COP 3503 FALL 2012 Shayan Javed Lecture 11

1. COP 3503 FALL 2012ShayanJavedLecture 11 Programming Fundamentals using Java

2. Exception Handling

3. Errors • Syntax Errors • Logic Errors • Runtime Errors

4. Syntax Errors • Arise because language rules weren’t followed.

5. Syntax Errors • Arise because language rules weren’t followed. • Detected by the compiler • javac for Java • g++ for C++

6. Logic Errors • Program compiles and runs, but results are wrong.

7. Logic Errors • Program compiles and runs, but results are wrong. • Detected and fixed through testing.

8. Logic Errors • Program compiles and runs, but results are wrong. • Detected and fixed through testing. • Arise because logic coded by the programmer was incorrect.

9. Logic Errors • Program compiles and runs, but results are wrong. • Detected and fixed through testing. • Arise because logic coded by the programmer was incorrect. • Example: wrote c = a - b instead of c = a + b

10. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out

11. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out • Examples of Code Errors: • Divide by zero

12. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out • Examples of Code Errors: • Divide by zero • Array out of bounds

13. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out • Examples of Code Errors: • Divide by zero • Array out of bounds • Accessing a null pointer (reference)

14. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out • Examples of Code Errors: • Divide by zero • Array out of bounds • Accessing a null pointer (reference) • Integer overflow

15. Runtime Errors • Occur when program is running – environment detects it and can’t carry it out • Examples of Code Errors: • Divide by zero • Array out of bounds • Accessing a null pointer (reference) • Integer overflow • Programs crash when such exceptions are not handled

16. Errors int[] numbers = { 1.5, 5, 7 }; System.out.prntln(numbers[numbers.length]); Try to point out all the errors in this code

17. Errors int[] numbers = { 1.5, 5, 7 }; System.out.prntln(numbers[numbers.length]); Syntax Error(s)

18. Errors int[] numbers = { 1.5, 5, 7 }; System.out.prntln(numbers[numbers.length]); Syntax Error(s)

19. Errors int[] numbers = { 1.5, 5, 7 }; System.out.prntln(numbers[numbers.length]); Syntax Error(s) Runtime Error(s)

20. Exception An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program’s instructions.

21. Exception Handling in Java • Mechanism for handling exceptions by detecting and responding to them in a systematic, uniform and reliable manner.

22. Exception Handling in Java • Mechanism for handling exceptions by detecting and responding to them in a systematic, uniform and reliable manner. • Any exceptions not handled within the Java program are “caught” by the Java Runtime Environment (JRE)

23. Exception • A method in Java throws Exceptions • “Something went wrong”

24. Exception • A method in Java throws Exceptions • “Something went wrong” • Exceptions are Objects • Every Exception is a subclass of the Exception class

25. Unchecked Exceptions/Errors

26. System Errors

27. System Errors • Thrown by the Java Virtual Machine (JVM)

28. System Errors • Thrown by the Java Virtual Machine (JVM) • Represented by the Error class

29. System Errors • Thrown by the Java Virtual Machine (JVM) • Represented by the Error class • Describes internal system errors

30. System Errors • Thrown by the Java Virtual Machine (JVM) • Represented by the Error class • Describes internal system errors • Rarely occur – if they do you can’t do much other than terminating

31. Runtime Exceptions (Unchecked)

32. Checked Exceptions

33. Checked Exceptions Need to explicitly deal with Checked Exceptions: try and catch them, or throw them

34. Exception Handling • Keywords: • try some code, catch any Exceptions

35. Exception Handling • Keywords: • try some code, catch any Exceptions • or throw an Exception

36. Exception Handling • Keywords: • try some code, catch any Exceptions • or throw an Exception • finally execute some code

37. Exception Handling • Java forces you to deal with checked Exceptions

38. Exception Handling • Java forces you to deal with checked Exceptions • Two ways to deal with them: void p1 () { try { riskyMethod(); } catch (Exception ex) { .... } } (a)

39. Exception Handling • Java forces you to deal with checked Exceptions • Two ways to deal with them: void p1 () { try { riskyMethod(); } catch (Exception ex) { .... } } (a) void p1 () throws Exception { riskyMethod(); } (b)

40. Exception Handling • Remember the clone() method?

41. Exception Handling • Remember the clone() method? • Can be written in two ways: Object clone() { try { return super.clone(); } catch (CloneNotSupportedException ex) { .... } } (a)

42. Exception Handling • Remember the clone() method? • Can be written in two ways: Object clone() { try { return super.clone(); } catch (CloneNotSupportedException ex) { .... } } (a) Object clone() throws CloneNotSupportedException { return super.clone(); } (b)

43. Exception Handling • In the first case, we are catching and handling the Exception

44. Exception Handling • In the first case, we are catching and handling the Exception • In the second case we are throwing it – needs to be caught and handled by the calling method

45. Catching Exceptions • A try-catch statement: try { // Statement(s) which throw Exceptions } catch (Exception1exception1) { // Handles Exceptions of type Exception1 } catch (Exception2exception2) { // Handles Exceptions of type Exception2 } catch (Exceptionexception) { // Handles Exceptions of type Exception // ALL Exceptions } // Any code after the try-catch block

46. Catching Exceptions • A try-catch statement: try { // Statement(s) which throw Exceptions } catch (CloneNotSupportedException exception1) { // Handles Exceptions of type CloneNotSupportedException } catch (NullPointerException exception2) { // Handles Exceptions of type NullPointerException } catch (Exceptionexception) { // Handles Exceptions of type Exception // ALL Exceptions } // Any code after the try-catch block

47. Catching Exceptions • A try-catch statement: try { Circle clone = circle1.clone(); } catch (CloneNotSupportedException exception1) { // Handles Exceptions of type CloneNotSupportedException } catch (NullPointerException exception2) { // Handles Exceptions of type NullPointerException } catch (Exceptionexception) { // Handles Exceptions of type Exception // ALL Exceptions } // Any code after the try-catch block

48. Catching Exceptions • ThrownExceptions have to be eventually caught somewhere in your code

49. Exception Information • So an Exception has been caught – what can we do with it? try { // Statements which throw Exceptions } catch (Exceptionexception) { // ALL Exceptions }

50. Exception Information • Some useful methods in the Throwable class: