1. Exception Handling Exceptions and language features to handle them

2. Exception  unusual condition that occurs during execution of a program may be error or not may be hardware or software based detected by  hardware(end-of-file)  OS(file-not-found)  PL(division-by-zero)  program(test for zero denominator)

3. Exception handling  response to exception being detected terminate program ignore exception (temporarily) ‘handle’ exception within program – special code to execute

4. Programmed exception handling  who handles the exception? procedure executing when exception is raised calling procedure  provides instructions to called procedure by passing procedure parameter or reference  receives notification by return value from called procedure  special exception communication features in the language

5. design of exception handling  possible functions defining exceptions detecting and identifying exceptions propagating exceptions intercepting exceptions binding exceptions to handlers resolving exceptions

6. design of exception handling  features complete structure (eg Pascal procedure, java class) or code segment? one general handler or many specific handlers? program continuation options programmer control – defining & detecting exceptions, defining handlers?

7. use of exception handling features  simplify code – remove error handling ‘clutter’  focus programmer attention on program correctness  give more program control in response to errors  use exceptions as legitimate design tool – not just for errors

8. open questions  should hardware errors be part of exception handling process?  should there be a default exception handling system?  should programmers have power to disable exceptions?

9. PL/I  system pre-defined exceptions e.g. ZERODIVIDE, ENDFILE(source)  system default handlers – SYSTEM  to use defaults: ON SUBSCRIPTRANGE SYSTEM; /* redundant */... (SUBSCRIPTRANGE): X(I) = Y(I+1) * 2;  some exceptions always on, some default on, some default off

10. PL/I programmer can:  create new exceptions CONDITION BADINPUT; /* declaration */  raise (throw) the exception: SIGNAL (X > 100) BADINPUT;  trap and handle the exception like a system exception

11. PL/I to handle programmer exceptions OR to override SYSTEM handlers  exception handlers are code segments execute in environment where coded ON BEGIN; END;  scope of control to end of block to next handler with same exception

12. PL/I  e.g. user defined exception  declare, handle and throw CONDITION BADINPUT ON BADINPUT BEGIN; END;... SIGNAL (X>100) BADINPUT;

13. PL/I exception handling problems  dynamic binding of handlers to exceptions is flexible but can be misleading: wrong handler can respond to raised exception e.g. – several handlers for SUBSCRIPTRANGE associated with different arrays  complex continuation rules (what to do after handler executes)

14. Ada  handlers are code segments in procedure or block scope, at the end of the block  static binding of exception to handler  after handler executes, procedure or block terminates, returns control to caller  unhandled exceptions propegated through dynamic links until handler found  if no handler found, task or program terminates

15. C++  try – throw – catch structure like java  system exceptions and programmer exceptions are distinct – programmer can’t handle system exceptions  anything can be thrown as exception (primitive or object) – no predefined exception classes  caught exceptions can be rethrown to caller

16. Java  try – throw – catch structure with a finally block option  exceptions are legitimate objects in the Throwable hierarchy some ‘handling’ can occur in the exception object constructor as well as the catch  continuation: catch handlers can rethrow exceptions or throw different ones hander can use return to terminate method otherwise execution continues after try/catch block

17. Java Exception classes Object  Throwable Error Exception  RuntimeException

18. class Throwable  only objects that can be thrown by jvm or by program  contains snapshot of execution stack for this thread string message (maybe) cause (chain)- reference to another throwable (trace history when one exception is caught and another is thrown) - recursive

19. class Error extends Throwable  constructors with / without message with / without cause  methods all inherited from Throwable  Errors should not be caught - serious abnormal conditions that should cause termination (“unchecked”)

20. class Exception extends Throwable  constructors with / without message with / without cause  methods all inherited from Throwable  Exceptions must be caught* - abnormal but manageable conditions that should be handled by program (“checked”) * except RuntimeExceptions, next slide

21. class RuntimeException extends Exception  constructors with / without message with / without cause  methods all inherited from Throwable  RuntimeExceptions do not have to be caught - errors that should not occur (“unchecked”)

