Download Note Set 16

CSE 1341 - Honors
Principles of Computer Science I
Spring 2008
Mark Fontenot
[email protected]
Note Set 16
Note Set 16 Overview
 Exception Handling
exception
 Exception – an indication of a problem that occurs during a
program’s execution
 Exception handling allows the graceful handling of errors in a
program.
 Goal – allow the program to continue executing as if no
problem had been encountered
 Can somewhat separate error handling code from regular
processing code
The standard
Do Something (processing)
Check to see if it worked (error checking)
Do something else
Check to see if it worked
Do Something
Check to see if it worked
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// Fig. 13.1: DivideByZeroNoExceptionHandling.java
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// An application that attempts to divide by zero.
import java.util.Scanner;
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public class DivideByZeroNoExceptionHandling
{
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// demonstrates throwing an exception when a divide-by-zero occurs
public static int quotient( int numerator, int denominator )
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{
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} // end method quotient
Attempt to divide;
denominator may be zero
return numerator / denominator; // possible division by zero
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public static void main( String args[] )
{
Scanner scanner = new Scanner( System.in ); // scanner for input
System.out.print( "Please enter an integer numerator: " );
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int numerator = scanner.nextInt();
System.out.print( "Please enter an integer denominator: " );
int denominator = scanner.nextInt();
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int result = quotient( numerator, denominator );
Read input; exception occurs if
input is not a valid integer
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System.out.printf(
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"\nResult: %d / %d = %d\n", numerator, denominator, result );
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} // end main
26 } // end class DivideByZeroNoExceptionHandling
Please enter an integer numerator: 100
Please enter an integer denominator: 7
Result: 100 / 7 = 14
Please enter an integer numerator: 100
Please enter an integer denominator: 7
Result: 100 / 7 = 14
Please enter an integer numerator: 100
Please enter an integer denominator: 0
Exception in thread "main" java.lang.ArithmeticException: / by zero
at
DivideByZeroNoExceptionHandling.quotient(DivideByZeroNoExceptionHandling.java:10)
at
DivideByZeroNoExceptionHandling.main(DivideByZeroNoExceptionHandling.java:22)
Please enter an integer numerator: 100
Please enter an integer denominator: hello
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at java.util.Scanner.nextInt(Unknown Source)
at
DivideByZeroNoExceptionHandling.main(DivideByZeroNoExceptionHandling.java:20)
Stack Trace
Exception Handling Code
try {
//put code here that might
// throw an exception
//if exception is thrown, move to catch
}
catch (InputMismatchException e) {
//Handle the exception case here – do what is
//needed to gracefully recover from the
//exception
}
Exception Parameter
catch (Exception e) {
//Can handle multiple types of exceptions
}
Try Block
Exception
handlers
Uncaught exception – exception for which there is no
exception handler
- exception type doesn’t match any of the catch blocks
ExceptionTest
public class ExceptionTest {
public static void main (String [] args) {
int x = 3;
int y = 0;
throws ArithmeticException
int z = x / y;
System.out.println(z);
}
}
run:
Exception in thread "main" java.lang.ArithmeticException: / by zero
at ExceptionTest.main(ExceptionTest.java:9)
Java Result: 1
ExceptionTest2
public class ExceptionTest2 {
public static void main (String [] args) {
int x = 3;
int y = 0;
int z = 0;
try {
z = x / y;
} catch (ArithmeticException e) {
System.out.println("Exception Caught");
}
System.out.println(z);
}
}
run-single:
Exception Caught
0
With Scanner
import java.util.*;
public class ExceptionTest3 {
public static void main (String [] args) {
Scanner s = new Scanner(System.in);
int x = 0;
int y = 0;
try {
x = s.nextInt();
} catch (InputMismatchException e) {
e.printStackTrace();
}
}
}
Use Java API
JAVA API lists the exceptions thrown by each method
Termination Model of Exception Handling
try {
}
code line 1
code line 2
code line 3
catch (Exception e) {
handler 1
handler 2
}
code line 4
code line 5
if line 2 throws exception,
control transfers to exception
handler
then to line of code after
handlers.
Objectville
 All exception classes directly or indirectly extend
java.lang.Exception.
 It is possible to create your own exception classes by
extending java.lang.Exception.
Objectville
 Unchecked Exceptions
 Subclass of RuntimeException
 Not required to be handled
 Checked Exceptions
 Subclass of Exception
 Must be handled or explicitly
or declared w/ throws
throws
public int myFunction () throws MySpecialException {
//Code here that could throw a MySpecialException
//but you don’t want to handle it here for some reason
//so you have to declare that this method could also throw
//MySpecialException – a checked exception
}