1. Question of the Day  A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

2. Question of the Day  A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

3. Question of the Day  A landscaper plants 5 rows of 4 trees each, but only uses 10 trees. How is this possible?

5. Error Handling Goals

6.  What should we do when an error occurs?  Should alert system to the error  Stop immediately and execute code handling error  Minimize amount of rewritten code  How error handled & fixed may change in subclass  Unknown future uses of code demands flexibility  Changing printing of error message is not possible

7. Exceptional Circumstances  ex-cep-tion  ex-cep-tion : n.  Situation or case not conforming to the general rule  Proper way to signal error in object-oriented code  Seen Java exceptions in CSC111 already  Most of these were unchecked (not solvable)  ArrayIndexOutOfBoundsException -- accessing a nonexistent array entry  NullPointerException -- used null reference

8. Exception Classes Java  Exception is class defined by Java  Java already defines many subclasses of Exception  New Exception subclasses can be written, also  Error handling uses instances of these classes  Classes are not magic and work like all others  Classes can include fields, methods, & constructors  Exception instances not special, either  Must instantiate before use  Use anywhere, not strictly for error handling

9. Error Handling Codes  throw exception upon detecting problem  Handle problem by catch ing exception  Do not need to catch an exceptions  If it is never caught, program will crash  Not a bad thing if the error was unfixable, however

10. Throwing an Exception public class BankAccount { private float balance; // Lots of code here… float withdraw(float amt) throws ReqException{ if (amt balance) { ReqException re = new ReqException(); re.setBadRequest(amt, balance); throw re; } balance -= amt; return balance; } }

11. Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it

12. Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed

13. Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed  Catch the exception and throw a new one

14. Handling Exceptions  Once an exception is thrown, methods can:  Include code to catch exception and then ignore it  Catch & handle exception so error is fixed  Catch the exception and throw a new one  Ignore exception so passed on to calling method

15. Handling Exceptions  Exception can be thrown anywhere & anytime uncaught  throws lists method’s fixable uncaught exceptions void controller() throws LostPlane {…} int scheduler() throws NoFreeTime {…}  Calling methods now aware of possible errors  If it would like, could catch and correct errors  List exception in own throws clause otherwise void plan(int i) throws NoFreeTime { // Lots of interesting code here… scheduler(); }

16. try {…} Blocks  Can only catch exceptions thrown in try block void cantCatch() throws MistakeExcept,MyBadExcept{ try { // There is code here that does something interesting… System.err.println(“No exceptions”); } catch (Oops oop) { // Here be code to fix the exception } methodThatMightThrowMistakeExcept(); throw new MyBadExcept(); }

17. try {…} Blocks

18.  Can catch some (or all) exceptions from try at least 1  Each try needs at least 1 catch void catchSome() throws MyBadException { try { methodThatMightThrowExceptions(); throw new MyBadExcept(); } catch (Oops oop) { oop.printStackTrace(); System.err.println(“Method ends normally.”); } catch (MistakeException me) { System.err.println(“Boy was I dumb!”); } }

19. try {…} Blocks  If exception thrown, stop try & go to catch void catchAll() { try { methodThatMightThrowOops(); methodThatShouldThrowOops(); throw new MyBadExcept(); } catch (Oops oop) { oop.printStackTrace(); System.err.println(“Oops was handled.”); } catch (MyBadExcept mbe) { mbe.printStackTrace(); System.err.println(“MyBad fixed.”); } }

20. Not Handling Exceptions float withdraw(float amt) throws BadReqEx { if (amt > balance) { BadReqEx re = new BadReqEx(amt,balance); throw re; } balance -= amt; return balance; } void forcedWithdrawal(float amount) { callPolice(); addDyePacks(); withdrawal(amount); } public void robbedByJesseJames(float amt) { forcedWithdrawal(amt); }

21. Not Handling Exceptions float withdraw(float amt) throws BadReqEx { if (amt > balance) { BadReqEx re = new BadReqEx(amt,balance); throw re; } balance -= amt; return balance; } void forcedWithdrawal(float amount) { callPolice(); addDyePacks(); withdrawal(amount); } public void robbedByJesseJames(float amt) { forcedWithdrawal(amt); } Unfair - by not stating exception, caller hasn't chance to handle

22. Handling Exceptions void forcedWithdrawal(float amount) throws BadReqEx{ callPolice(); addDyePacks(); withdrawal(amount); } public void northfieldMN(float amt) { try { forcedWithdrawal(amt); } catch (BadReqEx bre) { formPosse(); killSomeGangMembers(); } finally { giveLollipop(); } }

23. Handling Exceptions void forcedWithdrawal(float amount) throws BadReqEx{ callPolice(); addDyePacks(); withdrawal(amount); } public void northfieldMN(float amt) { try { forcedWithdrawal(amt); } catch (BadReqEx bre) { formPosse(); killSomeGangMembers(); } finally { giveLollipop(); } }

24. 2 Types of Exceptions Checked ExceptionUnchecked Exception

25. 2 Types of Exceptions  Subclass of Exception must  throws must list uncaught  Use for fixable errors  Java forces methods to consider them  Only useful if fixable  Subclass of RuntimeException  Can be listed in throws  “You are hosed”  Usually can’t be fixed  Can ignore in method  Unless it is caught, these will still crash program Checked ExceptionUnchecked Exception

26. Tracing Example public static void generate() throws TraceException { TraceException te = new TraceException(); throw te; System.out.println(“Ending gE”); } public static void handler(boolean callIt) { try { System.out.println(“Starting cE”); if (callIt) { generate(); } System.out.println(“Ending cE”); } catch (TraceException te) { System.out.println(“Caught te”); } } public static void main(String[] args) { handler(false); handler(true); }

27. Tracing Example public static void generate() throws TraceException { TraceException te = new TraceException(); throw te; System.out.println(“Ending gE”); } public static void handler(boolean callIt) { try { System.out.println(“Starting cE”); if (callIt) { generate(); } System.out.println(“Ending cE”); } catch (TraceException te) { System.out.println(“Caught te”); } } public static void main(String[] args) { handler(false); handler(true); }

28. Tracing Example public static void generate() throws TraceException { TraceException te = new TraceException(); throw te; } public static void handler(boolean callIt) { try { System.out.println(“Starting cE”); if (callIt) { generate(); } System.out.println(“Ending cE”); } catch (TraceException te) { System.out.println(“Caught te”); } } public static void main(String[] args) { handler(false); handler(true); }

29. Your Turn  Get into your groups and complete activity

30. For Midterm  You can use on this midterm:  You can always use your textbook & notes IF  Printout of slides IF has notes on that day's slides  At the same time, you may NOT use:  Computer, calculator, cell phone, or similar  Copies of daily activities and/or solutions  Friends, Romans, countrymen or their ears  To be certain rules are followed, when test ends  Hand in all printed material you had with you

31. How to Prepare for Midterm DODON'T  Make cheat sheets for the test  Review how parts of Java work  Add post-its to important pages  Memorize  Drink case of 40s before test  Use post-its as clothing