Creating your own Exception Subclasses

Java defines a class called Exception, which represents generic unexpected events.Subclasses define specific unexpected events.There are two general categories of exceptions: checked and unchecked. Unchecked exceptions descend from Run time Exception and Error, which are subclasses of Exception.
You should leave unchecked exceptions to Java.That is, you should always throw checked exceptions, never subclasses of Run time Exception or Error. You can look through the APIs for the different types of checked and unchecked exceptions that Java defines. 

The difference between a checked and unchecked exception is that you do not have to wrap the call to a method that might throw an unchecked exception in a try/catch block, while you must do so for a checked exception. But unchecked exceptions will halt your thread just the same as checked exceptions if you do not catch them.

Your subclass don't need to actually implement anything .it is their existence in the type system that allows you to use them as exceptions.The Exception class does not define any methods of its own .It does of course inherit those methods provided by throw able .Thus all exceptions ,including those that you create have the methods defined by throw able available to them .

Here is a listing of some of the more common exceptions in Java and where they might occur:

Method	Description
Throwable fillInStackTrace	Returns a Throwable objectthat contains a completed stack trace.This object can be rethrown
Throwable initCause(Throwable causeexc)	Associates causeExc with the invoking exception as a cause of the invoking exception.Returns a reference to the exception
String getLocalized Message()	Returns a localizes description of the exception
String getMessage()	Return a description of the exception
void printStackTrace(PrintWriter stream)	Sends the stac trace to the specified Stream
void printStackTrace()	Displays the stack trace
void printStackTrace(PrintStream stream)	Sends the stack trace to the specified Stream
void setStackTrace(StackTraceElement elements[])	Sets the stack trace to the elements passed in elements.This method specialized applications,not normal use
String toString	Returns a String object containing a description of the exception .This method is called by prinln() when outputting a throwable object
AclNotFoundException	ActivationException
AlreadyBoundException	ApplicationException
AWTException	BackingStoreException
InvalidMidiDataException	InvalidPreferencesFormatException
GeneralSecurityException	GSSException
Throwable getCause()	Returns the exception that underlies the current exception .if there is no underlying exception null is returned
BadAttributeValueExpException	BadBinaryOpValueExpException
BadLocationException	BadStringOperationException
ClassNotFoundException	CloneNotSupportedException
RefreshFailedException	RemarshalException
RuntimeException	SAXException
TimeoutException	TooManyListenersException
LastOwnerException	LineUnavailableException
TransformerException	TransformException
DataFormatException	DatatypeConfigurationException
DestroyFailedException	ExecutionException
ExpandVetoException	FontFormatException
BrokenBarrierException	CertificateException
InstantiationException	InterruptedException
IntrospectionException	InvalidApplicationException
IllegalAccessException	IllegalClassFormatException
InvalidTargetObjectTypeException	InvocationTargetException
IOException	JAXBException
JMException	KeySelectorException
MarshalException	MidiUnavailableException
MimeTypeParseException	MimeTypeParseException
NamingException	NoninvertibleTransformException
NoSuchFieldException	NoSuchMethodException
NotBoundException	NotOwnerException
ScriptException	ServerNotActiveException
SOAPException	SQLException
StackTraceElement[] getStackTrace()	Returns an array that contains the stack trace.One element at atime as an array of StackTraceElement. The method called before the exception was thrown.This method is found in the first element of the array.The StackTraceElement class gives your program access to information about each element in the trace
ParseException	ParserConfigurationException
PrinterException	PrintException
PrivilegedActionException	PropertyVetoException
UnmodifiableClassException	UnsupportedAudioFileException
UnsupportedCallbackException	UnsupportedFlavorException
UnsupportedLookAndFeelException	URIReferenceException
XMLSignatureException	XMLStreamException
URISyntaxException	UserException
XAException	XMLParseException

Program : ExceptionDemo.java

import java.io.*;
class Myexception extends Exception
{
    private int detail;
    Myexception(int a)
    {
     detail=a;
    }
    public String toStrng()
     {
      return "Myexception["+detail+"]";
     }

}
class ExceptionDemo
{
    static void compute (int a) throws Myexception
    {
     System.out.println("called compute ("+a+")");
     if(a>10)
     throw new Myexception(a);
     System.out.println("Normal exit");
    }
    public static void main(String arg[])
     {
         try
        {

            
         compute(1);
         compute(20);
        }
         catch(Myexception e)
        {
         System.out.println("caught "+e);
        }
     }
}

This exaple defines a subclass of Exception called MyException .This subclass is quite simple.It has only a constructor plus an overloaded toString() method that displays the value of the exception .The ExceptionDemo class defines a method named compute() that throws a MyException object.The exception is thrown when compute() integet parameter is greater than 10 .The main method sets up an exception handler for MyException ,then calls compute() with a legal value and an ilegal one to show both paths through the code

Output

Called compute(1)

Normal exit

Called compute(20)

Caught MyException[20]