Java Exception

A Java exception is an object that describes an exceptional (that is, error) condition that has occurred in a piece of code. When an exceptional condition arises, an object representing that exception is created and thrown in the method that caused the error. That method may choose to handle the exception itself, or pass it on. Either way, at some point, the exception is caught and processed. Exceptions can be generated by the Java run-time system, or they can be manually generated by your code

Program statements that you want to monitor for exceptions are contained within a try block. If an exception occurs within the try block, it is thrown. Your code can catch this exception (using catch) and handle it. System-generated exceptions are automatically thrown by the Java run-time system. To manually throw an exception, use the keyword throw. Any exception that is thrown out of a method must be specified as such by a throws clause. Any code that absolutely must be executed after a try block completes is put in a finally block.

A general form of an exception-handling block: try {

// block of code to monitor for errors

}

catch (ExceptionType1 exOb) {

// exception handler for ExceptionType1

}

// ...

finally {

// block of code to be executed after try block ends

}

Here, ExceptionType is the type of exception that has occurred.

It is possible for your program to throw an exception explicitly, using the throw statement. The general form of throw is shown here:

throw ThrowableInstance;

Here, ThrowableInstance must be an object of type Throwable or a subclass of Throwable. Primitive types, such as int or char, as well as non-Throwable classes, such as String and

Object, cannot be used as exceptions. There are two ways you can obtain a Throwable object: using a parameter in a catch clause, or creating one with the new operator.

public class throwexample {

static void add(){

//int a=0,b=25;

try{

throw new NullPointerException("demo"); } catch(NullPointerException e){

System.out.println(" Caught inside demoproc() "); throw e;

}

}

public static void main(String args[])

{

try{

add();

}

catch (NullPointerException e){ System.out.println("Recaught"+e);

}

}

}

If a method is capable of causing an exception that it does not handle, it must specify this behavior so that callers of the method can guard themselves against that exception. Athrows clause lists the types

of exceptions that a method might throw. This is necessary for all exceptions, except those of type Error or RuntimeException, or any of their subclasses. All other exceptions that a method can throw must be declared in the throws clause

type method-name(parameter-list) throws exception-list

{

// body of method

}

package day1;

public class throwsexceptionexample {

static void throwone() throws IllegalAccessException { System.out.println("Inside throws");

throw new IllegalAccessException("Demo");

}

public static void main(String args[])

{

try{

throwone();

}

catch (IllegalAccessException e){ System.out.println("caught"+e);

}

}

}