1	CS110 Lecture 21 Thursday, April 15, 2004 Announcements hw9 due tonight hw9 due Thursday, April 22 exam Tuesday, April 27 Agenda Questions Error handling (continued) in Bank in Juno 7
2	Exceptions Java’s object oriented mechanism for error handling more powerful, more flexible than using return Java keywords try, catch, throw,throws in class: banking system, Juno with Exceptions for hw: improve Exception handling in Juno 7.5 Model in client: instead of testing a returned value try, hoping for the best prepare to pick up the pieces if necessary (catch) in code where the error may occur: detect error create a new Exception and throw it
3	Bank version 7 Suppose a customer tries to withdraw more than is in her account. Eventually BankAccount.java line 143 executes: if (newBalance < 0) { throw new InsufficientFundsException … Read code backward looking for messages (method invocations) to trace methods that are active at that moment in order to see where that Exception is caught
4	Who calls whom? method class line incrementBalance BA.java 144 withdraw BA.java 77 processTrans… Bank.java 173 visit Bank.java 96 main Bank.java 450
5	Method invocation stack At any moment while a program is running you can trace the sequence of active methods from the currently executing statement back to main() That sequence is the method invocation stack It’s called the call stack in C - often in Java too (because it’s easier to say) The call stack is dynamic, changing as the program runs (the program itself is static - fixed at compile time) There will be a call stack question on the exam
6	Stack The call stack grows each time a message invokes a method shrinks each time a method returns main() is always the first thing pushed on to the stack and the last to pop off: when main is done the program is done In CS a stack is a last in first out collection push adds an item to the stack pop removes one The call stack push a method when it’s invoked pop a method when it returns
7	Exceptions and the stack When error detected (BA.java line 143): if (newBalance < 0) throw new InsufficientFundsException … Normal flow control stops - JVM looks for the nearest catch, which may be in the running method somewhere up the call stack
8	Going back through the stack incrementBalance throws an InsufficientFundsException and does not catch it (no try block here) The incrementBalance message was sent from BankAccount withdraw method, which doesn’t catch the Exception either (no try block) The withdraw message was sent from Bank processTransactionsForAccount method – inside a try block. So control transfers to the matching catch block, which handles the Exception
9	Keyword throws – lawyers at work private final void incrementBalance( int amount ) throws InsufficientFundsException { if ( . . . ) { throw new InsufficientFundsException(); } } Since incrementBalance might throw an InsufficientFundsException and (if it does) it does not catch it, it must declare its intention to throw it on up the stack to its caller by asserting throws InsufficientFUndsException throws means might throw, not does throw
10	Keyword throws – lawyers at work public int withdraw( int amount ) throws InsufficientFundsException { incrementBalance( -amount ); } Since withdraw might see an InsufficientFundsException thrown by incrementBalance it must either catch it // it doesn’t or declare its intention to throw it on up the stack to its caller by asserting throws InsufficientFUndsException
11	Exceptions in the Java API No throws cause needed for these, but you can catch them if you suspect one may be thrown NoSuchElementException thrown for you by JVM when you try to ask for a element after an Iterator is done NullPointerException thrown for you by JVM when you try to send a message to an Object that does not exist ClassCastException thrown for you by JVM when you try to cast an Object to a type it isn’t an instance of ArrayIndexOutOfBoundsException …
12	Error handling in Juno 7 JunoExceptions are caught in the try/catch in CLIShell interpret lines 73-89 Note how different kinds of Exceptions are dealt with in order ExitShellException is thrown (only) by doIt in LogoutCommand class BadShellCommandException knows what command was bad so we can give help Catch a generic JunoException and print its message Then deal with truly unexpected errors … catch any Exception. At least Juno won’t crash.
13	Error detection in Juno Errors detected in the various doIt methods doIt declaration in abstract ShellCommand class says that it throws JunoException Example: TypeCommand.java, where we have written all the error detection
14	mars:> type # no filename try { filename = args.nextToken(); } catch (NoSuchElementException e) { throw new BadShellCommandException( this ); } Catch JVM’s Exception and throw one of your own that makes more sense in this application
15	mars:> type foo try { // retrieve foo from current directory // cast to TextFile and print contents } catch (NullPointerException e) { // throw a JunoException } catch (ClassCastException e) { // throw a JunoException }
16	Method invocation stack At any moment while a program is running you can trace the sequence of active methods from the currently executing statement back to main() That sequence is the method invocation stack It’s called the call stack in C - often in Java too When a method throws an Exception the JVM looks for a surrounding try block so it can resume execution in the corresponding catch The search begins locally and works its way back through the call stack
17	Call stack method class next call on line doIt TypeCommand interpret Shell 74 CLIShell Shell 50 constructor Shell 41 interpret LoginInterpreter 89 CLILogin LoginInterpreter 60 constructor Juno 66 main Juno 190
18	Keyword throws – lawyers at work public void doIt( ... ) throws JunoException { ... } Since doIt might throw a JunoException it must catch it // it doesn’t or declare its intention to throw it on up the stack to its caller by asserting throws JunoException throws means might throw, not does throw Built in Exceptions don’t need a throws clause If uncaught they crash the program
19	RumpelStiltskin Exception examples in a short standalone program The fairy tale: guess my name examples/RumpelStiltskin.java > Java RumpelStiltskin > Java RumpelStiltskin foo > Java RumpelStiltskin foo bar > Java RumpelStiltskin RumpelStiltskin
20	Java RumpelStiltskin Design (pseudocode) If there is no command line argument print usage message end the program Two possible implementation strategies test for args[0], proceed based on test result assume args[0] is there, catch Exception if not
21	Test first strategy if (args.length == 0 ) { System.out.println( "usage: java RumpelStiltskin guess"); System.exit(0); // leave program gracefully } // continue normal processing
22	Exception strategy try { System.out.println(”Are you " + args[0] +'?'); rumpelstiltskin.guessName(args[0]); System.out.println("Yes! How did you guess?"); System.exit(0); // leave program gracefully } // come here right away if there is no args[0] catch (IndexOutOfBoundsException e) { System.out.println( "usage: java RumpelStiltskin guess"); System.exit(0); // leave program gracefully }
23	java RumpelStiltskin foo sorry - foo is not my name Intentionally generate a NullPointerException, see what the Exception's toString method returns java.lang.NullPointerException Experiment with the printStackTrace() method: BadGuessException at java.lang.Throwable.<init>(Compiled Code) at java.lang.Exception.<init>(Compiled Code) at BadGuessException.<init>(Compiled Code) at Wizard.guessName(Compiled Code) at Wizard.makeMischief(Compiled Code) at RumpelStiltskin.main(Compiled Code) Look for a second command line argument, see what happens if it's not there: java.lang.ArrayIndexOutOfBoundsException: 1 at RumpelStiltskin.main(Compiled Code)
24	hw10 Add error handling to register and login Error detection for all shell commands Count fraction of Juno that’s there for error handling