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Exception

An exception is an event that occurs duringthe execution of a program that disrupts thenormal flow of instructions.

Java handles exceptions using fivekeywords try, catch, finally, throws,throw.

Programming errors User/environmental errors System errors


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A Java exception is an object that describesan exceptional condition that has occurred ina piece of code.

When an exceptional condition arises,

an object representing that exception iscreated and

thrown in the method that caused theerror.


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An exception can be caught to handle it orpass it on.

Exceptions can be generated by the Java run-time system, or they can be manuallygenerated by your code.


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Object

Class Object is the root of the classhierarchy.

Every class has Object as a superclass.

All objects, including arrays, implement themethods of this class.

The Object class provides several methodsthat can be useful to any of its subclasses.


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public class Test

{

...

}

is equivalent to

public class Test extends Object

{...

}


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clone() - used to make copies of an object -

equals(Object o) - used to test whether the object is equal to

o. The default is that they both point to the same object Thismethod is often overriden.

toString() - provide a string representation of this object.Defualt is a string consisting of

name of the class + @ + hash code

(where the hash code is in hex format) of the object. Usuallyoverriden to provide more information.

finalize() - called by the garbage collector when there are nomore references to this object.

getClass() - gets the runtime class name of the object

hashCode() - generate a hash code value unique for this


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The fololwing can only be called from within a synchronized method:

notify() - called by a thread that owns this object's lock to tell awaiting thread, as arbitrarily chosen by the VM, that the lock is nowavailable.

notifyAll() - similar to notify() but wakes all waiting threads and thenthey compete as usual for the lock.

wait() - the thread that owns the lock on this object will release thelock and then wait for a notify() or notifyAll() to get the lock back.

wait(long msecs) - same as wait() but if no notify comes within thespecified time, it wakes up and starts competing for the lock on thisobject.

wait(long msecs, int nanosecs) - same as wait(long msecs) butspecified to the nanosec.


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The Throwable class

The Throwable class is the superclass of allerrors and exceptions in the Java language.

Only objects that are instances of thisclass (or one of its subclasses) are thrownby the Java Virtual Machine or can bethrown by the Java throw statement.

Similarly, only this class or one of itssubclasses can be the argument type in acatch clause.


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class Exception

The class Exception represents exceptionsthat

a program faces due to abnormal or

special conditions during execution. Exceptions can be of 2 types:

Checked (Compile time Exceptions) Unchecked (Run time Exceptions).


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Runtime exceptions

Runtime exceptions represent programmingerrors that manifest at runtime.

For exampleArrayIndexOutOfBounds,NullPointerException and so on.


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Error

Errors are irrecoverable conditions thatcan never be caught.

Example: Memory leak, LinkageError etc.Errors are direct subclass of Throwableclass.


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Program statements that you want tomonitor for exceptions are contained withina try block.

If an exception occurs within a try block, it

is thrown.

Code within catch block catch theexception and handle it .

System generated exceptions areautomatically thrown by the Java run-time

system


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To manually throw an exception, use the

keyword throw

Any exception that is thrown out of amethod must be specified as such by athrows clause

Any code that absolutely must be executed

before a method returns is put in a finallyblock


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How do you handle exceptions?

To handle the exception, write a try-catch block.

To pass the exception up the chain,declare a throws clause in your methodor class declaration.


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Uncaught Exceptions

What happens when Exceptions are nothandled?

class Exec0

{public static void main(String ar[])

{

int a = 42 / 0;

}} java.lang.ArithmeticException : / by

zero

at Exec0.main(Exec0.java:5)

Output


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When the Java run-time system detects the

attempt to divide by zero, it constructs a new exception object and then throws this exception.

This causes the execution of Exec0 to stop,because once an exception is thrown itmust be caught by exception handler and

dealt with immediately.


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The output generated for the above exampleprogram is :

java.lang.ArithmeticException : / by zero

at Exec0.main(Exec0.java:5)Here Exec0 ----> the class name

main -----> the method name

Exec0.java ----> the file name.

5 -----> the line number

ArithmeticException ---> a type of exception


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Using try and catchclass Exec2{

public static void main(String ar[]){

int a;try{

a=32/0;System.out.println( This will not be printed\n);

}catch(ArithmeticException e){System.out.println(\n Division by zero\n);

System.out.println(\n Exception caught = +e);}

System.out.println(\n After catch statement\n);

}}


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Multiple Catch Clausesclass multicatch{

public static void main(String ar[]){try{

int a = ar.length;System.out.println("a = "+a);int b = 42/a;int c[]={1}; c[42]=99;

}catch(ArithmeticException e){

System.out.println("Divide by 0 : "+e);}catch(ArrayIndexOutOfBoundsException e){

System.out.println("Array Index Out Of Bounds : "+e);

}} }


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Unreachable Code Error

When multiple catch statements are

used , it is important to remember thatexception subclasses must come before anyof their superclasses.

