Java exception

JAVA
Exception
Handling
Prepared by
Miss. Arati A. Gadgil

2
Errors and Error Handling
An Error is any unexpected result obtained from a program during
execution.
Unhandled errors may manifest themselves as incorrect results or
behavior, or as abnormal program termination.Errors should be handled
by the programmer.
Traditional Error Handling
1.Every method returns a value (flag) indicating either success, failure,
or some error condition. The calling method checks the return flag and
takes appropriate action.
Downside: programmer must remember to always check the return
value and take appropriate action. This requires much code (methods
are harder to read) and something may get overlooked.

3
Traditional Error Handling
2. Create a global error handling routine, and use some form of
“jump” instruction to call this routine when an error occurs.
Downside: “jump” instruction (GoTo) are considered “bad
programming practice” and are discouraged. Once you jump to the
error routine, you cannot return to the point of origin and so must
(probably) exit the program.
Exceptions act similar to method return flags in that any method may
raise and exception should it encounter an error.
Exceptions act like global error methods in that the exception
mechanism is built into Java; exceptions are handled at many levels
in a program, locally and/or globally.

Exception
Due to design errors or coding errors, our programs may fail in
unexpected ways during execution.
It is our responsibility to produce quality code that does not fail
unexpectedly.
Consequently, we must design error handling into our programs.
An exception is a special type of error object that is created when
something goes wrong in a program.
After Java creates the exception object, it sends it to program, this action
called throwing an exception. It's up to our program to catch the
exception.

5
An exception is a representation of an error condition or a situation
that is not the expected result of a method.
Exceptions are built into the Java language and are available to all
program code.
Exceptions isolate the code that deals with the error condition from
regular program logic.
When an error is detected, an exception is thrown
Any exception which is thrown, must be caught by and exception
handler
If the programmer hasn't provided one, the exception will be caught
by a catch-all exception handler provided by the system.

6
The default exception handler may terminate the application.
Exceptions can be rethrown if the exception cannot be handled by the
block which caught the exception
Java has 5 keywords for exception handling:
1.try
2.catch
3.finally
4.throw
5.Throws

7
Exception -Class Hierarchy
Throwable
+ Throwable(String message)
+ getMessage(): String
+ printStackTrace():void
Error Exception
Errors: An error represents a condition
serious enough that most reasonable
applications should not try to catch.
- Virtual Machine Error
- out of memory
- stack overflow
- Thread Death
- Linkage Error
Exceptions: An error which reasonable
applications should catch
- Array index out of bounds
- Arithmetic errors (divide by zero)
- Null Pointer Exception
- I/O Exceptions
See the Java API Specification for more.

8
Exceptions -Checked and Unchecked
Java allows for two types of exceptions:
1.Checked.
If your code invokes a method which is defined to throw checked
exception, your code MUST provide a catch handler
The compiler generates an error if the appropriate catch handler is not
Present
2.Unchecked
These exceptions can occur through normal operation of the virtual
machine. You can choose to catch them or not.
If an unchecked exception is not caught, it will go to the default catch-all
handler for the application
All Unchecked exceptions are subclassed from RuntimeException

9
Common Java Exceptions.
ArithmeticException :Caused by math errors such as division by zero
ArrayIndexOutOfBoundsException:Caused by bad array indexes
ArrayStoreException: Caused when a program tries to store the wrong
type of data in an array
FileNotFoundException: Caused by an attempt to access a nonexistent
file
IOException: Caused by general I/O failures, such as inability to read
from a file
NullPointerException: Caused by referencing a null object
NumberFormatException: Caused when a conversion between strings
and numbers fails

10
OutOfMemoryException: Caused when there's not enough memory to
allocate a new object
SecurityException: Caused when an applet tries to perform an action not
allowed by the browser'ssecurity setting
StackOverflowException: Caused when the system runs out of stack
space
StringIndexOutOfBoundsException:Caused when a program attempts to
access a nonexistent characterposition in a string

11
Exceptions –Syntax
try
{ // Code which might throw an exception
// ...
}
catch(FileNotFoundException x)
{ // code to handle a FileNotFound exception }
catch(IOException x)
{ // code to handle any other I/O exceptions }
catch(Exception x)
{ // Code to catch any other type of exception }
finally
{ // This code is ALWAYS executed whether an exception was
thrown
// or not. A good place to put clean-up code. ie. close
// any open files, etc...
}

12
Try-Catch Mechanism
Wherever your code may trigger an exception, the normal code logic
is placed inside a block of code starting with the “try” keyword:
After the try block, the code to handle the exception should it arise is
placed in a block of code starting with the “catch” keyword.
You may also write an optional “finally” block. This block contains
code that is ALWAYS executed, either after the “try” block code, or
after the “catch” block code.
Finally blocks can be used for operations that must happen no matter
what (i.e. cleanup operations such as closing a file)

