Exception handling and throw and throws keyword in java.pptx

exception handling in java

1. ⦿ Th e Exception Handlin g in Jav a i s o n e of the p o wer f ul m e chanism to han d l e t h e ru n time er r ors s o that nor m a l flow of the application can be maintained. ⦿ w e wil l lear n ab o ut Java exceptions, its ty p e an d the di f fe re n ce b etwe e n checked and unchecked exceptions. ⦿ Dictionar y Meaning: Exception i s an abnormal condition.

2. ⦿ Th e core adv a ntage of e xcepti o n han d lin g i s to maintain the norma l flow of the application .

3. ⦿ Su p p o s e the r e ar e 1 state m e n ts i n your progra m an d t h ere occurs a n e xcepti o n a t stateme n t 5 , the re s t of t h e code will not b e e xecut e d i . e. state m e n t 6 to 10 wil l not be ex e cuted . ⦿ I f w e per f orm exception han d ling , the r e s t of t h e state m e n t wil l b e e xecut e d. T h a t i s wh y w e use exc e p t io n han d ling i n Java .

4. ⦿ Checked Exception ⦿ Unchecked Exception ⦿ Error

5. 1 ) Checked Exce p tion T he c l as s es w h i ch d i r e ctly i n h e r i t T hro w abl e c l as s exce pt Runt i meE x cep t i o n an d E r ror a r e k n o w n a s c h ecked exce p t i o ns e. g . I OExce p t i o n , S Q LEx cep t i o n e t c Checked exceptions are checked at compile-time. 2 ) Unch e cke d E x c e p t ion The classes which inherit RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time, but they are checked at runtime. 3 ) Er r or E r ror i s i r r eco v e r able e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.

6. In the above example, 100/0 raises an ArithmeticException which is handled by a try-catch block.

7. ⦿ Except i on ha n dl i ng allo w s us to c ontrol the normal fl o w of the progra m b y us i ng except i on handl i ng i n p r ogra m . ⦿ I t throws a n excepti on when e ve r a ca l l i ng method encounters a n er r or prov i d i ng that the cal l i ng method takes care of that er r or. ⦿ I t als o g i ve s us the scop e of o rgan i z i ng and d i ff e r e nt i at i ng betw e en d i ff e r e nt er r or types us i ng a s e parat e bl o ck of co d es . Th i s i s d o ne w i th t h e help of tr y - catch bl o cks

8. T h e r e ar e giv e n som e sc e na r io s wh e re unc h eck e d e xcepti o ns may o c cu r . T h ey ar e a s follow s : 1 ) A sc e na r i o wh e re Arit h m e ticException occurs I f w e divid e an y n umber b y zero, there occurs an ArithmeticException. int a=50/0;//ArithmeticException

9. 2 ) A sc e na r i o wh e re NullPoi n te r Exception occurs If we have a null value in any variable , perf or m in g an y ope r ation on the variable throws a NullPointerException. Strin g s = nul l ; System.out.println(s.length());//NullPointerEx ception

10. 3) A scenario where NumberFormatException occurs The wron g formatt i ng of an y value may occur NumberFormatException. Supp o s e I have a str i ng var i abl e that has character s , convert i ng th i s var i abl e i nto d i g i t w i l l oc c ur NumberFormatE x cept i o n . Str i ng s= " ab c "; int i = Integer . pa r se I n t ( s);/ / Num b erFormatE x c ep tion

11. 4 ) A sce n ari o wh e re ArrayIndexOutOfBoundsException occurs I f yo u ar e ins e rting an y valu e i n the wrong in d ex, i t woul d re s ult in ArrayIndexOutOfBoundsException as show n below: int a[] = ne w int [ 5 ]; a[1 ]=50 ; // Ar r ayInd e xOut O fBou n dsExc epti on

12. ⦿ The JVM f i rs t l y ch e cks whet h er the except i on i s handled or no t . ⦿ I f e xcept i on i s not handled, JVM pr o v i de s a default exception handler that performs the fol l ow i ng ta s ks: ⦿ Prints out exception description. ⦿ Pr i nts the stac k trace ( H i er a rchy o f m ethods where the exception occurred). ⦿ Causes the program to terminate. ⦿ Bu t i f except i on i s handled b y the application programmer, normal flow of the application is maintained i.e. rest of the co d e i s ex e cuted.

