1. CSE 116/504 – Intro. To Computer Science for Majors II
Lecture 13: Exceptions
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2. Writing OO Methods Legos the goal for every class you will ever write
When used by others, interactions clearly defined
Consistent across objects; states differ but hooks do not
No other classes required; each instance self-contained
Unlimited potential; does not assume specific uses

3. How do we create this perfection?
Writing OO Methods
Legos the goal for every class you will ever write
When used by others, interactions clearly defined
Consistent across objects; states differ but hooks do not
No other classes required; each instance self-contained
Unlimited potential; does not assume specific uses
How do we create this perfection?

4. Limit Data Lifespan Locals “live” for method or block within method
Name may be reused, but no relationship exists
Parameters pass data between methods
But receiving method must be called by sender
Instance vars live between calls or even longer
Values available with instance (or longer if static)

5. Locals always best choice Parameters & fields only as necessary
OO Key Concept #3
Locals always best choice
Parameters & fields only as necessary

6. Locals always best choice
OO Key Concept #3
Locals always best choice
Parameters & fields only as necessary (which is often)

7. Common Nightmare

8. Prevent the Nightmare
Everyone tries keeping certain details private

9. Prevent the Nightmare Everyone tries keeping certain details private
Exposure limited to appropriate areas

10. Prevent the Nightmare Everyone tries keeping certain details private
Exposure limited to appropriate areas
Limit how & why changes occur

11. Prevent the Nightmare Everyone tries keeping certain details private
Exposure limited to appropriate areas
Limit how & why changes occur
Stop others from seeing changes

12. Prevent the Nightmare Exact same holds for objects
Make fields private unless for very important reason:
private int fieldName;
Controls interactions since field accesses all in 1 file or

13. Declare fields private
OO Key Concept #4
Do NOT enable mistakes
Declare fields private
(unless absolutely needed)

14. Declare fields private
OO Key Concept #4
Do NOT enable mistakes
Declare fields private
(unless absolutely needed) (just rewrite the code if absolutely needed)

16. Error Handling Goals

17. Error Handling Goals
What do we do when an error occurs?

18. Error Handling Goals What do we do when an error occurs?
Almost certainly copyright infringement, but this GIF is from via

19. Error Handling Goals What do should we do when an error occurs?
Alert application to error & stop execution
Execute code handling error and crash if none exists
Still lazy! Minimize code eventually rewritten
How error handled & fixed WILL change with time
Fixing stupid calls not story that we can always write
How can we do this in object-oriented way?

20. Exceptional Circumstances
ex-cep-tion: n.
Situation or case not conforming to the general rule
Proper way to signal error in object-oriented code

21. Exceptional Circumstances
ex-cep-tion: n.
Situation or case not conforming to the general rule
Proper way to signal error in object-oriented code

22. Exception Classes Java
Exception is class defined by Java
Java predefines many subclasses of Exception
Unfortunately seen many Exception subclasses
All are RuntimeExceptions -- unfixable errors
IndexOutOfBoundsException
NullPointerException
StackOverflowException
Java libraries define these Exceptions for us
But like all of Java, just another class for us to use

23. 2 Types of Exceptions
Checked Exception
Unchecked Exception

24. 2 Types of Exceptions Checked Exception Unchecked Exception
Subclass of Exception
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

25. 2 Types of Exceptions Will crash program if uncaught
Checked Exception
Unchecked Exception
Subclass of Exception
throws must list uncaught
Use for fixable errors
Java forces methods to consider them
Only useful if fixable
Will crash program if uncaught
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
Will crash program if uncaught

26. Usage identical for Checked & Unchecked Exceptions
2 Types of Exceptions
Usage identical for Checked & Unchecked Exceptions

27. Writing Exception classes
Error handling based upon Exception classes
Classes are not magic just subclass of Exception
Classes can include fields, methods, & constructors
Must create instances to have objects to use
Also find Exception instances also not magic
Can be used in error handling, but not required
Like any other Java objects, can be used anywhere

28. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception { 1

29. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Convince your neighbor your answer is correct

30. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception { 1

31. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Package does NOT matter; Can create a new Exception class anywhere (plus I made up this package)

32. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Name does NOT matter; Names usually ___Exception but can name class anything

33. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Exception is a class; implements for interfaces

34. Which Is Exception Declaration?
package java.except; public class Slide {
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Only requirement: must inherit from Exception (inheritance can be indirect)

35. Throwing an Exception
public class BankAccount { private float balance; // Lots of code here… float withdraw(float amt) { if (amt < 0) { throw new RequestExcepton(); } else if (amt > balance) { RequestExcepton re = new RequestExcepton(); re.setBadRequest(amt, balance); throw re; } balance -= amt; return balance; } }

36. Correct Way to Raise Exception?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
throws new SlideException(); 1

37. Correct Way to Raise Exception?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
throws new SlideException();
Convince your neighbor your answer is correct

38. Correct Way to Raise Exception?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
throws new SlideException(); 1

39. Correct Way to Raise Exception?
SlideException se; throw se;
public class SlideException{
public class Slide implements Exception {
public class Slide extends Exception {
Almost correct but must instantiate Exception to throw

40. Correct Way to Raise Exception?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
public class Slide extends Exception {
Also close
but Exception instance needed and none is created here!

