What are Immutable and Mutable Objects (String vs StringBuffer)

 public static void main(String[] arg) { 
 /* String */ 
  
   String a = "abc"; 
   String b = "def"; 
   String c = "def"; 
  
   System.out.println(a.hashCode()); 
   a += "xyz"; 
   System.out.println(a.hashCode()); 
   System.out.println(b.hashCode()); 
   System.out.println(c.hashCode()); 
   System.out.println(a); 
  
 /* StringBuffer */ 
  
   StringBuffer sb = new StringBuffer("wer"); 
   System.out.println(sb.hashCode()); 
   sb.append("ert"); 
   System.out.println(sb.hashCode()); 
 } 

Output:
-------

96354
-1424366089
99333
99333
abcxyz
17523401
17523401

Explanation:
------------

Mutable and Immutable objects :
The objects whose values can be changed in the same address, are Mutable objects.
The objects whose values cannot be changes in the same address, are Immutable objects.

Example : String and StringBuffer

The main difference, String is Immutable whereas StringBuffer is Mutable.
String is Notsynchronized whereas StringBuffer is Synchronized.
String doesnt have an append method whereas StringBuffer has.


Important Note:
In the code above, when a String "def" is assigned to 'b', it is stored at address "99333".
Again, if I assign the same String "def" to "c", it points to the same address "99333".

Converting a substring to Integer in C++

#include < iostream >
#include < sstream >
#include < cctype >

int main()
{
std::string s = "dsdhjsahdk dsdjdsaj 36782367 sdjdhak";

int j = 0;
std::stringstream cstr;
cstr << s;
int i = 0;
char ch;

while( cstr >> ch )
{
if( std::isdigit(ch) )
{
cstr.putback(ch);
break;
}

if( ( j = s.find(" ", j) )== std::string::npos )
{
cstr.seekg(-1);
break;
}

cstr.seekg(++j, std::ios::beg);

}

if(cstr)
{
cstr >> i;
}
std::cout << i;

return 0;

}

Diff between calling a start()->run() method AND calling run() method directly without start() method

If the run() method of a Thread is called directly without start() method. It fails to obey properties of Thread. It acts as a normal method.

Example: without using start()

 class ThreadA extends Thread { 
   ThreadA() { 
   } 
  
   public void run() { 
     for (int i = 0; i < 5; i++) { 
       System.out.println("I am in run method " + i + " " + this.getName()); 
       try { 
         this.sleep(500); 
       } catch (InterruptedException ie) { 
         System.out.println(ie); 
       } 
     } 
   } 
 } 
  
 public class Threadtest { 
   public Threadtest() { 
   } 
  
   public static void main(String[] args) { 
     Thread t1 = new Thread(new ThreadA(), "t1"); 
     Thread t2 = new Thread(new ThreadA(), "t2"); 
     t1.run(); 
     t2.run(); 
   } 
 } 

The output will be:
I am in run method 0 Thread-0
I am in run method 1 Thread-0
I am in run method 2 Thread-0
I am in run method 3 Thread-0
I am in run method 4 Thread-0
I am in run method 0 Thread-1
I am in run method 1 Thread-1
I am in run method 2 Thread-1
I am in run method 3 Thread-1
I am in run method 4 Thread-1

But,
if the lines t1.run() and t2.run() are replaced by t1.start() and t2.start() then the output is,

I am in run method 0 Thread-0
I am in run method 0 Thread-1
I am in run method 1 Thread-1
I am in run method 1 Thread-0
I am in run method 2 Thread-0
I am in run method 2 Thread-1
I am in run method 3 Thread-1
I am in run method 3 Thread-0
I am in run method 4 Thread-0
I am in run method 4 Thread-1

How to handle exceptions in JSP?

You can catch exceptions in a JSP page like you would do in other Java classes. Simply put the code which can throw an exception/s between a try..catch block.

<%
try {
// Code which can throw can exception
} catch(Exception e) {
// Exception handler code here
}
%>


There is yet another useful way of catching exceptions in JSP pages. You can specify error page in the 'page' directive. Then if any exception is thrown, the control will be transferred to that error page where you can display a useful message to the user.

To demonstrate the run-time exception handling feature of JSP pages, we will build three pages.

* Form.html - Display a Form to the user to enter his or her age.
* FormHandler.jsp - A JSP Page which receives this value and prints it on the user screen.
* ExceptionHandler.jsp - An exception handler JSP page which is actually an error page to which control will be
passed when an exception is thrown.


Form.html page
--------------
<html>
<head>

</head>
<body>

<form action="FormHandler.jsp" method="post">
Enter your age ( in years ) :
<input type="text" name="age" />
<input type="submit" value="Submit" />
</form>

</body>
</html>


FormHandler.jsp
---------------
<%@ page errorPage="ExceptionHandler.jsp" %>
<html>
<head>

</head>
<body>

<%-- Form Handler Code --%>
<%
int age;
age = Integer.parseInt(request.getParameter("age"));
%>

<%-- Displaying User Age --%>

<p>Your age is : <%= age %> years.</p>

<p><a href="Form.html">Back</a>.</p>

</body>
</html>


ExceptionHandler.jsp
--------------------
<%@ page isErrorPage="true" import="java.io.*" %>
<html>
<head>
<title>Exception Occurred!</title>
</head>
<body>

<%-- Exception Handler --%>
<font color="red">
<%= exception.toString() %><br>
</font>

</body>
</html>


Conclusion:
-----------
Although you can provide your own exception handling within JSP pages, it may not be possible to anticipate all situations. By making use of the page directive's errorPage attribute, it is possible to forward an uncaught exception to an error handling JSP page for processing.

Abstract classes and interfaces

Abstract classes
Abstract classes can have both abstract methods and concrete methods.
The abstract class should be preceded by an "abstract " keyword.
The abstract methods should also be preceded by an "abstract " keyword.
The abstract class may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.

 abstract class Test { 
   int x; 
   int y; 
  
   abstract void method1(); 
   abstract void method2(); 
  
   void concretemethod1() { 
     System.out.println("I am a concrete method"); 
   } 
   void concretemethod2() { 
     System.out.println("I am a concrete method"); 
   } 
 } 

Interfaces
Interfaces can have only abstract methods.
The data-fields inside interfaces are 'public static final' by default.
So, the constants are declared inside interfaces (coding convention).
A class can extend only parent class, but an interface can extend many interfaces.

 interface Test { 
   int x = 0; 
   void method1(); 
   void method2(); 
 } 

What is MVC?

MVC - Model View Architecture. It is a type of Design pattern, followed extensively.

Model1(MVC1):

Browser -> JSP -> Bean -> DB
Browser <- JSP <- Bean <- DB

Model2(MVC2):

Browser -> Servlet -> JSP -> Bean -> DB
Browser <- JSP <- Bean <- DB

Here, servlet will instantiate the Bean