13. ⦿ This small program includes an expression that intentionally causes a divide-by-zero error: cla s s Exc { publ i c stati c v o i d main ( Str i ng arg s [ ] ) { i nt d = ; i nt a = 4 2 / d ; } } W hen the Java run - time sy s tem detect s the attemp t to d i v i de b y ze ro, i t construc t s a new exception object and then throws this exception. Th i s cause s the execut i on of Exc to st o p, becau s e once a n ex c ept i on has bee n thro w n , i t mus t be c aug h t b y a n excep t i o n ha n d l er an d dea l t w i th immediately

14. ⦿ Sta c k Tra c e i s a l i s t of method c a l l s fr o m the point when the application was started to the poi n t whe r e the exc e ption w a s t h r ow n . The most re c ent me t hod c a l l s a r e a t the top. ⦿ A sta c k tra c e i s a v ery h e lpf ul debuggi n g tool. I t i s a l is t of the me t hod ca l l s that the appl i cati o n wa s i n the mid d l e of w hen an Exc eption wa s t h r ow n . ⦿ This i s v ery use f ul be c au s e i t doe s n't o n l y s h ow yo u w he r e the er r or happe n ed, bu t al s o how the progr a m ended up i n that pla c e of the code.

15. ⦿ I n som e cas e s , more than one exc e pt i on could be raised by a single piece of code. To handl e th i s type of s i tuat i on, yo u can sp e c i fy t w o or mo re catch cla u s e s , each catch i ng a d i ff e r e nt type of except i on. ⦿ W h en a n except i on i s thrown, each catch stat e ment i s i nsp e cted i n order, a nd the f i rst one whose type matches that of the except i on i s ex e cuted. Aft e r one catch st a tem e nt ex ecute s , the other s are bypassed, and execution continues after the try/catch block.

16. ⦿ I n som e cas e s , more than one exc e pt i on could be raised by a single piece of code. To handl e th i s type of s i tuat i on, yo u can sp e c i fy t w o or mo re catch cla u s e s , each catch i ng a d i ff e r e nt type of except i on. ⦿ W h en a n except i on i s thrown, each catch stat e ment i s i nsp e cted i n order, a nd the f i rst one whose type matches that of the except i on i s ex e cuted. Aft e r one catch st a tem e nt ex ecute s , the other s are bypassed, and execution continues after the try/catch block.

17. Arithmetic Exception occurs rest of the code

18. ⦿ At a time only one exception occurs and a t a time only one ca t ch bl o ck is executed. ⦿ All catch block s m u s t b e o r d e r e d f rom most specifi c to m ost g e n eral, i . e. ca tch for Arith m eticExcepti o n must come befor e cat c h for E xcepti o n

19. ⦿ The try statemen t c a n be neste d . That i s , a try statement can be inside the block of another try. Eac h time a t ry statemen t i s entere d , the context of that excepti on i s pu s hed on the stack. ⦿ I f a n inne r try statemen t d o es not have a c a tch handler for a particular exception, the stack is unw ound an d the next try statement’ s cat c h hand l ers ar e i nspe cted for a ma t ch. ⦿ Th i s continu e s unti l one of the ca t ch statements succeeds, or until all of the nested try statements are exh a uste d . I f no c a tch state m ent ma t c h es, t h en the Ja v a run - time s y st e m w i ll hand l e the excepti on

20. ⦿ Jav a finally bloc k i s a bloc k that is used to e xecute importan t code such as closing conn e cti o n, strea m etc. ⦿ Java finally block is always executed wh e ther exc e p t io n i s h andle d or not. ⦿ Java finally bl o ck follows try or ca tch block.

21. ⦿ For each try bloc k th e re can b e z e r o or more catch blocks, but only one finally block. ⦿ Th e fi n all y bloc k wil l not b e e xecut e d if progra m exits( e ithe r b y calling System.exit() or by causing a fatal error that causes the proces s to abort ) .

22. ⦿ The Java throws ke y w o rd i s us e d to declare a n except i on. I t g i ve s a n i nformat i on to the progra m mer that the r e may oc c ur an except i on s o i t i s bette r for the progra m mer to provide the exception handling code so that normal fl o w can be ma i nta i ned. ⦿ Exception Handling is mainly used to handle the checked except i ons. I f there oc c urs an y unchecked except i on such as NullPointerException, it is programmers fault that he i s not perform i ng c heck up before the code being used.

23. Syntax of java throws retur n _ty p e m e thod_ n ame ( ) throws exc e pt ion_class_name{ //method code } unchecke d Exception : und e r you r control s o correct you r code. error : beyon d you r control e . g. yo u are unable to do anything if there occurs Virtual M achine Erro r or StackOverflowError

24. ⦿ Now C h ec k ed Exc e ptio n can be pro p agate d (fo r warde d i n call stack ) . ⦿ It provides information to the caller of the m e thod abou t t h e exc e ption