22. Java – exception classes  unchecked exceptions ( Error, RuntimeException and subclasses) assumed throwable and propagatable from any class  checked exceptions (Exception) complier verifies that any exceptions a method throws are either caught or declared in header  which and why?? the official wordthe official word

23. extending Error AnnotationFormatError, AssertionError, AWTError, CoderMalfunctionError, FactoryConfigurationError, FactoryConfigurationError, IOError, LinkageError, ServiceConfigurationError, ThreadDeath, TransformerFactoryConfigurationError, VirtualMachineError

24. extending Exception - catch these AclNotFoundException, ActivationException, AlreadyBoundException, ApplicationException, AWTException, BackingStoreException, BadAttributeValueExpException, BadBinaryOpValueExpException, BadLocationException, BadStringOperationException, BrokenBarrierException, CertificateException, ClassNotFoundException, CloneNotSupportedException, DataFormatException, DatatypeConfigurationException, DestroyFailedException, ExecutionException, ExpandVetoException, FontFormatException, GeneralSecurityException, GSSException, IllegalAccessException, IllegalClassFormatException, InstantiationException, InterruptedException, IntrospectionException, InvalidApplicationException, InvalidMidiDataException, InvalidPreferencesFormatException, InvalidTargetObjectTypeException, InvocationTargetException, IOException, JAXBException, JMException, KeySelectorException, LastOwnerException, LineUnavailableException, MarshalException, MidiUnavailableException, MimeTypeParseException, MimeTypeParseException, NamingException, NoninvertibleTransformException, NoSuchFieldException, NoSuchMethodException, NotBoundException, NotOwnerException, ParseException, ParserConfigurationException, PrinterException, PrintException, PrivilegedActionException, PropertyVetoException, RefreshFailedException, RemarshalException, RuntimeException, SAXException, ScriptException, ServerNotActiveException, SOAPException, SQLException, TimeoutException, TooManyListenersException, TransformerException, TransformException, UnmodifiableClassException, UnsupportedAudioFileException, UnsupportedCallbackException, UnsupportedFlavorException, UnsupportedLookAndFeelException, URIReferenceException, URISyntaxException, UserException, XAException, XMLParseException, XMLSignatureException, XMLStreamException, XPathException

25. extending RuntimeException AnnotationTypeMismatchException, ArithmeticException, ArrayStoreException, BufferOverflowException, BufferUnderflowException, CannotRedoException, CannotUndoException, ClassCastException, CMMException, ConcurrentModificationException, DOMException, EmptyStackException, EnumConstantNotPresentException, EventException, IllegalArgumentException, IllegalMonitorStateException, IllegalPathStateException, IllegalStateException, ImagingOpException, IncompleteAnnotationException, IndexOutOfBoundsException, JMRuntimeException, LSException, MalformedParameterizedTypeException, MirroredTypeException, MirroredTypesException, MissingResourceException, NegativeArraySizeException, NoSuchElementException, NoSuchMechanismException, NullPointerException, ProfileDataException, ProviderException, RasterFormatException, RejectedExecutionException, SecurityException, SystemException, TypeConstraintException, TypeNotPresentException, UndeclaredThrowableException, UnknownAnnotationValueException, UnknownElementException, UnknownTypeException, UnmodifiableSetException, UnsupportedOperationException, WebServiceException

26. syntax of exceptions - Java … preceding code try{ …code may throw Exceptions } catch (MyException mE){ \\ exception handler … exception handling code } catch (MyNextException nE){ … handling by throwing new Exception } … more exception handlers finally{ … must be executed } … following code

27. syntax of exceptions - Java … preceding code try{ …code may throw Exceptions } catch (MyException mE){ … exception handling code } catch (MyNextException nE){ … handling by throwing new Exception } … more exception handlers finally{ … must be executed } … following code control flow with 1.no exception in try 2.exception caught by MyException 3.exception caught by MyNextException 4.exception not caught control flow with 1.no exception in try 2.exception caught by MyException 3.exception caught by MyNextException 4.exception not caught