exeption handling

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Exception Handling

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Exception Handling in Java

Types of Errors

1.Compile time

All syntax errors identified by java compiler.

No class file is created when this occurs.

So it is necessary to fix all compile time errors for successful compilation.

Egs:

Missing of semicolon,

use of = instead of ==

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2.Run time

Some pgms may not run successfully due to wrong logic or errors like stack overflow.

Some of the Common run time errors are:

Division by 0

Array index out of bounds

Negative array size etc..

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Exception is a condition caused by a run time error in the program. When the java interpreter identifies an error such as division by 0 it creates an Exception object and throws it

Definition:

An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.Is defined as a condition that interrupts the normal flow of operation within a program.

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Java allows Exception handling mechanism to handle various exceptional conditions. When an exceptional condition occurs, an exception is said to be thrown.

For continuing the program execution, the user should try to catch the exception object thrown by the error condition and then display an appropriate message for taking corrective actions. This task is known as Exception handling

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This mechanism consists of :

1. Find the problem(Hit the Exception)2. Inform that an error has occurred(Throw the Exception)3. Receive the error Information(Catch the Exception)4. Take corrective actions(Handle the Exception)

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In Java Exception handling is managed by 5 key words:

try

catch

throw

throws

finally

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In Java Exception handling is managed by 5 key words:

try

catch

throw

throws

finally

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Throwable

Exception error

InterruptedException(checked exception)

RuntimeException(unchecked exception)

Exception Hierarchy

Package java.lang

Exception Handling in JavaUnchecked exception:

These exception need not be included in an method’s throws list

The compiler does not check to see if a method handles or throws these exception

these are subclasses of RuntimeExceptionThe compiler doesn't force client programmers either

to catch the exception or declare it in a throws clause.Class Error and its subclasses also are unchecked.

Checked exception: Must be included in an method’s throws list if that method can

generate one of those exceptions and does not handle it itself These exception defined by java.lang

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Exception Meaning

ArithmeticException Arithmetic error, such as divide-by-zero

ArrayIndexOutOfBoundsException Array index is out-of-bounds

ArrayStoreException Assignment to an array element of an incompatible type

ClassCastException Invalid cast

EnumConstantNotPresentException An attempt is made to use an undefined enumeration value

IllegalArgumentException Illegal argument used to invoke a method

IllegalMonitorStateException Illegal monitor operation, such as waiting on an unlocked thread

IllegalStateException Environment or application is in incorrect state

IllegalThreadStateException Requested operation not compatible with current thread state

IndexOutOfBoundsException Some type of index is out-of-bounds

NegativeArraySizeException Array created with a negative size

Java’s unchecked RuntimeException subclasses defined in java.lang


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Exception Meaning

NullPointerException Invalid use of a null reference

NumberFormatException Invalid conversion of a string to a numeric format

SecurityException Attempt to violate security

StringIndexOutOfBoundsException Attempt to index outside the bounds of a string

TypeNotPresentException Type not fount

UnsupportedOperationException An unsupported operation was encountered


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Java’s checked Exception defined in java.lang

Exception Meaning

ClassNotFoundException Class not found

CloneNotSupportedException Attempt to clone an object that does not implement the Cloneable interface

IllegalAccessException Access to a class is denied

InstantiationException Attempt to create an object of an abstract class or interface

InterruptedException One thread has been interrupted by another thread

NoSuchFieldException A requested field does not exist

NoSuchMethodException A requested method does not exist


class Ex{

public static void main(String args[]){ int a=0;

int b=2/a;

}

}

Java.lang.ArithmeticException: / by zero at Ex.main

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try & catch

try Block

Statement that causes Exception

Catch Block

Statement that causes Exception

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try{

Statement:

}

catch(Exception-type e){

statement;

}

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Exception Handling in Javaclass Ex{ public static void main(String args[]){

int d,a;try{ d=0;

a=10/d; System.out.println("from try");

}catch(ArithmeticException e){

System.out.println("divsn by Zero");}

System.out.println("after catch"); }}

Once an exception is thrown , program control transfers out of the try block into the catch block.

Once the catch statement is executed pgm control continues with the next line following the entire try/catch mechanism.

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We can display the description of an Exception in a println statement by simply passing the exception as an argument.

catch(ArithmeticException ae){

System.out.println(“Exception:”+ae);

}

o/p

Exception:java.lang.ArithmeticException: /by zero

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Common Throwable methods

• getMessage(); All throwable objects can have an associated error message. Calling this message will return the message if one present.

• getLocalizedMessage(); gets the localized version of error message.

• printStackTrace(); sends the stack trace to the system console. This is a list of method calls that led to the exception condition. It includes line number and file names too. Printing of the stack trace is the default behavior of a runtime exception when u don’t catch it ourselves.

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Multiple catch Statement

some cases, more than one exception could be raised by a single piece of code.

such cases we can specify two or more catch clauses, each catching a different type of exception.

when an exception thrown, each catch statement is inspected in order, and the first one whose type matches that of the exception is executed.

After 1 catch statement is executed, the others are bypassed and execution continues after the try/catch block.

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class Ex{public static void main(String

args[]){int d,a,len;try{ len=args.length;

a=10/len;int c[]={1};c[10]=23;

}

catch(ArithmeticException e){System.out.println("divsn by

Zero"+e);}catch(ArrayIndexOutOfBoundsExcept

ion ae){System.out.println("Array index"+ae);}System.out.println("after catch");}} 23


In multiple catch statement exception subclasses must come before any of their superclasses.

Because a catch statement that uses a superclass will catch exception of that type plus any of its subclasses.

Thus a subclass would never be reached if it came after its superclass.

Further java compiler produces an error unreachable code.

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finally

It creates a block of code that will be executed after try/catch block has completed and before the code following try/catch block.

It will execute whether or not an exception is thrown

finally is useful for:

• Closing a file

• Closing a result set

• Closing the connection established with database

This block is optional but when included is placed after the last catch block of a try

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Form:

try

{}

catch(exceptiontype e)

{}

finally

{} finally

finally

Catch block

try block

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exeption handling

Documents

methods throws

5 key words

stack trace

string args

checked exception

error message

error

statement