Lecture 21 1

CS110 Lecture 21Thursday, 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

Lecture 21 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

Lecture 21 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

Lecture 21 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

InsufficientFundsException thrown here (line 144)

caught in catch (line 204) after try (lines 161-203)

Lecture 21 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

Lecture 21 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

Lecture 21 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

Lecture 21 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

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Keyword throws – lawyers at workprivate 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

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Keyword throws – lawyers at workpublic 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

Lecture 21 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 …

Lecture 21 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.

Lecture 21 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

Lecture 21 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

Lecture 21 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

}

Lecture 21 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

Lecture 21 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

JunoExceptions may be thrown here

caught in catch (line 79 or 83) after try (lines 73-75)

Lecture 21 18

Keyword throws – lawyers at workpublic 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

Lecture 21 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

Lecture 21 20

Java RumpelStiltskin• Design (pseudocode)

If there is no command line argumentprint usage messageend the program

• Two possible implementation strategies– test for args[0], proceed based on test result– assume args[0] is there, catch Exception if not

Lecture 21 21

Test first strategyif (args.length == 0 ) {

System.out.println(

"usage: java RumpelStiltskin guess");

System.exit(0); // leave program gracefully

}

// continue normal processing

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Exception strategytry {

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

}

Lecture 21 23

java RumpelStiltskin foosorry - foo is not my name

Intentionally generate a NullPointerException,see what the Exception's toString method returnsjava.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)

Lecture 21 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