Java

N.SURATHAVANI(Info
Tech)
EXCEPTION
HANDLING
NADAR SARASWATHI
COLLEGE OF ARTS AND
SCIENCE,THENI.

EXCEPTION HANDLING
FUNDAMENTALS:
A Jvava exception is an object that describes an
exceptional(that is,error) condition that has
occurred in a piece of code.
Exceptions can be generated by the Java run-
time system, or they can be manually generated
by your code.
Exceptions thrown by Java relate to
fundamental errors that violate the rules of the
Java language or the constraints of the Java
execution environment.

Key words:
 try
catch
throw
finally.

General form of an exception-
handling block:
try{
//block of code to monitor for errors
}
catch(Exception Type1 exOb){
//exception handler for ExceptionType1
}
catch(Exception Type2 exOb){
//exception handler for ExceptionType2
}
//...
finally{
//block of code to be executed after try blocks ends
}

EXCEPTION TYPES
All exception types are subclasses of the built-in
class Throwable.
Thus, Throwable is at the top of the exception
class
hierarchy.
Branches
Headed
by
exceptio
n
Topped
by Error

Headed by Exception:
This class is used for exceptional conditions
that user programs should catch.
This is also the class that you will subclass to
create your own custom exception types.There
is an important subclass of Exception, called
Runtime Exception.

Topped by Error:
The other branch is topped by Error, which
defines exceptions that are not excepted to
be caught under normal circumstances by
your program.
Exceptions of type Error are used by the
Java run-time system to indicate errors
having to do with the run-time environment,
itself.

Try catch block:
Although the default exception handler provided
by the java run-time system is useful for
debugging.
To guard against and handle a run-time
error,simply enclose the code that you want to
monitor inside a try block.
Immediately following the try block, include a
catch clause that specifies the exception type
that you wish to catch,

Example program:
class Exc2{
public static void main(String args[ ]){
int d,a;
try { // monitor a block of code.
d=0;
a=42/d;
System.out.println(“This will not be printed”);
} catch (ArithmeticException e){//catch divide-by-
zero error
System.out.println(“Division by zero”);
}
System.out.println(“After catch statement”);
}
}

THROW:
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.
throw ThrowableInstance;

Two ways can obtain a
Throwable object:
parameter in a catch clause.
creating one with the
new operator.

It is possible for your program to throw an
exception explicity.
Throwable Instance must be an object of type
throwable.
The flow of execution will stop automatically
after
encounter the “throw keyword”.
The nearest try and catch block inspected
immediatly.

 The flow of execution stops immediately
after the throw statement; any subsequent
statements are not executed.
The nearest enclosing try bock is inspected
to
see if it has a catch statement that matches
the
type of exception.
If it does find a match, control is transferred
to
that statement.If not,then the next enclosing

Example program:
// Demonstrate throw.
Class ThrowDemo{
Static void demoproc()
{
try
{
Throw new NullPointerException(“demo”);
}
catch(NullPointerException e)
{
System.out.println(“Caught inside demoproc.”);
Throw e; // rethrow the exception

}
}
Public static void main(String args[])
{
Try
{
Demoproc();
}
catch(NullPointerException e)
{
System.out.println(“Recaught: ” +e);
}
}
}

throws
Capable of causing an exception. It didn’t
handle by
catch block.
You can do by using “throws” declaration.
Throws clause lists the types of explicitly that a
method might throw.
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.

Java

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