Exception Handling in Java Programming: Techniques and Best Practices

1. CSc2310 tutoring session, week 8Fall, 2012 • Using Exceptions • Homework 4 HaidongXue 5:30pm—8:30pm 10/23/2012 and 10/24/2012

2. CSc2310 Tutoring • Time: 5:30pm-8:30pm • Tutor: HaidongXue • Website: http://www.cs.gsu.edu/~hxue1/csc2310_Tutoring/index.html There are 2 sections: 1. Review • Using Exceptions • Homework 4 2. Q&A • Answer your questions about java programming

3. Using exceptions • How to deal with the potential errors in a program? • E.g.: a class to calculate the area for a rectangle: class RecArea{ public static calRecArea ( double width, double height){ return width*height; } }

4. Using exceptions • A potential problem here is: • The input width may be negative • The input height may be negative • How to prevent unwanted consequences? • class RecArea{ • public static double calRecArea( double width, double height){ • return width*height; • } • }

5. Using exceptions • Method 1 – print some error info • class RecArea{ • public static double calRecArea( double width, double height){ • if (width<0 || height<0) • System.out.println(“invalid input!”); • return width*height; • } • } • Drawback: • The user of RecArea may annoyed by the unwanted message, and has no way to remove it. • The user of RecArea may neglect it.

6. Using exceptions • E.g.: • class RecAreaUser{ • public void main( String[] args){ • …. • // assuming we have house area width as w and height as h • // and need to calculate the tax here • double tax = 10 * RecArea.calRecArea(w, h); • } • } The author of RecArea is Haydon; the author of RecAreaUser is you. You may have designed a very good user interface; however, sometimes there could be a very unfriendly “invalid input!” on the screen; what’s worse, you have no way remove them. (height and width may obtained from the final a final user in run time). In a project, we do not like components like this RecArea. Moreover, you may still use a negative area to calculate tax. (A logic error)

7. Using exceptions • Method 2 – return some value to indicate the problem • class RecArea{ • public static double calRecArea( double width, double height){ • if (width<0) return -1; // -1 means invalid width • if (height<0) return -2; // -2 mean invalid height • return width*height; • } • }

8. Using exceptions Before exceptions, it was a very popular method: • class RecAreaUser{ • public void main( String[] args){ • …. • // assuming we have house area width as w and height as h • // and need to calculate the tax here • double area = RecArea.calRecArea(w, h); • double tax = 0; • if (area == -1 ) {/*do something for invalid width*/} • elseif(area == -2 ) {/*do something for invalid height*/} • else • tax = 10 *area; • } • } Drawback: the author of RecAreaUser may also neglect the problem, and use -1 or -2 as the area.

9. Using exceptions • Method 3 – Using exceptions • class RecArea{ • public static double calRecArea ( double width, double height) throws Exception{ • if (width<0 || height <0) throw new Exception(“invlid input”); • return width*height; • } • } The user of RecArea cannot neglect this potential exception.

10. Using exceptions publicclassRecAreaUser{ publicstaticvoid main(String[] args){ Scanner s = new Scanner(System.in); while(true) { System.out.println("=================================="); System.out.println("Type a for calculating tax;"); System.out.println("Type q to quit."); String command = s.nextLine(); if(command.equalsIgnoreCase("a") ){ System.out.print("The width of your house area: "); doublewidth = s.nextDouble(); s.nextLine(); System.out.print("The height of your house area: "); doubleheight = s.nextDouble(); s.nextLine(); doubletax; // you can try to remove the try and catch block, you will see a compile error try{ tax = RecArea.calRecArea(width, height); System.out.println("The tax you need to pay: " + tax); } catch(Exception e){ System.out.print("Sorry, when calculating your house area, we see a error: "); System.out.println(e.getMessage()); } } elseif( command.equalsIgnoreCase("q")) break; else continue; } } }

11. Using exceptions • Throw an exception • Throw a exception when needed throw [Exception Object]; • Catch an exception try{ [code may throw exceptions] } catch (Exception e){ [do something with this exception] }

12. HW4 - ClassifyScores • A use case: Enter a number: 230 Wrong number Enter a number: 23 Enter a number: 1 Enter a number: 23 Enter a number: 12 Enter a number: 99 Enter a number: 100 Enter a number: -1 Decile Scores ------- -------- 0-9 1 10-19 1 20-29 2 30-39 0 40-49 0 50-59 0 60-69 0 70-79 0 80-89 0 90-100 2

13. HW4 - ClassifyScores • Where to put the scores? • Using an array int[] scores = new int[10]; for (int i = 0; i < 10; i++) scores[i] = 0;

14. HW4 - ClassifyScores intindex = n / 10; if (index == 10) index = 9; // only for the number of 100 e.g.: When n = 0, index will be 0; When n = 11, index will be 1; When n = 23, index will be 2; When n = 34, index will be 3; … When n = 92, index will be 9; When n = 100, index will be 9;

15. HW4 - ClassifyScores • Finished code is at my website: http://www.cs.gsu.edu/~hxue1/csc2310_Tutoring/code/ClassifyScores.java

16. HW4 – NFLTeam5, NFLGameDay5 • Create the player array in constructor: public NFLTeam5(String eName) { win = 0; loss = 0; TeamName= eName; nTotalNumPlayers= 0; sPlayerArray= new String[100]; }

17. HW4 – NFLTeam5, NFLGameDay5 • Add “addAPlayer”method public void addAPlayer(String playerName) { sPlayerArray[nTotalNumPlayers] = playerName; nTotalNumPlayers++; } • Finish the code to construct “Steelers” Finished code are at my website: http://www.cs.gsu.edu/~hxue1/csc2310_Tutoring/code/NFLTeam5.java http://www.cs.gsu.edu/~hxue1/csc2310_Tutoring/code/NFLDay5.java

18. Please let me know your questions. I will be here till 8:30pm