Exceptions Handling the unexpected

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Checked and Unchecked draw drawOne draw drawOne 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 DCS – SWC

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

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

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

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

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

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

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

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

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

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