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Exceptions Handling the unexpected

Exceptions Handling the unexpected. Motivation. So far, most of our code has been somewhat n ä ive We have assumed that nothing goes wrong… User enters correct input We never address outside the boundaries of an array …and so on. Motivation. Of course, the real world works differently

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Exceptions Handling the unexpected

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  1. ExceptionsHandling the unexpected

  2. Motivation • So far, most of our code has been somewhat näive • We have assumed that nothing goes wrong… • User enters correct input • We never address outside the boundaries of an array • …and so on DCS – SWC

  3. Motivation • Of course, the real world works differently public class BankAccount { public void withdraw(double amount) { if (amount > balance) // Now what? } ... } DCS – SWC

  4. Motivation • Possible actions depend on our interpre-tation of this situation • Legal – just do business logic • Illegal, and we know what to do – perhaps just do nothing • Illegal, and we do not know what to do! • Error detection and error handling are often separated in code! DCS – SWC

  5. Motivation • Problem: Can become very complex to ”drag” error handling code around in code for business logic • Error-handling may be application-specific • GUI-application: Pop-up window • Other: Write to a log file • Business logic code should not choose strategy for error handling! DCS – SWC

  6. Motivation • Next problem: Error handling code is often very ”far away” from error detction askUser doAction makeWithdraw withdraw main ? Error detected Error handling DCS – SWC

  7. Motivation • Management of errors can be broken down into several tasks: • Detection – realising an error situation has occurred • Signaling – making the surrounding code aware that an error has been detected • Capturing – taking responsibility for handling the error • Handling – performing the error handling actions DCS – SWC

  8. Exceptions • The mechanism for crossing the gap of method calls is exceptions • An exception in itself is ”just another class” • We can create exception objects just as we can create other objects • An exception object contains information about the type of error which occurred • Java contains several built-in exception classes, forming an inheritance hierarchy DCS – SWC

  9. Throwing and catching • Exception can be thrown and catched • What does that mean!? • A very different flow than usual method calls • An exception is thrown up through the chain of method calls DCS – SWC

  10. Throwing and catching askUser doAction makeWithdraw withdraw main Who will catch the exception? Error detected – throw! DCS – SWC

  11. Throwing and catching • A throw can look like this in Java: public void withdraw(double amount) { if (amount > balance) { IllegalArgumentException ex = new IllegalArgumentException(”...”); throw ex; } balance = balance – amount; } NOTE! DCS – SWC

  12. Throwing and catching • An exception is now thrown; this changes the flow of code immediately! • Remaining code in the method throwing the exception is not executed • Somebody must catch the exception • In order to catch the exception, we must write an exception handler DCS – SWC

  13. Throwing and catching • General exception handler structure: try { // Code which may throw an exception ... } catch (ExceptionType ex) { // Proper handling of exception ... } DCS – SWC

  14. Throwing and catching try { myAccount.withdraw(1000); myAccount.getTransactions(); ... } catch (IllegalArgumentException ex) { System.out.println(ex.getMessage()); ex.printStackTrace(); } DCS – SWC

  15. Throwing and catching • Things to note: • Error detection (throw) and error handling (try/catch) is usually not in the same method • The catch statement only catches exceptions of the specified type • Information about the error is found implicitly – by the type of the exception – and explicitly from e.g the text stored in the object DCS – SWC

  16. Throwing and catching • Throw early, catch late! • If you cannot fix a pro-blem correctly, throw an exception • Only catch an exception if you really know how to fix the problem DCS – SWC

  17. Throwing and catching try { myAccount.withdraw(1000); myAccount.getTransactions(); ... } catch (Exception ex) { // do nothing... } Tempting, but bad…! DCS – SWC

  18. Checked and Unchecked • How do I know what exceptions some piece of code can throw…? • Difficult to code a method correctly without this knowledge • Two types of exceptions exist • Checked exception • Unchecked exception DCS – SWC

  19. Checked and Unchecked • Checked exception • Used for problems beyond the control of the programmer • Corrupted file, network problems, etc.. • Compiler insists that you explicitly decide what to do about it • Option 1: Re-throw the exception • Option 2: Handle the exception, using a catch clause matching the exception DCS – SWC

  20. Checked and Unchecked // Suppose draw() can throw DrawException draw(Figure f); // Compiler will not like this! public void drawOne(Figure f) { getScreen().draw(f); } DCS – SWC

  21. Checked and Unchecked // Option 1: Re-throw the exception // (i.e. do nothing…) public void drawOne(Figure f) throws DrawException { getScreen().draw(f); } DCS – SWC

  22. Checked and Unchecked // Option 2: Handle the exception public void drawOne(Figure f) { try { getScreen().draw(f); } catch (DrawException de) { // Code for handling the problem } } DCS – SWC

  23. drawOne draw Option 1: drawOne does nothing, so it must annonce that it will (re)throw the exception draw drawOne Option 2: drawOne handles the exception, so it is ”consumed” by drawOne Checked and Unchecked DCS – SWC

  24. Checked and Unchecked • Unchecked exception • Used for problems which the programmer should be able to prevent • Null reference, out of bounds reference,… • Why do we have these…? Accidents do happen…! • Unchecked exceptions are not announced DCS – SWC

  25. The finally Clause • Sometimes we need to execute some specific code after an exception occurs • Typically ”clean-up” code – close a file connection, a database connection, etc. • Where do we put this code…? • In exception handlers? Difficult, who actually catches the exception… • In a finally clause! DCS – SWC

  26. The finally Clause PrintWriter out = new PrintWriter(filename); ... try { writeData(out); } finally { // This code will always be executed, // even if the above code throws an exception out.close(); } DCS – SWC

  27. The finally Clause • The code in the finally clause is guaran-teed to be executed, in one of these ways: • If no exceptions are thrown: After completing the last statement in the try block • If an exception is thrown: • Execute code in finally clause • Exit to exception handler DCS – SWC

  28. The finally Clause try { try { } finally { } } catch { } try { } catch { } finally { } NOT SO GOOD GOOD DCS – SWC

  29. Making your own exceptions • Throw exceptions that are as specific as possible – also in terms of type • Many built-in exceptions to choose from • Can be appropriate to create your own exceptions • Just extend existing class DCS – SWC

  30. Making your own exceptions public class InsufficientFundsException extends RunTimeException { public InsufficientFundsException() {} public InsufficientFundsException(String message) { super(message); } } DCS – SWC

  31. Exceptions vs. Flow control • Exceptions change the linear flow of code, just like if, while, etc. • However, they are only intended for error detection and handling • Do not use exceptions as a substitute for ordinary flow control DCS – SWC

  32. Exceptions vs. Flow control GOOD for (int i = 0; i < noOfElements; i++) myArray[i] = i; try { for (int i = 0; /* No Test?? */ ; i++) myArray[i] = i; } catch (ArrayIndexOutOfBoundsException e) {} BAD DCS – SWC

  33. Exceptions summary • Throw early, catch late • Only catch, if you can handle the problem correctly • You must deal with checked exceptions • Use try, catch and finally appropriately • Make your own exception classes, if you really need them • Exceptions are not for flow control DCS – SWC

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