exception handling in java-123456789.ppt

2
Introduction
 Rarely does a program runs successfully at
its very first attempt.
 It is common to make mistakes while
developing as well as typing a program.
 Such mistakes are categorised as:
 syntax errors - compilation errors.
 semantic errors– leads to programs producing
unexpected outputs.
 runtime errors – most often lead to abnormal
termination of programs or even cause the
system to crash.

3
Common Runtime Errors
 Dividing a number by zero.
 Accessing an element that is out of bounds of an
array.
 Trying to store incompatible data elements.
 Using negative value as array size.
 Trying to convert from string data to a specific data
value (e.g., converting string “abc” to integer value).
 File errors:

opening a file in “read mode” that does not exist or no read
permission

Opening a file in “write/update mode” which has “read
only” permission.

4
Without Error Handling – Example 1
class NoErrorHandling{
public static void main(String[] args){
int a,b;
a = 7;
b = 0;
System.out.println(“Result is “ + a/b);
System.out.println(“Program reached this line”);
}
}
Program does not reach here
No compilation errors. While running it reports an error and stops without
executing further statements:
java.lang.ArithmeticException: / by zero at Error2.main(Error2.java:10)

5
Traditional way of Error Handling -
Example 2
class WithErrorHandling{
int a,b;
a = 7; b = 0;
if (b != 0){
System.out.println(“Result is “ + a/b);
}
else{
System.out.println(“ B is zero);
}
System.out.println(“Program is complete”);
}
}
Program reaches here

6
Error Handling
 Any program can find itself in unusual
circumstances – Error Conditions.
 A “good” program should be able to
handle these conditions gracefully.
 Java provides a mechanism to handle
these error condition - exceptions

7
Exceptions
 An exception is a condition that is caused
by a runtime error in the program.
 Provide a mechanism to signal errors
directly without using flags.
 Allow errors to be handled in one central
part of the code without cluttering code.

8
Exceptions and their Handling
 When the JVM encounters an error such as
divide by zero, it creates an exception object
and throws it – as a notification that an error
has occurred.
 If the exception object is not caught and
handled properly, the interpreter will display an
error and terminate the program.
 If we want the program to continue with
execution of the remaining code, then we
should try to catch the exception object thrown
by the error condition and then take
appropriate corrective actions. This task is
known as exception handling.

9
Common Java Exceptions
 ArithmeticException
 ArrayIndexOutOfBoundException
 ArrayStoreException
 FileNotFoundException
 IOException – general I/O failure
 NullPointerException – referencing a null object
 OutOfMemoryException
 SecurityException – when applet tries to perform an
action not allowed by the browser’s security setting.
 StackOverflowException

10
Exceptions in Java
 A method can signal an error condition
by throwing an exception – throws
 The calling method can transfer control
to a exception handler by catching an
exception - try, catch
 Clean up can be done by - finally

11
Exception Handling Mechanism
try Block
Statements that causes
an exception
catch Block
Statements that
handle the exception
Throws
exception
Object

12
Syntax of Exception Handling Code
…
…
try {
// statements
}
catch( Exception-Type e)
{
// statements to process exception
}
..
..

13
With Exception Handling - Example
3
class WithExceptionHandling{
int a,b; float r;
a = 7; b = 0;
try{
r = a/b;
System.out.println(“Result is “ + r);
}
catch(ArithmeticException e){
}
System.out.println(“Program reached this line”);
}
}
Program Reaches here

14
Multiple Catch Statements
 If a try block is likely to raise more than one type of exceptions, then
multiple catch blocks can be defined as follows:
…
…
try {
// statements
}
catch( Exception-Type1 e)
{
// statements to process exception 1
}
..
..
catch( Exception-TypeN e)
{
// statements to process exception N
}
…

15
finally block
 Java supports definition of another block called finally that be used to
handle any exception that is not caught by any of the previous statements.
It may be added immediately after the try block or after the last catch block:
…
try {
// statements
}
catch( Exception-Type1 e)
{
// statements to process exception 1
}
..
..
finally {
….
}
 When a finally is defined, it is executed regardless of whether or not an
exception is thrown. Therefore, it is also used to perform certain house
keeping operations such as closing files and releasing system resources.

16
Catching and Propagating
Exceptions
 Exceptions raised in try block can be
caught and then they can be thrown
again/propagated after performing
some operations. This can be done by
using the keyword “throw” as follows:
 throw exception-object;

17
4
class WithExceptionCatchThrow{
int a,b; float r; a = 7; b = 0;
try{
r = a/b;
}
throw e;
}
}
}
Program Does Not
reach here
when exception occurs

18
5
class WithExceptionCatchThrowFinally{
int a,b; float r; a = 7; b = 0;
try{
r = a/b;
}
throw e;
}
finally{
}
}
}
Program reaches
here

19
 Problem Statement :
 Consider the example of the Circle class
 Circle class had the following constructor
public Circle(double centreX, double centreY,
double radius){
x = centreX; y = centreY; r = radius;
}
 How would we ensure that the radius is not zero
or negative?
User-Defined Exceptions

20
Defining your own exceptions
import java.lang.Exception;
class InvalidRadiusException extends Exception {
private double r;
public InvalidRadiusException(double radius){
r = radius;
}
public void printError(){
System.out.println("Radius [" + r + "] is not valid");
}
}

21
Throwing the exception
class Circle {
double x, y, r;
public Circle (double centreX, double centreY,
double radius ) throws InvalidRadiusException {
if (r <= 0 ) {
throw new InvalidRadiusException(radius);
}
else {
x = centreX ; y = centreY; r = radius;
}
}
}

22
Catching the exception
class CircleTest {
try{
Circle c1 = new Circle(10, 10, -1);
System.out.println("Circle created");
}
catch(InvalidRadiusException e)
{
e.printError();
}
}
}

23
User-Defined Exceptions in
standard format
class MyException extends Exception
{
MyException(String message)
{
super(message); // pass to superclass if parameter is not handled by used defined exception
}
}
class TestMyException {
…
try {
..
throw new MyException(“This is error message”);
}
catch(MyException e)
{
System.out.println(“Message is: “+e.getMessage());
}
}
}
Get Message is a method defined in a standard
Exception class.

24
Summary
 A good programs does not produce
unexpected results.
 It is always a good practice to check for
potential problem spots in programs and
guard against program failures.
 Exceptions are mainly used to deal with
runtime errors.
 Exceptions also aid in debugging programs.
 Exception handling mechanisms can effectively
used to locate the type and place of errors.

25
Summary
 Try block, code that could have exceptions /
errors
 Catch block(s), specify code to handle various
types of exceptions. First block to have
appropriate type of exception is invoked.
 If no ‘local’ catch found, exception propagates
up the method call stack, all the way to main()
 Any execution of try, normal completion, or
catch then transfers control on to finally block

exception handling in java-123456789.ppt

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