Lecture 09 Exceptions

8/11/2019 Lecture 09 Exceptions

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Lecture09


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Exce tional event

Error that occurs during runtime Cause normal program flow to be disrupted

Examples

Divide by zero errors Accessing the elements of an array beyond its range

Invalid input

Opening a nonexistent file

Heap memory exhausted


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An exception in Java is an object that is created when anabnormal situation arises in your program

nature of the problem

An Exception is always an object of some subclass of the

Java provides a very well defined hierarchy of Exceptions to deal

with situations which are unusual.

the class Throwable

Class Error


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Classification of Exceptions

ClassNotFoundException

IOException

Exception

r me c xcep on

NullPointerException

RuntimeException

IndexOutOfBoundsException

Many more classes

ThrowableObject

Many more classes

IllegalArgumentException

LinkageError

Error VirtualMachineError

Many more classes


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Error class

Used by the Java runtime system to handle errorsoccurring in the runtime environment

Generally beyond the control of user programs Exam les

Out of memory errors

Hard disk crash

Exce tion class Conditions that user programs can reasonably deal with

Usually the result of some flaws in the user program code

Division by zero error

Array outofbounds error


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Checked Exceptions vs. Unchecked

and their subclassesError,RuntimeExceptionare known as unchecked exce tions. All other

exceptions are known as checked exceptions,

meanin that the com iler forces theprogrammer to check and deal with the

exce tions.

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Unchecked Exceptions

In most cases, unchecked exceptions reflect programminglogic errors that are not recoverable. For example,

is thrown if you access an objectNullPointerExceptiona

through a reference variable before an object is assigned to it;

is thrown if you access anIndexOutOfBoundsExceptionan

element in an array outside the bounds of the array.

ese are e og c errors a s ou e correc e n e

program.

.

To avoid cumbersome overuse of try-catch blocks, Java does

7

.


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class DivByZero {

public static void main(String args[]) {

System.out.println(3/0);

System.out.println(Pls. print me.);

Exception in thread "main java.lang.ArithmeticException: \ by zero at. .

Default exception handler

Provided by Java runtime

r n s ou excep on escr p on

Prints the stack trace

Causes the program to terminate


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What Happens When an Exception

Occurs? When an exce tion occurs within a method the method

creates an exception object and hands it off to the runtime

system

called throwing an exception

Exception object contains information about the error, including its


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For all subclasses of Exception Class(except

RuntimeExce tion ou must include code to deal with

them

If your program has the potential to generate an exception

of such a type, you have got two choices

Handle the exception within the method

Register that your method may throw such an exception (You

are passing the exception on)

I you o ne t er your co e won t comp e


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Catching Exceptions:

The trycatch Statements

Syntax:

try {

}

catch ( ) {< an er xcept on ype occurs>

}


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class DivByZero {


try {System.out.println(3/0);

System.out.println(Please print me.);

}

catch (ArithmeticException exc) {

//Division by zero is an ArithmeticException

System.out.println(exc);

}System.out.println(After exception.);

}


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class MultipleCatch {


try

int den = Integer.parseInt(args[0]);System.out.println(3/den);

catch (ArithmeticException exc) {

System.out.println(Divisor was 0.);

catch (ArrayIndexOutOfBoundsException exc2) {

System.out.println(Missing argument.);

}System.out.println(After exception.);

}

}


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Nested Try

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

tr {

int a = Integer.parseInt(args[0]);

try {int b = Inte er. arseInt ar s 1

System.out.println(a/b);

} catch (ArithmeticException e) {S stem.out. rintln Div b zero error!"

}

}

System.out.println(Need 2 parameters!");

}


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class NestedTryDemo2 {

static void nestedTry(String args[]) {

int a = Integer.parseInt(args[0]);

int b = Integer.parseInt(args[1]);

System.out.println(a/b);

} catch (ArithmeticException e) {

System.out.println("Div by zero error!");

}}

public static void main(String args[]){

try {

nes e ry args ;

} catch (ArrayIndexOutOfBoundsException e) {

System.out.println("Need 2 parameters!");

}}


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try {

}catch ( ) {

}finally {


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different scenarios: Forced exit occurs usin a return a continue or a

break statement

Normal completion Caught exception thrown

Exception was thrown and caught in the method

ncaug excep on rown Exception thrown was not specified in any catch block

in the method


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Examplec ass na y emo

static void myMethod(int n) throws Exception{

try {

for (int i=1; i


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public static void main(String args[]){

= =

try { FinallyDemo.myMethod(i);

}

catch (Exception e){

S stem.out. rint "Exce tion cau ht: "

System.out.println(e.getMessage());

}

ys em.ou .pr n n ;

}

}


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output

1st case

in finally try blk entered

2nd case


3rd case

RuntimeException: case 3!


after finally

4th case


Exception caught: 4!

Lecture 09 Exceptions

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