Java exception handling

Exception Handling in Java
 Exception
 Exception is an abnormal condition that arises at run time
 Event that disrupts the normal flow of the program.
 It is an object which is thrown at runtime.
 Exception Handling
 Exception Handling is a mechanism to handle runtime errors.
 Normal flow of the application can be maintained.
 It is an object which is thrown at runtime.
 Exception handling done with the exception object.

There are three categories of errors:
 Syntax errors - arise because the rules of the language have not been
followed. They are detected by the compiler.
 Runtime errors - occur while the program is running if the
environment detects an operation that is impossible to carry out.
 Logic errors - occur when a program doesn't perform the way it was
intended to.
Types of Errors

 Types of Exception:
 There are mainly two types of exceptions:
 Checked
 Unchecked – Eg. error
 The sun microsystem says there are three types of exceptions:
 Checked Exception - are checked at compile-time.
 Unchecked Exception - are not checked at compile-time rather
they are checked at runtime.
 Error

 Checked Exception - Classes that extend Throwable class except
RuntimeException and Error are known as checked exceptions.
Checked Exceptions means that compiler forces the programmer to
check and deal with the exceptions. e.g.IOException, SQLException
etc.
 Unchecked Exception - Classes that extends RuntimeException, Error
and their subclasses are known as unchecked exceptions e.g.
ArithmeticException,NullPointerException, ArrayIndexOutOf Bounds
Exception etc.
 Error is irrecoverable should not try to catch. e.g. OutOfMemoryError,
VirtualMachineError, AssertionError etc.

Exception Classes
LinkageError
Error
AWTError
AWTException
Throwable
ClassNotFoundException
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Several more classes
IllegalArgumentException

6
System Errors
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur.

7
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
Exceptions
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.

8
Runtime Exceptions
LinkageError
Error
AWTError
AWTException
Throwable
VirtualMachineError
IOException
Exception
RuntimeException
Object
ArithmeticException
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-
bounds array, and numeric errors.

Exception Handling Terms
 try – used to enclose a segment of code that may produce a
exception
 throw – to generate an exception or to describe an instance of an
exception
 catch – placed directly after the try block to handle one or more
exception types
 finally – optional statement used after a try-catch block to run a
segment of code regardless if a exception is generated

Exceptions –Syntax
try
{
// Code which might throw an exception
}
catch(Exceptionclass object1)
{
// code to handle an exception
}
{
}
finally
{
// ALWAYS executed whether an exception was thrown or not
}

class Simple
{
public static void main(String args[])
{
int data=50/0;
System.out.println("rest of the code...");
}
}
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero
Rest of the code is not executed (rest of the code..)statement is not printed.

 JVM first checks whether the exception is handled or not.
 If exception is not handled, JVM provides a default exception handler
that performs the following tasks:
 Prints out exception description.
 Prints the stack trace (Hierarchy of methods where the exception
occurred).
 Causes the program to terminate.
 if exception is handled by the application programmer, normal flow of
the application is maintained i.e. rest of the code is executed.

class Simple1
{
{
try
{
int data=50/0;
}
catch(ArithmeticException e)
{
System.out.println(e);
}
}}
Output:
Exception in thread main java.lang.ArithmeticException:/ by zero
rest of the code…

Multiple catch block:
 If more than one exception can occur, then we use multiple catch
blocks
 When an exception is thrown, each catch statement is inspected in
order, and the first one whose type matches that of the exception is
executed
 After one catch statement executes, the others are bypassed

Multiple Catch Exceptions –Syntax
try
{
// Code which might throw an exception
}
{
}
{
}

Nested try Statements
 A try statement can be inside the block of another try
 Each time a try statement is entered, the context of that exception
is pushed on the stack
 If an inner try statement does not have a catch, then the next try
statement’s catch handlers are inspected for a match
 If a method call within a try block has try block within it, then then
it is still nested try

try
{
statement 1;
statement 2;
try
{
statement 1;
statement 2;
}
catch(Exception e)
{
}
}
catch(Exception e)
{
}
Nested Try Block

class Excep6
{
{
try
{
try
{
S.o.p("going to divide");
int b =39/0;
}
{
}
try
{
int a[]=new int[5];
a[5]=4;
}
catch(ArrayIndexOutOfBoundsException e)
{
}
System.out.println("other statement");
}
catch(Exception e)
{
System.out.println("handeled");
}
System.out.println("normal flow..");
}
}

 Finally block
 is a block that is always executed.
 To perform some important tasks such as closing connection, stream etc.
 Used to put "cleanup" code such as closing a file, closing connection etc.
 Finally creates a block of code that will be executed after a try/catch block
has completed
 Finally block will be executed whether or not an exception is thrown.
 Each try clause requires at least one catch or finally clause.
 Note: Before terminating the program, JVM executes finally block(if any).
 Note: finally must be followed by try or catch block.

class Simple
{
{
try
{
int data=25/0;
System.out.println(data);
}
{
}
finally
{
System.out.println("finally block is always executed");
}
}}

 throw keyword
 keyword is used to explictily throw an exception / custom exception.
throw new ExceptionName("Error Message");
 Throw either checked or uncheked exception.
throw new ThrowableInstance
 ThrowableInstance must be an object of type Throwable /
subclass Throwable
 There are two ways to obtain a Throwable objects:
 Using a parameter into a catch clause
 Creating one with the new operator

public class bank
{
{
int balance = 100, withdraw = 1000;
if(balance < withdraw)
{
//ArithmeticException e = new ArithmeticException("No money please");
//throw e;
//throw new ArithmeticException();
throw new ArithmeticException("No Money");
}
else
{
System.out.println("Draw & enjoy Sir, Best wishes of the day");
}
}
}

import java.io.*;
public class Example
{
public static void main(String args[]) throws IOException
{
DataInputStream dis=new DataInputStream(System.in);
int x = Integer.parseInt(dis.readLine());
if(x < 0)
{
throw new IllegalArgumentException();
throw new IllegalArgumentException
("You have entered no"+" "+ x +" "+ "which is less than 0");
}
else
{
System.out.println("The no is"+x);
}
}
}

throws
 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
type method-name parameter-list) throws exception-list
{
// body of method
}
 It is not applicable for Error or RuntimeException, or any of their
subclasses

unchecked Exception: under your control so correct your code.
error: beyond your control e.g. you are unable to do anything
E.g: VirtualMachineError or StackOverflowError.
throw keyword throws keyword
throw is used to explicitly throw an
exception.
throws is used to declare an exception.
checked exception can not be propagated
without throws.
checked exception can be propagated
with throws.
throw is followed by an instance. throws is followed by class.
throw is used within the method.
throws is used with the method
signature.
You cannot throw multiple exception
You can declare multiple exception e.g.
public void method()throws
IOException,SQLException.

class NumberRangeException extends Exception
{
String msg;
NumberRangeException()
{
msg = new String("Enter a number between 20 and 100");
}
}
Create our Own Exception:

public class My_Exception
{
public static void main (String args [ ])
{
try
{
int x = 10;
if (x < 20 || x >100) throw new NumberRangeException( );
}
catch (NumberRangeException e)
{
System.out.println (e);
}
}
}

Java exception handling

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Java exception handling