unit 6 - exception handling

7/23/2019 Unit 6 - Exception Handling

http://slidepdf.com/reader/full/unit-6-exception-handling 1/40

Basic JavaCopyright © 2008 Pratian Technologies

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Unit 6 – !ception "an#ling

Pratian Technologies $%n#ia& Pvt. 't#.www.pratian.com

Basic Java





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Topics

Ø rror "an#ling in Conventional 'ang(agesØ !ceptionsØ !ception "an#ling in Java

Ø !ception "ierarchyØ Throwa)le ClassØ rror an# !ceptionØ Types o* !ceptionsØ Throwing !ceptionsØ

+eclaring !ceptions per ,etho#Ø Catching !ceptionØ +esigning (ser-#e*ine# !ception classes

Ø ,etho# verri#ing an# !ceptions




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rror Con#ition§ (r applications sho(l# )e )(ilt to s(rvive /error

con#itions.

§ The program 1 mo#(le m(st han#le the error sit(ation

grace*(lly an# not (st terminate a)r(ptly

3hat /error are we tal4ing a)o(t 5




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rror Con#ition§ n error is a con#ition #(e to which a program cannot

normally contin(e e!ec(tion an# nee#s to )e a)r(ptly

terminate#.




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rror han#ling in tra#itional lang(ages§ Tra#itional error han#ling metho#s incl(#e

§ Boolean *(nctions $which ret(rn T7U1'9&.

§ %nteger *(nctions $ret(rns –: on error&.§ n# other ret(rn arg(ments an# special val(es.

int main () {

int res;

if (can_fail () == -1) { cout << "Something failed!" << endl;

return 1;

}

if(div(1res) == -1) {

cout << "ivision #$ %ero!" << endl; return &;

}

return ;

}

Can %

(n#erstan#

what wentwrong with

(st the

ret(rn

val(e5

m % *orce#

to receive

theret(rning

val(e *rom a

*(nction5

%s error

han#ling

#one in aseries o* i*

then )loc45

%s my co#emaintaina)l

e5




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Pro)lems with tra#itional approach

Caller is not *orce# to

receive the val(e

ret(rne# )y the *(nction

(nction can ret(rn only

one val(e ; not #etails

o* the error con#ition ,(ltiple i*-else lea#s tocl(ttere# co#e ; is

#i**ic(lt to maintain


http://slidepdf.com/reader/full/unit-6-exception-handling 7/40Basic JavaCopyright © 2008 Pratian Technologies

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!ception han#ling in Java§ Javas s(pport *or error han#ling is #one thro(gh

!ceptions.

§ 'hat is an ce*tion+§ n exception is an event that occ(rs #(ring the e!ec(tion

o* a program that #isr(pts the normal *low o* instr(ctions.

§ %n the wa4e o* s(ch an event< the J=, creates an

e!ception o)ect< that contains in*ormation a)o(t the error<

incl(#ing its type an# state o* the program when the error

occ(rre#.

§ Creating an e!ception o)ect an# noti*ying the caller o*

the metho# is calle# throwing an exception.



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!ception han#ling mechanism

,ain

,etho# where

error occ(rre#

,etho# Call

Throws

e!ception

'oo4ing *or

appropriate han#ler

9ample 'isting> ivisionrroremo,ava

http://c/Users/Radhika/Desktop/Examples/DivisionErroDemo.java

http://c/Users/Radhika/Desktop/Examples/DivisionErroDemo.java



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3hy (se e!ceptions5§ !ception han#ling provi#es the programmer with

many a#vantages over tra#itional error-han#ling

techni?(es>

Can %

(n#erstan#what went

wrong an#

where when

an error is

thrown5

m % *orce#

to han#lee!ceptions5

%s error

han#ling#one in a

series o* i*

then )loc45

%s my co#e

maintaina)l

e5

%s it easy to

(n#erstan#

how to

han#le

e!ceptions5



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To s(mmari@e a#vantages o* !ception

han#ling§ !ception han#ling provi#es the programmer with

many a#vantages over tra#itional error-han#ling

techni?(es>

§ %t provi#es a means to separate error-han#ling co#e *rom

*(nctioning program co#e.