41. Correct Way to Raise Exception?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
throws new SlideException();
Wrong keyword used: must use throws

42. Which Is Exception Declaration?
SlideException se; throw se;
throw new SlideException();
throw SlideException();
throws new SlideException();
This is correct. Another way to write:
SlideException se = new SlideException(); throw se;

43. After the throw

44. After the throw
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45. try {…} catch {…} Blocks
Only exceptions in try block can be caught
And not all exceptions: only ones with matching catch
try { // Will handle any MistakeException raised in this area if (badThingHappening()) { throw new MistakeException(); } } catch (MistakeException oop) { // Handle and fix the problem }
// More code could be here, but this is all made up throw new MistakeException();

46. try {…} catch {…} Blocks
Only exceptions in try block can be caught
And not all exceptions: only ones with matching catch
try { // Will handle any MistakeException raised in this area if (badThingHappening()) { throw new MistakeException(); } } catch (MistakeException oop) { // Handle and fix the problem }
// More code could be here, but this is all made up throw new MistakeException();
If anything executed by try raises MistakeException

47. try {…} catch {…} Blocks
Only exceptions in try block can be caught
And not all exceptions: only ones with matching catch
try { // Will handle any MistakeException raised in this area if (badThingHappening()) { throw new MistakeException(); } } catch (MistakeException oop) { // Handle and fix the problem }
// More code could be here, but this is all made up throw new MistakeException();
If anything executed by try raises MistakeException
We can handle it in catch

48. try {…} catch {…} Blocks
Only exceptions in try block can be caught
And not all exceptions: only ones with matching catch
try { // Will handle any MistakeException raised in this area if (badThingHappening()) { throw new MistakeException(); } } catch (MistakeException oop) { // Handle and fix the problem }
// More code could be here, but this is all made up throw new MistakeException();
If anything executed by try raises MistakeException
We can handle it in catch
But not when outside try

49. try {…} catch {…} Blocks
Can catch some (or all) exceptions from try
Multiple fine, but every try needs at least 1 catch
void catchSome() { try { methodThatMightThrowExceptions(); } catch (Oops oop) { // Handle and fix the problem for oop } catch (MistakeException me) { // Handle and fix the problem for me } }

50. throws Declaration Exception can be thrown anywhere & anytime
throws lists method’s uncaught exceptions void controller() throws LostPlane {…} int scheduler() throws NoFreeTime {…}
throws provides warning to calling method
Allows it to add try-catch block if it can handle error
Exception not required, just warning it could happen

51. throws Declaration Warning of potential errors not just those thrown
Only care that Exception possible, not just started
Does not matter if passed along from caller’s perspective
If Exception can arise, should list in method’s throws
int scheduler() throws NoFreeTime {…}
void plan(int i) throws NoFreeTime { scheduler(); }

52. 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);

53. 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);
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54. 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 does not get chance to fix

55. Error Handling Story (1)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method also handled its errors, so not a problem
throw statement included in try code block
try had catch block for this Exception instance
The code in that catch block handled the problem
No other code needed – problem was solved
... And they lived happily ever after

56. Error Handling Story (1)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method also handled its errors, so not a problem
throw statement included in try code block
try had catch block for this Exception instance
The code in that catch block handled the problem
No other code needed – problem was solved
... And they lived happily ever after

57. Error Handling Story (1)
public int parseNum(String s) { try { return Integer.parseInt(s); } catch (NumberFormatException nfe) { return 0; } }

58. Error Handling Story (2)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method called in try with catch for this Exception
The code in that catch block handled the problem
No other code needed – problem was solved
... And they lived happily ever after

59. Error Handling Story (2)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method called in try with catch for this Exception
The code in that catch block handled the problem
No other code needed – problem was solved
... And they lived happily ever after

60. Error Handling Story (2)
float secureWithdraw(float amt){ try { return withdraw(amt); } catch (BadReqEx var) { lockDoors(); callPolice(); return 0; } } float withdraw(float amt) throws BadReqEx { if (amt > balance) { throw new BadReqEx(amt,balance); } balance -= amt; return balance; }

61. Error Handling Story (3)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method call similar: not in try or not with catch
Exception instance goes to method which called that
Keeps going up calls until matching try-catch found

62. Error Handling Story (3)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method call similar: not in try or not with catch
Exception instance goes to method which called that
Keeps going up calls until matching try-catch found
... And they lived happily ever after

63. Error Handling Story (3)
float oceansEleven(){ try { planHeist(); hireCrew(); return robVaults( f); } catch (BadReqEx var) { return rewardFromServing(); } } float withdraw(float amt) throws BadReqEx { if (amt > balance) { throw new BadReqEx(amt,balance); } balance -= amt; return balance; }

64. Error Handling Story (4)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method call similar: not in try or not with catch
Exception instance kept going through to main()
No matching try-catch found in any method call
... And so the program crashed

65. Error Handling Story (4)
Once upon a time… an error was detected
Method created instance from subclass of Exception
Exception instance used in throw statement
Method did not handle error; passed up call chain
throw either not in try or lacked matching catch
Method call similar: not in try or not with catch
Exception instance kept going through to main()
No matching try-catch found in any method call
... And so the program crashed

66. 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(); }

67. 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(); }

68. Locals always best choice
OO Key Concept #3
Locals always best choice
Parameters & fields only as necessary (which is often)

69. Declare fields private
OO Key Concept #4
Do NOT enable mistakes
Declare fields private
(unless absolutely needed) (just rewrite the code if absolutely needed)

70. Usage identical for Checked & Unchecked Exceptions
2 Types of Exceptions
Usage identical for Checked & Unchecked Exceptions

71. After the throw
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72. For Next Lecture Reading is online (via website) for Friday
Even if not in book, reading remains required
ADT and design pattern presented in one swoop
How should Collections provide data access
Week #5 weekly assignment due Monday
Get started early; project phase #1 due Monday night