate output as follows (assuming that the file foo.bar does not exist):

main: Starting Demo1 with file name = foo.bar
main: Oops, FileNotFoundException caught
f2: Oops, FileNotFoundException caught
f2: finally block
f2: Returning from f2
main: Demo1 ended

The example program tries to create two file input streams. Therefore two methods f1and f2 are implemented. First, the main program calls f1 method. Because the constructor of the FileInputStream throws a FileNotFoundException the method f1must be defined with the throws FileNotFoundException in the method definition. The thrown exception is not handled in the method but forwarded to the invoker. Note, that the system output before the return statement is never executed.

So the invoker, in our example the main program, must catch this exception. The method f1 is called in a try block. The following catch blocks catch either aFileNotFoundException or a general Exception. In our example, the exception is caught in the first catch block and the system output is generated. Since the exception in f1 is caught and handled, the execution of the program is not terminated.

Second, the example program creates another FileInputStream invoking the f2method. This method catches the FileNotFoundException, so this exception must not be forwarded to the invoker. The try ... catch statement is followed by a finallyblock. This block is always executed, regardless whether or not an exception occurs within the try block. This generates the system output in our example. Again, the exception is caught and the program executes, this generates the system output inf2 and main.

This was a straight forward example with caught exceptions. What happens with uncaught exceptions? Change the fileName assignment in the main method: Comment out the first assignment and activate the second String fileName = null; then compile and execute Demo1 again. The generated output is:

main: Starting Demo1 with file name = null
main: Oops, genreal exception caught
f2: finally block
java.lang.NullPointerException
java.io.FileInputStream Demo1.f2(java.lang.String)
void Demo1.main(java.lang.String[])
Exception in thread main

This time, we try to create two FileInputStream objects using null instead of a file name. Invoking f1 will generate a NullPointerException which is caught in the main program. Next f2 is called which could handle only the FileNotFoundException but not a NullPointerException. Nevertheless the finally block is executed and then the control returns to the main program. Because there is no try ... catch statement around the call to f2 and no matching catch block is found, the thread is terminated. Note, that f2 and main can not execute the system output statements any more.

                         How to Track Changes in a Word 2007 Document For Dummies

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Implementing an equals() Method

                            Overriding equals()

You learned about the equals() method in earlier chapters, where we looked at the wrapper classes. We discussed how comparing two object references using the == operator evaluates to true only when both references refer to the same object (because == simply looks at the bits in the variable, and they're either identical or they're not). You saw that the String class and the wrapper classes have overridden the equals() method (inherited from class Object), so that you could compare two different objects (of the same type) to see if their contents are meaningfully equivalent. If two different Integer instances both hold the int value 5, as far as you're concerned they are equal. The fact that the value 5 lives in two separate objects doesn't matter. 
When you really need to know if two references are identical, use ==. But when you need to know if the objects themselves (not the references) are equal, use the equals() method. For each class you write, you must decide if it makes sense to consider two different instances equal. For some classes, you might decide that two objects can never be equal. For example, imagine a class Car that has instance variables for things like make, model, year, configuration—you certainly don't want your car suddenly to be treated as the very same car as someone with a car that has identical attributes. Your car is your car and you don't want your neighbor Billy driving off in it just because, "hey, it's really the same car; the equals() method said so." So no two cars should ever be considered exactly equal. If two references refer to one car, then you know that both are talking about one car, not two cars that have the same attributes. So in the case of a Car you might not ever need, or want, to override the equals() method. Of course, you know that isn't the end of the story.



                                        Implementing an equals() Method



public class EqualsTest {
public static void main (String [] args) {
Moof one = new Moof(8);
Moof two = new Moof(8);
if (one.equals(two)) {
System.out.println("one and two are equal");
}
}
}
class Moof {
private int moofValue;
Moof(int val) {
moofValue = val;
}
public int getMoofValue() {
return moofValue;
}
public boolean equals(Object o) {
if ((o instanceof Moof) && (((Moof)o).getMoofValue()
== this.moofValue)) {
return true;
} else {
return false;
}
}
}

How to convert a date String to a Date or Calendar object?

                How to convert a date String to a Date or Calendar object?

DateFormat formatter = new SimpleDateFormat("MM/dd/yy");
Date date = formatter.parse("01/29/02");

See SimpleDateFormat javadoc for more.And to turn it into a Calendar, do:

Calendar calendar = Calendar.getInstance();
calendar.setTime(date);

Sample Program:

import java.util.*;
import java.text.*;
public class DateToCalender {
public static void main(String[] args) {
 try {  String str_date="11-June-07";
  DateFormat formatter ; 
  Date date ; 
  formatter = new SimpleDateFormat("dd-MMM-yy");
  date = (Date)formatter.parse(str_date); 
 Calendar cal=Calendar.getInstance();
 cal.setTime(date);
 System.out.println("Today is " +cal );
  }
  catch (ParseException e)
  {System.out.println("Exception :"+e);  }  
 
 }
}