L14 Exception

8/3/2019 L14 Exception

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MIT AITI 2003

Lecture14

Exceptions

Javas Error handling mechanism


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Objectives(we will see if we have achieved them at the end of the day)

How to use methods that cause exceptions

How to respond to exceptions in your Java

programs

How to create methods that ignore an

exception, leaving it for another class to

handle

How to create your own exceptions


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SampleProgram (1)

-- Whats the output of the following program?

public class SampleProgram {

public static void main(String args[]) {

String[] greek = {"Alpha", "Beta", "Gamma"};

System.out.println(greek[3]);

}

}


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SampleProgram (2)

--The output

java.lang.ArrayIndexOutOfBoundsException

at SampleProgram.main(SampleProgram.java:5)

Exception in thread "main"


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SampleProgram (3)

--Notes Compiles successfully but encounters a problem when it

runs

The Java interpreter made note of the exception by

displaying the error message and stopped the program.

An object of type ArrayIndexOutOfBoundException is

created to alert the useryou have used an array element

that isnt within the arrays boundaries.

ArrayIndexOutOfBoundException is a subclass of

Exception


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Some words about Exceptions

All exceptions are subclasses ofException.

Some exceptions are comparable to compiler

errors, e.g. ArrayIndexOutOfBoundsException Others must be dealt with every time a program

runsyou cant deal with or you want to handle

them within a Java class

Exception handling is done using these five

statements: try, catch, finally, throw and throws


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SumNumbers (1)

public class SumNumbers {

public static void main(String[] arguments) {

float sum = 0;for (int i=0; i


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SumNumbers (2)

java SumNumbers 8 6 7 5 3 0 9

Output:

Those numbers add up to 38.0

Java SumNumbers 1 3 5xOutput?


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SumNumbers (3)

java.lang.NumberFormatException: 5x

at java.lang.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1176)

at java.lang.Float.parseFloat(Float.java:183)

at SumNumbers.main(SumNumbers.java:7)

Exception in thread "main"


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Catching Exceptions in a try-

catch Block


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NewSumNumbers(1)

public class NewSumNumbers {


float sum = 0;

for (int i=0; i


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NewSumNumbers(2)

Java NewSumNumbers 1 3 5x

Output:

5x is not a number.

Those numbers add up to 4.0


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Catching Several Different

Exceptions


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public class DivideNumbers {


if (arguments.length == 2) {

int result =0;

try {result = Integer.parseInt(arguments[0])/Integer.parseInt(arguments[1]);

System.out.println(arguments[0] + " divided by " + arguments[1] + " equals " + result);

}

catch (NumberFormatException e) {

System.out.println("Both arguments must be numbers.");

}catch (ArithmeticException e) {

System.out.println("You cannot divide by zero");

}

}

}

}


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Handling Something After an

Exception


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try-catch-finally block

try{

//statement(s) that might cause the exception

}catch (Exception e) {

//what to do when the exception occurs

}

finally {//statements to execute no matter what

}


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Throwing Exceptions


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float sum = 0;

for (int i=0; i


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public static void main(String[] arguments) throws NumberFormatException {

float sum = 0;

for (int i=0; i


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Create your own exception class


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Writing Your Own Exception Classes

Writing your own exception class is simple.

New exception classes allow you to handle a new type oferror separately.

Exception classes extendjava.lang.Exception.

public class DataFormatException

extends java.lang.Exception {

public DataFormatException(){ super(); }

public DataFormatException(String s)

{ super( s ); }

}


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Exception Objects -Structure 2 Constructors

the default constructor with no argument

the constructor that takes a string (errormessage) argument

getMessage() method

return the error message string(the argument of the 2nd constructor)


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Creating and Throwing

your own Exception instances Exceptions are objects. You must create a

new instance of an exception before you

can throw itif ( dArray.length == 0 )

throw new

IllegalArgumentException();


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Declaring an Exception If a method can throw an exception, you can always declare the type

of the exception in the header after the keyword throws

public static double average( double [] dArray )

throws IllegalArgumentException The compiler requires you to declare the possible exception throw if

the exception class is not derived from RuntimeException orError. These are called checked exceptions (checked by thecompiler).

Exceptions derived from RuntimeException are calledunchecked exceptions and their declaration is optional.


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Throwing an Exception

-- An Examplepublic static double average( double [] dArray )

throws IllegalArgumentException{

if ( dArray.length == 0 )throw new IllegalArgumentException();else

{

double sum = 0.0;

for ( int i = 0; i < dArray.length; i++ )sum += dArray[ i ];

return sum / dArray.length;}

}


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Exceptions, Errors, and

RuntimeExceptions Throwable have two special subclasses: Error and Exception

Error: a catastrophic failure (your program will not be able torecover)

Exception: a non-catastrophic failure (your program shouldrecover or exit gracefully)

RuntimeException (unchecked Exception): a specialsubclass of Exception. Methods and constructors throwing

RuntimeExceptions do not have to declare this fact. All other exceptions (checked exception): methods orconstructors throwing checked exception should declare thisfact.


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Checked vs. Unchecked Exceptions

In principle, checked exceptions are those which you, theprogrammer, are supposed to be able to fix at runtime, such as aFileNotFoundException. Very often these are generated insystem code by user, not programmer error.

Unchecked exceptions (RuntimeExceptions) are supposed to beable to occur anywhere (so hard to check for) and to be the result ofprogrammer error (so the best way of handling them is to fix theprogram).

Good examples of unchecked exceptions: NullPointerException,ArrayIndexOutOfBoundsException.

Bad example of unchecked exceptions, NumberFormatException,thrown e.g. by Float.parseFloat(String s).


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Recap:

When things go wrong What happens when things go wrong?

How do I create alternative ways to handle

atypical circumstances? Do you know the followings?

How to use methods that cause exceptions

How to respond to exceptions in your Java programs

How to create methods that ignore an exception,

leaving it for another class to handle

How to create your own exceptions


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Expecting the unexpectedtypes of Unexpected Circumstances

Catastrophic failures: can not be prevented, but

certain systems need to design in mechanisms to

minimize the damage that they cause Some failures: can be anticipated and avoided

through simple checks and guards.

Other failures: must be handled as they arise,often use Javas exception handling mechanism.


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How would you classify these

situations? Someone tripped over the power cord of

the computer on which your program was

running. Divisor is 0 for a division operation.

The arguments dont match the signature of

the methods.


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Whats wrong here?

public static int [] add(int [] n1, int [] n2) {

int [] n3 = new int[n1.length];

for (int k = 0; k


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Lets write an exception to handle the problem

public class MyArrayException extends Exception{

public MyArrayException()

{super();}

public MyArrayException(String s)

{super(s);}

public String getMessage(){

System.out.println(Oops you cant add the arrays);

return super.getMessage();

}}


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Now, rewrite the function

public static int [] add(int [] n1, int [] n2)

throws MyArrayException {

if (n1.length ! = n2.length)

throw new MyArrayException(Wrong!);else {

int [] n3 = new int[n1.length];

for (int k = 0; k


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So, what will happen here?

int [] a = { 3, 5, 6};

int [] b = { 4, 9};

try {

int [] c = MyArrayTest.add(a, b);

}

catch (MyArrayException e) {System.out.println(e.getMessage());

}


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The Output

Oops you cant add the arrays

Wrong!

L14 Exception

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