Lecture 20 1

CS110 Lecture 20Tuesday, April 13, 2004

• Announcements– hw9 due Thursday, April 15– exam Tuesday, April 27

• Agenda– Questions– Error handling (JOI Chapter 7 finally!)

Lecture 20 2

Error handlingWrite code so that running program can

– recognize when an error has occurred– tell someone– do something sensible

public void withdraw( int amount ){ if ( amount <= balance ) { incrementBalance( - amount );

} else { // an error; what now … ? }}

Lecture 20 3

Help! What next … ?

• First idea: print a messagepublic void withdraw( int amount )

{ if ( amount <= balance ) { incrementBalance( - amount );

} else { System.out.println(“sorry”); }}

Lecture 20 4

To print or not to print?

• Good first idea – but …– in banking system, i/o uses the ATM, but here atm.println( ) fails because BankAccount object doesn’t know about ATM (and shouldn’t)

• In general, printing is a bad idea

• Better idea: tell someone, not necessarily the user

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Tell your client: return a valuepublic boolean withdraw( int amount ){ if ( amount <= balance ) { incrementBalance( - amount ); return true;

} return false;}

• Client tests:if (! account.withdraw(amount) ) // print if appropriate

// else return failure to your client

• Client knows that something happened, but not what happened

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Use better return value public String withdraw( int amount ){

if ( amount <= balance ){ incrementBalance( - amount ); return “” // or null

} return “insufficient funds”; }

• Client tests return value, has more information– can print, or parse, or return something to whoever

called it

Lecture 20 7

Return an int• Some designs/languages use int return code • C and Unix shell convention:

– return 0 when all goes well– return some n > 0 to report an error

• Design encourages a message catalog:– look up message String using return code as a key (catalog

might be an ArrayList)– provide a catalog in user’s native language, for

internationalization (I18N)• You could invent an Object to return

Lecture 20 8

Two System tricks• If you must print, use System.err rather than System.out

• System.err goes to screen even when System.out is redirected with >

• Terminal class implements errPrintln as well as println

• If you must shut down immediately: if (trueDisaster) System.exit(911);

operating system can check return code from the JVM

Lecture 20 9

When you detect an error, consider

• printing something

• returning something the client can examine

• setting the value of some global (class) variable that a client can look at

• shutting the program down

• throwing an Exception a client can catch

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Error handling is painful• Imagining all possible errors is tricky

– you routinely miss some until code breaks– that’s why you have to test!

• Deciding what to do is often difficult (particularly if original design did not plan for handling errors)

• Error handling code – spreads out over the whole program – can make clean logic complicated and ugly– often accounts for 1/4 to 1/3 of real software

Lecture 20 11

Exceptions• Java’s object oriented mechanism for error handling• more powerful, more flexible than using return • Java keywords try, catch, throw• in class: banking system, JOI with Exceptions• for hw: improve Exception handling in Juno 7

• 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

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Exceptions (client side)• In client code (for example, Bank.java)

try { account.withdraw( amount ));} catch ( InsufficientFundsException e ) { // look at e, take appropriate action}// processing continues

• If all goes well, catch block code never executes• If withdraw method in BankAccount has a problem it

creates a new InsufficientFundsException object and throws it, to be caught here

Lecture 20 13

Exceptions (service provider)

• Thrown where error happens (The idea in BankAccount.java, not real code …):public int withdraw( int amount )

{

if ( amount > balance ) {

throw new InsufficientFundsException();

}

incrementBalance( -amount ); return amount;

}

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Real Bank example• 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 20 15

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 20 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

(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)

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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

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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

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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

Lecture 20 20

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– catch it // it doesn’t– or declare its intention to throw it on up the stack to its caller

by asserting throws InsufficientFUndsException