Exceptions Handling the unexpected

RHS – SWC 2 The Real World 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

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

RHS – SWC 4 The Real World 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!

RHS – SWC 5 The Real World 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!

RHS – SWC 6 The Real World Next problem: Error handling code is often very ”far away” from error detction main askUserdoActionmakeWithdrawwithdraw Error detected Error handling ?

RHS – SWC 7 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

RHS – SWC 8 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

RHS – SWC 9 Throwing and catching main askUserdoActionmakeWithdrawwithdraw Error detected – throw! Who will catch the exception?

RHS – SWC 10 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!

RHS – SWC 11 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

RHS – SWC 12 Throwing and catching General exception handler structure: try { // Code which may throw an exception... } catch (ExceptionType ex) { // Proper handling of exception... }

RHS – SWC 13 Throwing and catching try { myAccount.withdraw(1000); myAccount.getTransactions();... } catch (IllegalArgumentException ex) { System.out.println(ex.getMessage()); ex.printStackTrace(); }

RHS – SWC 14 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

RHS – SWC 15 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

RHS – SWC 16 Throwing and catching try { myAccount.withdraw(1000); myAccount.getTransactions();... } catch (Exception ex) { // do nothing... } Tempting, but bad…!

RHS – SWC 17 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!

RHS – SWC 18 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(); }

RHS – SWC 19 The finally Clause The code in the finally clause is guaran- teed to be executed, in one of these ways: –After completing the last statement in the try block –After completing the last statement in a catch clause, if an exception is caught –When an exception was throw in the try block and not caught

RHS – SWC 20 The finally Clause try { } catch { } finally { } try { try { } finally { } catch { } GOOD NOT SO GOOD

RHS – SWC 21 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

RHS – SWC 22 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

RHS – SWC 23 Checked and Unchecked // Suppose draw() can throw DrawException draw(Figure f); // Compiler will not like this! public void drawOne(Figure f) { getScreen().draw(f); }

RHS – SWC 24 Checked and Unchecked // Option 1: Re-throw the exception // (i.e. do nothing…) public void drawOne(Figure f) throws DrawException { getScreen().draw(f); }

RHS – SWC 25 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 }

RHS – SWC 26 Checked and Unchecked drawdrawOne 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

RHS – SWC 27 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

RHS – SWC 28 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

RHS – SWC 29 Making your own exceptions public class InsufficientFundsException extends RunTimeException { public InsufficientFundsException() {} public InsufficientFundsException(String message) { super(message); }

RHS – SWC 30 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

RHS – SWC 31 Exceptions vs. Flow control for (int i = 0; i < noOfElements; i++) myArray[i] = i; try { for (int i = 0; /* No Test?? */ ; i++) myArray[i] = i; } catch (ArrayIndexOutOfBoundsException e) {} GOOD BAD

RHS – SWC 32 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