Handling Errors with Exception (in Java)

1. Handling Errors with Exception (in Java) Project 10 CSC 420

2. 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

3. 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

4. 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

5. 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)); }

6. 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 ( . . . )

7. 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"); } }

8. 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

9. 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

10. Web sources http://java.sun.com/docs/books/tutorial/essential/exceptions/index.html http://www.javaworld.com/javaworld/jw-08-2000/jw-0818-exceptions.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