Exception Handling

Presentation on

Presented By :Sumit Lole

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Agenda

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What is Exception?

Exception Hierarchy

Exception vs. Error

Types of Exception

Default Exception Handler

Exception Handling

Creating Your own Exception

Exception

Definition: 

“An exception is an event, which occurs during

the execution of a program, that disrupts the

normal flow of the program's instructions”

An exception occurs when our code asks the

JVM to perform the technically impossible and

unanticipated (by the compiler)!

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Exception Hierarchy

All exception types are subclasses of the built-in class Throwable

Throwable has two subclasses :

Exception

Error

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Throwable

Error Exception

RuntimeException

IOExceptionThe Java

Exception Hierarchy

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Error

Error hierarchy describes internal errors

and resource exhaustion.

Don’t throw an object of this type!

Relatively rare

Mostly beyond programmers control

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Types of Exceptions in Java

Checked exceptions

All exception inherited from Exception

Class

Unchecked exceptions

All exception inherited from

RuntimeException Class

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Default Exception Handler ?

Default handler prints a stack trace from

the point at which the exception occurred,

and terminates the program

Example: BasicDemo.java9

Exception Handling

1. Anticipate the error by the user/system.

2. Return the program to a safe state that

enables the user to execute other

commands.

3. Inform the user of the error’s cause.

4. Allow the user to save work and terminate

the program gracefully.

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Ways to throw an Exception

1. Calling a method that throws a checked

exception

2. Code detects an error and generates checked

exception with throw statement

3. Programming error e.g. a[-1] =0; generates

an unchecked exception

ArrayIndexOutOfBoundsException

4. JVM or runtime library internal error

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Keywords for Java Exceptions

throws

throw

try

catch

finally

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Catching Exceptions

“What goes up must come down”

Catching exceptions is a bit trickier

An uncaught exception will terminate

the program …

… print the exception type to screen …

… print the stack trace.

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Using try and catch

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Placed the code (which is suppose to throw an

exception) inside a block of code starting with the

“try” keyword

The catch clause should follow immediately the

try block.

Once the catch statement has executed, program

control continues with the next line in the program

following the entire try/catch mechanism

Example : InitialDemo.java

Nested Try StatementA 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

Example : ExceptionChaining.java

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What information does java.lang.Exception contain The type of exception -- the exception class

Where the exception occurred -- the stack

trace

Context and explanatory information --

the error message, and other state information

Example : StackTrace.java

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finallyIt is used to handle premature execution of a

method (i.e. a method open a file upon entry and

closes it upon exit)

finally creates a block of code that will be

executed after try/catch block has completed

and before the code following the try/catch block

finally clause will execute whether or not an

exception is thrown

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Some Important Exception that generally occurs…IOExceptionClassNotFoundExceptionRuntimeException

Most occurring exceptionCaused by programming error“If it’s a RuntimeException it’s your

fault!”Example :

ArrayIndexOutOfBoundExceptionIndexOutOfBoundArithmeticExceptionNullPointerException

Example : ExceptionExample.java18

throwIt is possible for your program to throw

an exception explicitlythrow ExceptionInstance

Here, ExceptionInstance must be an object of type Throwable or a subclass Throwable

There are two ways to obtain a Throwable objects:Using a parameter into a catch clauseCreating one with the new operator

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Creating Your Own ExceptionsJava provides over 40 categories of Exceptions

Java allows you to create your own ExceptionsExtend a subclass of Throwable

Exception class constructorsException()Exception(String message)Exception(String message, Throwable cause)

Exception(Throwable cause)

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Example

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throw statement : throw <object reference expression>;

Class DivisionByZeroException extends Exception{DivisionByZeroException(String msg) {

super(msg);}

}public void division(){

int num1 = 10;int num2 = 0;if(num2 == 0)

throw new DivisionByZeroException();System.out.println(num1/num2);

}

throwsIf 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 exceptiontype method-name parameter-list) throws exception-

list{

// body of method}

It is not applicable for Error or RuntimeException, or any of their subclasses

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Rule for Method Overriding

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While overriding, the new method definition in the subclass can only specify all or a subset of the exception classes specified in the throws clause of the overridden method in the superclass

Some Methods Inherited from Throwable

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fillStackTrace() returns ThrowablegetCause() returns ThrowablegetMessage() returns StringgetStackTrace() return

StackTraceElement[] - ThrowableinitCause(Throwable) printSatckTrace() return voidsetStackTrace(Throwable) toString() return String

Queries

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