Transcript Handling Errors with Exception (in Java)

Handling Errors with Exception (in
Java)
Project 10
CSC 420
What’s an exception?
“Exception” means exceptional event that occurs during the
execution of a program that disrupts the normal flow of
instructions
Many kinds of errors cause exception; hardware errors, hard disk
crash, or simple programming error
When such errors occurs in Java, the method creates an
exception object then hands it off to the runtime system; is called
throwing an exception in Java
The runtime system then finds a way to handle the
exception or finds a method that contains an
appropriate exception handler
Cont’d..
If the runtime system fails to find an appropriate
exception handler, then the program and the runtime
system terminates
Java has 3 advantages by using exceptions to manage
errors
-separating error handling code from regular code
-propagating errors in the call stack
-grouping error types and error differentiation
How do Java deal with it?
Catch method catches an exception by providing
an exception handler
Specify is a method is the ability to throw an
exception if chose not to catch that exception
Checked exceptions are not runtime exceptions
and are checked by the compiler
How to write exception handlers?
Try block is the first step In building an exception
where you enclose that might throw an exception
{ System.out.println(“entering try statement"); out = new
PrintWriter( new FileWriter(“outFile.txt")); for (int i = 0; i
< size; i++) out.println(“value at: " + i + " = " +
victor.elementAt(i)); }
Cont’d..
Catch block is the association of exception
handlers with a try block by giving one or more
catch blocks after the try block
{
...
} catch ( . . . ) {
...
} catch ( . . . )
Cont’d
The finally block gives a device for cleaning
up the state of the method before allowing
control to a different part of the program
finally { if (out != null) { System.out.println(“closing
PrintWriter"); out.close(); } else {
System.out.println("PrintWriter not open"); } }
Throw statements
All java methods uses throw statements
Requires a single argument, a throwable object
throwable objects are instances of any subclass
of the Throwable in Java system
throw someThrowableObject ;
The compiler refuses to compile a program if you
try to throw an object that is not throwable
Codes used in describing Exceptions
Runtime –error that can occur in any code
Parentclass java.lang.RuntimeException
Error – serious errors such as out of memory
Parentclass java.lang.RuntimeError
Checked – can occur in some part of the code\
Parentclass java.lang. Exception
Web sources
http://java.sun.com/docs/books/tutorial/essential/excepti
ons/index.html
http://www.javaworld.com/javaworld/jw-08-2000/jw-0818exceptions.html
http://mindprod.com/jglossexception.html
http://citeseer.nj.nec.com/context/147839/0
http://www.eecs.umich.edu/gasm/papers/javaexc.html
http://www.isr.uci.edu/~cjensen/info/ExceptionHandler.htm