Lect3 Exceptions

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Software Engineering

2013

Lecture 3:Exception


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2013 FIT330 Software Engineering 2

Outline

Why exception?

Java error handling mechanism

checked & unchecked exceptions

Working with exceptions

esign issues


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Why exception?

Problem: how to design robustprocedures i!e! "ehave in a well#de$ined way when errors

occur

%euirements: not to terminate a"ruptly

provides an approximationo$ the normal "ehaviour

1


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Example: invalid input pro"lem

' "ehaviour is reuired $or invalid input:/*** @effects* if a is sorted in ascending order* returns true

* else returns false*/publicstaticbooleansorted(int[] a) { intprev = a[0];for(inti = 1; i < a.lengt; i!!) {

if(prev


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(wo common solutions

%eturns a )special) value: error is treated part o$ the normal $low

*enerates an special )signal):

error is an exceptional case +separate $rom thenormal $low,

?Which solution is better


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-olution . example

publicstaticbooleansorted(int[] a) {

if(a == null) returnfalse;

if(a.lengt


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-olution .)s limitations

/o valid special values $or total procedures 0alue may "e ignored "y caller

'n additional check is always reuired

%eturned value does not carry error details



-olution 1: Exception

(he special signal is called exception no returned values needed

2aller can "e $orced to handle exceptions

2aller may deligate the handling Exception is a data type that carry error details



-olution 1 example

publicstaticbooleansorted(int[] a)throws#ull$ointer%&ception { if(a == null)

thrownew#ull$ointer%&ception('sorted arra is null');if(a.lengt



Java error handling mechanism

se special )signal)

Execution errors include:

environmental errors: derived $rom %rrorclass

program errors: exceptions

/ew exception types can "e created:

checked or unchecked

4ethods are speci$ied with exceptions

2



Java exception type hierarchy

(hrowa"le

Error Exception

%untimeException +checked exceptions,

+unchecked exceptions,



2hecked vs! unchecked exception

2hecked exceptions: must "e handled

derived $rom %&ception

nchecked exceptions: need not "e handled +automatically handled "y

Java run#time,

used $or code sa$ety

su"types o$ unti+e%&ception



Working with exceptions

etermine the exception type 2reate a new exception type

-peci$y a procedure with exceptions

-peci$y a type with exceptions

(hrow an exception in the code

5andle an exception

3



etermine the exception type

(ypes o$ error: invalid input: e!g! array is null

logic: e!g! division "y 6ero

others: e!g! disk access $ailure

Exception depends on the )expectedness) o$the error:

+highly, expected: use checked exception

+mildly, unexpected: use unchecked exception

%euse exceptions where possi"le



5ow to name an exception?

Exception name 7 type o$ error 8 9Exception

Examples:

array is null ; #ull$ointer%&ception

division "y 6ero ; ,ivide-ero%&ception

disk access $ailure ; ,isccess%&ception



2reate a new exception

2reate a su"type o$ %&ception+i$ checked,or unti+e%&ception+i$ unchecked,


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Example

public class #on$ositive%&ception

extendsunti+e%&ception {

public%+pt%&ception(tring +sg) {

super(+sg);

"

"


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Example: fact

/*** @effects* if n is non2positive3* tro4s #on$ositive%&ception* else* returns te factorial of n

*/publicstaticintfact(intn) throws#on$ositive%&ception


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searc

/*** @requires ais sorted* @effects * if a is null* tro4s #ull$ointer%&ception

* else if & is not in a* tro4s #ot5ound%&ception* else* returns i suc tat a[i] = &.*/publicstaticintsearc(int[] a3 int&)

throws#ull$ointer%&ception3 #ot5ound%&ception


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Example: 6eicle

c7.veicle&.6eicle /ote:

6eiclethrows #ot$ossi8le%&ceptionin

constructor and setters 6eicleimplements 9o+para8leand provides

an implementation $or co+pare:o:

co+pare-#a+e: throws two run#time exceptions


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(hrow an exception

se the tro4statement Example:

/**

* (o+itted)

*/

publicstaticintfact(intn) throws#on$ositive%&ception {

if(n < 0)

thrownew#on$ositive%&ception( 'fact() input is not positive '! n);

returnc.#u+.fact(n);

"


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5andle an exception

-peci$y in tro4sor handle in the code -urrounds the using code with tr!!!catc

(hree techniues applied to the catc"lock:

Logging

4asking

%e$lecting

' com"ination o$ the a"ove techniues is alsocommon


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Example

int n = 10;try{

intf = fact(n);

ste+.out

.println('fact('!n!') '! f);" catch(#on$ositive%&ception e) {

// andle e&ception

"


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Log an exception

%ecord the exception details +e!g! to $ile, Example:

int n = 10;

try{

intf = fact(n);

ste+.out.println('fact('!n!') '! f);

" catch(#on$ositive%&ception e) {

// sould not appen3 log

ste+.err.println('%rror invalid input '!e.getessage());

// +ore details

// e.printtac:race();

"


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4ask an exception

When exceptions are expected as part o$ thelogic

5andles the exception to terminate normally

+i$ reuired, return an exceptional value O$ten used together with logging


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Example

/*** @effects* if n is non2positive* log error and return 21* else* return te factorial of n

*/publicstaticintco+pute5act1(intn) { try{ intf = fact(n); returnf; " catch(#on$ositive%&ception e) {

// log ste+.err.println('%rror invalid input '!

e.getessage()); // +as using 21 return21; "

"


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%e$lect an exception


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%e$lection example

/*** @effects* if n is non2positive* tro4 #ot$ossi8le%&ception* else* return te factorial of n*/publicstaticvoidco+pute5act(intn)

throws#ot$ossi8le%&ception { try{ intf = fact(n);

ste+.out.println('fact('!n!') '! f); " catch(#on$ositive%&ception e) { thrownew#ot$ossi8le%&ception( '9ould not co+pute fact('!n!')'); "

"


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esign issues

4ethod overriding with exceptions %euse exceptions where possi"le

2onsider the context o$ use:

partial procedures may use special values

total procedures need exceptions to de$ine"ehaviour $or all inputs

ont overuse exceptions: excessive exception handling degrades

per$ormance

4


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Overriding: header rule +review,

=nherited methods must "e header compatible 4ethod header includes:

signature: method name num"er and types o$parameters

return type

thrown exceptions +i$ any,

2ompati"ility means:

same signature

return type: same +Jdk @ .!A, or su"type +B7 .!C,

exceptions: +., su"set +1, new su"types or

unchecked


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Example: 6eicle(5

c7.veicle&.[-us39ar] /ote:

-us& 9arconstructors now throw

#ot$ossi8le%&ception -uch exceptions may "e omitted

(hese constructors may throw additional run#timeexceptions

?


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(otal procedure

/** * @effects* if a is null* tro4s #ull$ointer%&ception* else if a is sorted in asc order

* return true* else return false

*/ publicstaticbooleansorted(int[] a)

throws#ull$ointer%&ception


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Overuse example

/*** @requiresa = null

* @effects

*

* ifa is sorted in ascending order* return true

* else

* return false

*/booleansorted(int[] a)


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Which part is overusing?

booleansorted(int[] a) { intprev; try{ prev = a[0]; " catch(nde&?ut?f-ounds%&ception e) { returntrue;

"

for(inti = 1; i < a.lengt; i!!) { if(prev


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uestions?

Lect3 Exceptions

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