This is because a catch statement that usesa superclass will catch exceptions of thattype plus any of its subclasses.

Thus, a subclass would never be reached ifit came after its superclass.

In Java, unreachable code is an error.

Error !!!


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class SuperSubCatch {public static void main(String args[]) {

try {int a = 0;int b = 42 / a;

}

catch(Exception e) {System.out.println("Generic Exception catch.");}

catch(ArithmeticException e) { // ERROR -unreachable

System.out.println("This is never reached.");}

}}

Error !!!


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throw

It is possible for program to throw anexception explicitly, using the throwstatement.

The general form of throw is throw ThrowableInstance

ThrowableInstance must be an object oftype Throwable or a subclass of Throwable.

There are two ways to obtain a Throwableobjects: Using a parameter into a catch clause

Creating one with the newoperator


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class throwdemo{

static void demoproc()

{try{

throw new NullPointerException("demo");}

catch(NullPointerException e){

System.out.println("Caught within demoproc() ");throw e;

}}

Exception Re-thrown


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public static void main(String ar[]){

try{

demoproc();}

catch(NullPointerException e){

System.out.println("\n Recaught : "+e);}

}} OUTPUT

Caught within demoproc.Recaught: java.lang.NullPointerException: demo


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throw new NullPointerException("demo");

new is used to construct an instance ofNullPointerException.

All of Java's built-in run-time exceptions have at

least 2 constructors :one with no parameter and

one that takes a string parameter.


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A throws clause lists the types ofexceptions that a method might throw.

This is necessary for all exceptions, exceptthose of type Error or RuntimeException,or any of their subclasses.

All other exceptions that a method canthrow must be declared in the throws

clause.

If they are not, a compile-time error willresult.


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throws - Syntax

This is the general form of a methoddeclaration that includes a throws clause:

type method-name(parameter-list) throws

exception-list{

// body of method

}


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// This program contains an error and will notcompile.

class ThrowsDemo {static void throwOne() {System.out.println("Inside throwOne.");throw new IllegalAccessException("demo");

}public static void main(String args[]) {throwOne();

}}

ThrowsError.java:5: unreported exceptionjava.lang.IllegalAccessException; must be caught or declared tobe thrown

throw new IllegalAccessException("demo");^

1 error

Output

Program Containing Error


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To make this example compile, you needto make two changes.

First, you need to declare that throwOne( )throws IllegalAccessException.

Second, main( ) must define a try/catchstatement that catches this exception.


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class ThrowsDemo {static void throwOne() throws IllegalAccessException

{System.out.println("Inside throwOne.");

throw new IllegalAccessException("demo");}public static void main(String args[]) {

try {throwOne();

} catch (IllegalAccessException e) {System.out.println("Caught " + e);

}}

}

Correct - Program


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finally

finally creates a block of code that will be

executed after a try/catch block hascompleted and before the code following thetry/catch block.

The finally block will execute whether or notan exception is thrown.

If an exception is thrown, the finally blockwill execute even if no catch statementmatches the exception.


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static void procB() {


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static void procB() {try {System.out.println("inside procB");return;

}finally {

System.out.println("procB's finally");}

}

static void procC() {try {

System.out.println("inside procC");

}finally {

System.out.println("procC's finally");}

}


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public static void main(String args[]) {

try {procA();

}catch (Exception e) {

System.out.println("Exception caught");}

procB();procC();

}}

inside procAprocAs finally

Exception caughtinside procBprocBs finallyinside procC

procCs finally

Output


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In this example,

procA( ) prematurely breaks out of the tryby throwing an exception. The finallyclause is executed on the way out.

procB( )s try statement is exited via areturn statement. The finally clause isexecuted before procB( ) returns.

In procC( ), the try statement executes

normally, without error. However, thefinally block is still executed.

J B il i E i


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Javas Built-in Exceptions

Inside the standard package java.lang,Java defines several exception classes.

Unchecked Exception

Checked Exception

U h k d E i


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Unchecked Exception

NumberFormatExceptionNullPointerExceptionNegativeArraySizeExceptionIndexOutOfBoundsExceptionIllegalstateException

IllegalMonitorStateExceptionIllegalArgumentExceptionClassCastExceptionArrayStoreExceptionArrayIndexOutOfBoundsExceptionArithmeticExceptionSecurityException


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class ExceptionDemo {


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class ExceptionDemo { static void compute(int a) throws MyException { System.out.println("Called compute(" + a + ")");

if(a > 10)throw new MyException(a);System.out.println("Normal exit");

} public static void main(String args[]) {

try {compute(1);

compute(20);}

catch (MyException e) {System.out.println("Caught " + e);

}}

Date post:	07-Apr-2018
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