§ %t provi#es a mechanism *or propagating errors (p the

metho# call stac4< meaning that metho#s higher (p the call

chain can )e allowe# to han#le pro)lems originating lower

in the chain.

§ %t provi#es a way to organi@e an# #i**erentiate )etween

#i**erent types o* a)normal con#itions.

§ Both< application logic an# pro)lem han#ling logic< )ecome

simpler< cleaner an# clearer.



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Throwa)le class§ 3hen an error occ(rs in a metho# the caller o* the metho# is noti*ie#

)y throwing an e!ception o)ect< this o)ect sho(l# )e o* the typeclass Throwable.

§ Throwable has two s()classes< Error an# Exception.

.#ect

/hro0a#le

ce*tionrror

untimece*tion



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rrors

§ rrors are e!ceptional con#itions that are e!ternal to theapplication< an# that the application cannot anticipate or recover

*rom< there*ore rrors are not handled #$ the a**lication.§ or e!ample<

§ n application has to open a *ile an# write to it< #(e to somehard0are error < the application is (na)le to open the *ile we wo(l#get a ava.io.%rror.



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Types o* !ception

.#ect

/hro0a#le

ce*tionrror

untimece*tion



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Types o* !ception

§ !ceptions are o* 2 types§ 2hec3ed ece*tions

§ 4nchec3ed or runtime ece*tion



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Types o* !ception

§ 2hec3ed ece*tions § These are e!ceptional con#itions that a well written application

sho(l# anticipate an# recover *rom.§ 9ince these e!ceptions are anticipate# an# are recovera)le

con#itions< com*iler forces these e!ceptions to )e han#le#.

e.g.> ileAoto(n#!ception< %!ception< 9'!ception

§ 4nchec3ed or runtime ece*tion§

These are e!ceptional con#itions that are internal to theapplication< an# that the application cannot anticipate or recover*rom.

§ These (s(ally in#icate programming )(gs< s(ch as logical errors orimproper (se o* an P%.

§ 9ince these can )e *i!e# programmatically< com*iler does not

force these e!ceptions to )e han#le#. e.g.> A(llPointer!ception< rray%n#e!(t*Bo(n#s!ception



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"ow #o % han#le e!ceptions5

§ Ao nee# to import any ava pac4age§ Java programming lang(age s(pports< e!ception han#ling

)y provi#ing three e!ception han#ler components< these

are§ try § catch

§ finally

§ The try < catch an# finally )loc4s are (se# to write an

e!ception han#ler.




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try Bloc4

tr$

co#e

DDD..

E

try )loc4

§ ny co#e that might throw an e!ception is enclose#within a try )loc4. %t is the *irst step in constr(cting an

e!ception han#ler.

U it 6 ti " #li




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try Bloc4 !ample

p()lic int co(ntChars$9tring *ileAame&

int total F 0G try

ile7ea#er r F new ile7ea#er$*ileAame&G

while$ r.rea#y$&&

r.rea#$&G totalHHG

E

r.close$&G

E

E

U it 6 ti " #li




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catch Bloc4§ The catch )loc4 contains

co#e that is e!ec(te# i* an#when the e!ception occ(rs.

§ catch )loc4 is an e!ceptionhan#ler associate# with a try )loc4 an# han#les the type o*e!ception in#icate# )y its

arg(ment.

§ very try )loc4 is associate#with zero or more catchblock

try

E

catch$ExceptionType name&

Ecatch$ExceptionType name&

E

Unit 6 ception "an#ling




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catch Bloc4 !ample


int total F 0G try


while$ r.rea#y$&&

r.rea#$&G

totalHHG

E r.close$&G

E

catch$ileAoto(n#!ception e&

9ystem.o(t.println$Iile name# I H *ileAame H Inot *o(n#. I He&G

total F -:G

E catch$%!ception e&

9ystem.o(t.println$Una)le to per*orm %1< please try laterK&G

total F -:G

E

E

Unit 6 !ception "an#ling




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Try Catch Bloc4 !ample

*u#lic static dou#le divide(int a int #)

{

dou#le 5uotient=;

tr${

5uotient=a6#;

} catch (7rithmeticce*tion e){ S$stem,out,*rintln(" ivisor is 8ero");}

return 5uotient;

}





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Try catch Bloc4 !ample*u#lic static void main(String9: args)

