Exception Handling

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Exception Handling
throw
catch
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Exception
Exceptional (that is error) condition that has
occurred in a piece of code of Java program.

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Exception
 It will cause abnormal termination of program or wrong
execution result.
 Java provides an exception handling mechanism to handle
exceptions.
 Exception handling will improve the reliability of application
program.
 Java creates different type of objects in case of different
exceptional conditions that describe the cause of exception.

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Exception Definition
Treat exception as an object.
All exceptions are instances of a class extended
from Throwable class or its subclass.
Generally, a programmer can make new
exception class by extending the Exception
class which is subclass of Throwable class.

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Hierarchical Structure of Throwable Class
ObjectObject
ThrowableThrowable
ErrorError ExceptionException
RuntimeExceptionRuntimeException
...
...
...

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Exception Types
Error Class
o Abnormal conditions those
can NOT be handled are
called Errors.
Exception Class
o Abnormal conditions those
can be handled are called
Exceptions.

Java Exception class hierarchy
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ObjectObject
Error
ThrowableThrowable
ExceptionException
LinkageError
VirtualMachoneError
ClassNotFoundExceptionClassNotFoundException
FileNotFoundExceptionFileNotFoundException
IOExceptionIOException
AWTError
…
AWTExceptionAWTException
RuntimeException
…
ArithmeticException
NullPointerException
IndexOutOfBoundsException
Unchecked
CheckedChecked
NoSuchElementException
…
: try-catch block is mandatory
: try-catch block is optional

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Exception Handling
 Exception handling is managed by five keywords try, catch, throw,
throws, and finally.
 Exceptions can be generated by
o Java “run-time system” are called System-generated exceptions. It
is automatically raised by Java run-time system.
o Your code are called Programmatic Exceptions. It is raised by
throw keyword.
 When an exceptional condition arises, an object is created that contains
exception description.
 Handling is done with help of try-catch-finally block.
 Exception raised in try block is caught by catch block.
 Block finally is optional and always executed.

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try-catch-finally Statement
 try {
 // code
 } catch (ExceptionType1 identifier) {
 // alternate flow 1
 } catch (ExceptionType2 identifier) {
 // alternate flow 2
 } finally {
 //Resource release statements like close file ,
 //close N/W connection,
 //close Database connection, Release memory cache.
 }

Uncaught Exception
 public class TestArithmetic {
 public static void main(String[] args) {
o int k = 0;
o int i = 15;
o double div = i / k;
o System.out.println("Div is " + div);
 }}
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Output
java.lang.ArithmeticException: / by zero
at TestArithmetic .main(TestArithmetic .java:5)
JVM detects that number is divided by zero that will produce infinity.
Since infinity can be stored that is why it makes a new exception object
and then throws this exception.
Output
java.lang.ArithmeticException: / by zero
at TestArithmetic .main(TestArithmetic .java:5)
JVM detects that number is divided by zero that will produce infinity.
Since infinity can be stored that is why it makes a new exception object
and then throws this exception.

Stack Trace
Exception Output
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Exception class Exception Message
Exception Line Number
 JVM detects the attempt to divide by zero, it makes new
exception object.
 java.lang.ArithmeticException: / by zero
 at TestArithmetic .main( TestArithmetic .java:5)

Handle Exception
 public class TestArithmetic {
o int k = 0; int i = 15;
o try {
• double div = i / k;
• System.out.println("Div is " + div);
o } catch (ArithmeticException e) {
• System.out.println(“Divided by Zero");
o }
 }}
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Output
Divided by Zero
Notice that the call to println( ) inside the try block is never executed
Output
Divided by Zero
Notice that the call to println( ) inside the try block is never executed