13
throws keyword
The throws keyword is used in method declaration, in order to explicitly
specify the exceptions that a particular method might throw.
When a method declaration has one or more exceptions defined using
throws clause then the method-call must handle all the defined
exceptions.
When defining a method you must include a throws clause to declare
those exceptions that might be thrown but doesn’t get caught in the
method.
If a method is using throws clause along with few exceptions then this
implicitly tells other methods that – “ If you call me, you must handle
these exceptions that I throw”.
Syntax of Throws in java:
void MethodName() throws ExceptionName{ Statement1 ... ... }

14
throw Keyword
By default, when an exception condition occurs the system automatically
throw an exception to inform user that there is something wrong.
However we can also throw exception explicitly based on our own
defined condition.
Using “throw keyword” we can throw checked, unchecked and user
-defined exceptions.

15
public class ThrowExample
{
static void checkEligibilty(int stuage, int stuweight)
{
if(stuage<12 && stuweight<40)
{ throw new ArithmeticException("Student is not eligible
for registration");
}
else
{ System.out.println("Entries Valid!!"); }
}
public static void main(String args[])
{
System.out.println("Welcome to the Registration process!!");
checkEligibilty(10, 39);
System.out.println("Have a nice day..");
}
}

16
import java.io.*;
class Example
{
public static void main(String args[]) throws IOException
{
FileInputStream fis = null;
fis = new FileInputStream("B:/myfile.txt");
int k;
while(( k = fis.read() ) != -1)
{
System.out.print((char)k);
}
fis.close();
}
}
Declare the exception in the method using throws keyword.

17
import java.io.*;
class Example
{ public static void main(String args[])
{ FileInputStream fis = null;
try{
fis = new FileInputStream("B:/myfile.txt");
}catch(FileNotFoundException fnfe)
{
System.out.println("The specified file is not exist);
}
int k;
try{
while(( k = fis.read() ) != -1)
{ System.out.print((char)k); }
fis.close();
}catch(IOException ioe)
{ System.out.println("I/O error occurred: "+ioe); }
}

18
Throw vs Throws in java
1. Throws clause in used to declare an exception and thow keyword is
used to throw an exception explicitly.
2. If we see syntax wise than throw is followed by an instance variable
and throws is followed by exception class names.
3. The keyword throw is used inside method body to invoke an exception
and throws clause is used in method declaration (signature).
4.By using Throw keyword in java you cannot throw more than one
exception but using throws you can declare multiple exceptions. PFB the
examples.

19
User defined exception
User defined exceptions in java are also known as Custom exceptions.
Most of the times when we are developing an application in java, we
often feel a need to create and throw our own exceptions. These
exceptions are known as User defined or Custom exceptions.
class MyException extends Exception
{
String str1;
MyException(String str2)
{
str1=str2;
}
public String toString()
{
return ("Output String = "+str1) ;
}
}

20
class CustomException
{
public static void main(String args[])
{
try
{
throw new MyException("Custom");
// I'm throwing user defined custom exception above
} catch(MyException exp)
{
System.out.println("Hi this is my catch block") ;
System.out.println(exp) ;
}
}
}

21
Defining Your Own Exceptions
To define your own exception you must do the following:
Create an exception class to hold the exception data.Your exception class
must subclass "Exception" or another exception class
To create unchecked exceptions, subclass the RuntimeException class.
Minimally, your exception class should provide a constructor which takes
the exception description as its argument.
To throw your own exceptions:
If your exception is checked, any method which is going to throw the
exception must define it using the throws keyword
When an exceptional condition occurs, create a new instance of the
exception and throw it.

22
Rethrowing Exceptions
M2() throws an exception and M1() handles it and then rethrows it.
class MyException extends Exception { }
public class RethrowException
{
public static void main(String[] args)
{   try
       { M1(); }
          catch (Exception e)
    { System.out.println("main(): Caught " + e.toString()); }
}
public static void M1() throws MyException
{   try
       { M2(); }

23
catch (Exception e)
    {
System.out.println("M1(): Caught e.toString());
                     System.out.println("M1(): Rethrowing " + e.toString());
                     throw e;
                }
             }
public static void M2() throws MyException
{
System.out.println("M2(): Throwing MyException...");
throw new MyException();
}
}

24
Advantages of Exceptions
1: Separating Error-Handling Code from "Regular" Code
2: Propagating Errors Up the Call Stack
3: Grouping and Differentiating Error Types

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