{

int a=;

int #=;

tr${

a=nteger,*arsent(args9:);

#=nteger,*arsent(args91:);}

catch (um#erormatce*tion e)

{

S$stem,out,*rintln(" >lease enter an integer onl$ ,,,");

}

dou#le ans=divide(a#);

S$stem,out,*rintln("/he 5uotient is "? ans);

}

9ee listing > @andledivisionrroemo,ava


http://c/Users/Radhika/Desktop/Examples/HandledDivisionErroDemo.java

http://c/Users/Radhika/Desktop/Examples/HandledDivisionErroDemo.java




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!ercise§ 3rite some co#e that is capa)le o* generating e!ceptions

li4e rithmetic!ception< A(m)erormat!ception<

rray%n#e!(t*Bo(n#s!ception etc

3rite an e!ception han#ler< that is< enclose these line o*

co#e in try )loc4 an# write appropriate catch )loc4s to catch

#i**erent e!ception types.





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!ception "an#ler § %t is possi)le to write a generic catch )loc4 to catch any

type o* e!ception that might occ(r


int total F 0G

try


while$ int c F r.rea#$& LF -:&

totalHHGr.close$&G

E



E

catch$%!ception e& 9ystem.o(t.println$Una)le to per*orm %1< please try laterK&G

E

catch$!ception e&

9ystem.o(t.println$Una)le to contin(e processing< #(e to

some internal errorK&G

E

3henever an

!ception other than

ileAoto(n# or

%!ception occ(r<

this catch )loc4 isinvo4e#

9(ch a generic catch

)loc4 sho(l# )e

incl(#e# at the en#<

a*ter the speci*ic

catch )loc4s





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Clean (p co#e


int total F 0G try


while$ r.rea#y$&&

r.rea#$&G

totalHHG

E r.close$&G

E



total F -:G

r.close$&G E

catch$%!ception e&

9ystem.o(t.println$Una)le to per*orm %1< please try laterK&G

total F -:G

r.close$&G

E

+eveloper nee#s to

per*orm clean (p

that is< release the

*ile that was opene#

programmatically





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*inally Bloc4§ finally )loc4 is e!ec(te# irrespective o* whether the try )loc4

throws an error or not.§ finally )loc4 is g(arantee# to )e e!ec(te# an# can )e (se# *or any

clean-(p co#e.§ %* no catch )loc4 matches an e!ception thrown< control is trans*erre#

to the finally )loc4.§ There can )e only one finally )loc4 *or a try )loc4.

try

E

catch$ExceptionType name&

E

finall$

E





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*inally Bloc4 !ample*u#lic int count2hars(String fileame) {

int total = ;

ileeader r = ne0 ileeader(fileame);

tr$ { 0hile( r,read$()) {

r,read();

total??;

}

}

catch(ileotoundce*tion e){ S$stem,out,*rintln("ile named " ? fileame ? "not

found, " ? e);

total = -1;

}

catch(.ce*tion e){

S$stem,out,*rintln(".ce*tion occured "? "0hile

counting " ? e);

total = -1;

}

finall$ {

r,close(); }

Clean (p co#egoes into *inally

)loc4





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p g

System-Defined Exception





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p g

System-Defined Exception§ IndexOutOfBoundsException :

§ When beyond the bound of index in the object which use index, suchas array, string, and vector

§ ArrayStoreException :§ When assign object of incorrect type to ee!ent of array

§ "egativeArraySi#eException :§ When using a negative si#e of array

§ "u$ointerException : § When refer to object as a nu pointer

§ SecurityException :§ When vioate security% &aused by security !anager

§ Iega'onitorStateException :§ When the thread which is not owner of !onitor invoves wait or notify

!ethod





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p g

Throwing an !ception§ ll metho#s (se the throw statement to throw an e!ception.§ The throw statement re?(ires a throwa)le o)ect as arg(ment.

throwa)le o)ect are instances o* any s()class o* the Throwable class.

§ throw ca(ses the metho# to terminate an# ret(rns an e!ceptiono)ect to the caller.