Flow of execution
• try {
• a
• b //Throw Exception
• c
• } catch (Exception e) {
• d
• e
• } finally {
• f
• }
Normal Flow
a b c f
Exceptional Flow
 a b d e f
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Exception methods
 Object received in catch block contains two key methods
o e.getMessage(); //displays error message.
• / by zero
o e.printStackTrace(); //displays complete trace of exception
• java.lang.ArithmeticException: / by zero
• at TestArithmetic .main( TestArithmetic .java:5)
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Multiple catch blocks
 More than one exception could be raised by a single try
block. To handle this type of situation, you can specify two
or more catch blocks.
 String name = “Vijay”;
 try {
o System.out.println("Length of name is " + name.length());
o System.out.println("Charter at 7th position is " + name.charAt(6));
 } catch (StringIndexOutOfBoundsException e) {
o System.out.println("String abhi choti he!!");
 } catch (NullPointerException e) {
o System.out.println(“Sundar sa nam nahi he!");
 } finally {
o System.out.println(“Pandit hu me");
 }
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Multiple catch blocks (cont. )
 More than one exception could be raised by a single try
block. To handle this type of situation, you can specify two
or more catch blocks.
 String name = null;
 try {
 } catch (NullPointerException e) {
 } finally {
 }
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Parent catch
 try {
o System.out.println("Charter at 7 position is " + name.charAt(6));
 } catch (RuntimeException e) {
 } finally {
 }
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Generic Catch
 A catch block of Parent Class can handle exceptions of its sub classes.
It can be used as a generic catch to handle multiple exceptions in down
hierarchy.
 try {
 } catch (Exception e) {
o System.out.println(“Error ” + e.getMessage() );
 }
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Order of catch blocks: Parent & Child
 Catch block of a Child class must come first in the order, if
Parent’s class catch does exist.
 try {
o System.out.println("Charter at 7 position is " +
name.charAt(6));
 } catch (RuntimeException e) {
o System.out.println(“Error “ + e,getMessage());
 }
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System-Defined Exception
 It is raised implicitly by system because of illegal
execution of program when system cannot continue
program execution any more.
 It is created by Java System automatically.
 It is extended from Error class or RuntimeException class.

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System-Defined Exception
 IndexOutOfBoundsException:
o When beyond the bound of index in the object which use index, such as
array, string, and vector.
 ArrayStoreException:
o When incorrect type of object is assigned to element of array.
 NegativeArraySizeException:
o When using a negative size of array.
 NullPointerException:
o When referring to object as a null pointer.
 SecurityException:
o When security is violated.
 IllegalMonitorStateException:
o When the thread which is not owner of monitor involves wait or notify
method.

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Programmer-Defined Exception
Programmer can create
custom exceptions by
extending Exception or
its sub-classes.
Exceptions are raised by
programmer with help of
throw keyword.

Programmer Exception Class
 class LoginException extends Exception { //Custom exception
o public LoginException() {
• super("User Not Found");
o }
 }
 class UserClass { //Raise custom exception
 LoginException e = new LoginException();
 // ...
 if (val < 1) throw e;
 }
 }
 Exception is raised by throw keyword.
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Exception Occurrence
Raises implicitly by system.
Raises explicitly by programmer.
Syntax: throw exceptionObject
throw new LoginException();
Throwable class or
its sub class

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Exception propagation
 If there is no catch block to deal
with the exception, it is propagated
to calling method.
 Unchecked exceptions are
automatically propagated.
 Checked exceptions are propagated
by throws keyword.
 returntype methodName(params)
throws e1, ... ,ek { }
Called method
Calling method

Exception propagation ( Cont.)
o try {
• authenticate (“vijay”);
o } catch (LoginException exp) {
• System.out.println(“Invalid Id/Passwod”);
o }
 }
 public static void authenticate ( String login) throws LoginException{
o If( !“admin”.equals(login)){
• LoginException e = new LoginException();
• throw e;
o }
 }
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Disclaimer
This is an educational presentation to enhance the
skill of computer science students.
This presentation is available for free to computer
science students.
Some internet images from different URLs are
used in this presentation to simplify technical
examples and correlate examples with the real
world.
We are grateful to owners of these URLs and
pictures.
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Thank You!
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Exception Handling

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