*u#lic static dou#le divide(int a int #)

{dou#le 5uotient=;

tr${ if (#==)

{ thro0 ne0 7rithmeticce*tion();

}

5uotient=a6#;

}

catch (7rithmeticce*tion e)

{ S$stem,out,*rintln(e,getAessage()?"Bn ivisor is 8ero");

}

return 5uotient;

}

.9ee listing > /hro0ivisionrroemo,ava


http://c/Users/Radhika/Desktop/Examples/ThrowDivisionErroDemo.java

http://c/Users/Radhika/Desktop/Examples/ThrowDivisionErroDemo.java




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Propagation o* (nca(ght e!ception.





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§ metho# can speci*y to throw an e!ception )ya##ing a throws cla(se to the metho# #eclaration.

§ The throws cla(se comprises the throws 4eywor#*ollowe# )y a comma-separate# list o* all thee!ceptions thrown )y that metho#.

§ The cla(se goes a*ter the metho# name an#arg(ment list an# )e*ore the )race that #e*ines thescope o* the metho#.

p()lic voi# write'ist$& thro0s ileotoundce*tion

.ce*tion

DDDDDDD

E

The /throws cla(se





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p()lic class Propagating!ceptionTest

voi# metho#:$& throws %!ception

D..

metho#2$&G

DD

E

voi# metho#2$& throws %!ception

DD

throw new %!ception$&G DD.

E

p()lic static voi# main$9tringNO args& throws %!ception

Propagation o* (nca(ght e!ception.





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!ception han#ling mechanism

+ata)ase

B(siness 'ogic

Control

U%

retrieveUser%n*o$& throws !ception

retrieves+ata$&

E

E

(ser'oginorm$&

a(thenticateUser%nControl$&GE

Throws

e!ception

a(thenticateUser$& throws !ception

retrieveUser%n*o$&

E

a(thenticateUser%nControl$&

a(thenticateUser$&G

E

"an#les

!ception with

try1catch

9ee listing > /hro0semo,ava


http://c/Users/Radhika/Desktop/Examples/ThrowsDemo.java






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9ome Use*(l ,etho#s§ The Throwa)le class has some (se*(l metho#s that can

)e calle# on any !ception type§ 9tring get,essage$&

§ 7et(rns the message that was set while the e!ception o)ect was

create#< )y calling the parameteri@e# constr(ctor ce*tion(String message)

§ 9tring to9tring§ 7et(rns a 9tring representation o* the e!ception o)ect

§ voi# print9tac4Trace$&





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9tac4Trace

• *rintStac3/race() metho# helps to trace the origin o* the

!ception





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User #e*ine# !ceptions§ %* an e!ception cannot )e

represente# )y those in

the Java plat*orm< a (sercan #e*ine his own

e!ception.

§ The (ser #e*ine#e!ception class sho(l#

)e a s()class o*

!ception or any o* its

s() types.

*u#lic class

ProductNotFoundException

etends ce*tion

E

p()lic Pro#(ct getPro#(ct$int

pro#%#& throws

ProductNotFoundException

DDD

DDD

E


/




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/Throwing !ceptions§ Use the /throw cla(se to throw the (ser #e*ine# e!ception o)ect

§ The throw statement re?(ires a throwa)le o)ect as arg(ment.Throwa)le o)ects are instances o* any s()class o* the Throwable class.

§ throw ca(ses the metho# to terminate an# ret(rns an e!ceptiono)ect to the caller.

p()lic Pro#(ct getPro#(ct$int pro#%#& throwsPro#(ctAoto(n#!ception

11 7etrieve pro#(ct in*o *rom the #ata)ase

11 select Q *rom Pro#(ct where pro#Ri# F pro#%#

i*$L*o(n#&

thro0 ne0

>roductotoundce*tion();

ret(rn pro#(ctG

E


U # *i # ti




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User #e*ine# !ceptions

!ception

ge!ception

(t*ge'imit!ception %llegalgeormat!ception

TooSo(ng!ception Tool#!ception Aegativege!ception

9ee listing > 4serefinedce*tionsemo,ava


ti ti

http://c/Users/Radhika/Desktop/Examples/UserDefinedExceptionsDemo.java

http://c/Users/Radhika/Desktop/Examples/UserDefinedExceptionsDemo.java



Please try to limit the questions to the topics discussed during the session. Thank you.

(estion time

unit 6 - exception handling

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