2. 交通大學資訊工程學系 Programming in Java Exception Handling again 蔡文能 交通大學資訊工程學系 tsaiwn@csie.nctu.edu.tw http://www.csie.nctu.edu.tw/~tsaiwn/java//java/

3. 交通大學資訊工程學系 蔡文能 7- 第 2 頁 JavaMore Java Examples Agenda Exception handling Exception handling in Java Exception handling in C++

4. 交通大學資訊工程學系 蔡文能 7- 第 3 頁 JavaMore Java Examples Exception? exception  An exception is any unusual event, either erroneous or not, detectable by either hardware or software, that may require special processing exception handling  Without exception handling  When an exception occurs, control goes to the operating system, where typically an error message is displayed the program is terminated With exception handling  Programs are allowed to trap exceptions  There is a possibility to fix the problem and continuing execution

5. 交通大學資訊工程學系 蔡文能 7- 第 4 頁 JavaMore Java Examples What to do when Exception occurs? There are several ways a function may react when encountering an unexpected/erroneous situation (i.e., Exception )  Terminate the program Cannot be used in a program that cannot afford to crash  Return a value representing “error”  Raise a flag Many standard C library functions used global variable errno  Need to check the flag after every operation  Losses information  Call a function supplied to be called in case of error Raise a signal / Event How will it handle the error? char* String::getString(); //ok: return NUKK on error int atoi(const char *); //what to return for “bad-str”?

6. 交通大學資訊工程學系 蔡文能 7- 第 5 頁 JavaMore Java Examples When To Use Exceptions Required by compiler when using some statements (Java) Make code safer (catch fatal errors)‏ Fix the problem and try again Patch the problem and continue Make a complex error mechanism simpler

7. 交通大學資訊工程學系 蔡文能 7- 第 6 頁 JavaMore Java Examples Exception example try { readFromFile("datafile"); } catch (FileNotFoundException e) { System.err.println("Error: File not found"); } try { readFromFile("datafile"); } catch (FileNotFoundException e) { System.err.println("Error: File not found"); }

8. 交通大學資訊工程學系 蔡文能 7- 第 7 頁 JavaMore Java Examples Exception Handling: Some Options Print something Throw a new exception Re-Throw the exception Fix the problem Exit

9. 交通大學資訊工程學系 蔡文能 7- 第 8 頁 JavaMore Java Examples Exception Handling: Printing You can print a stack trace by calling the exception method printStackTrace() Sometimes it's better to send error messages to stderr :  System.err.println("Error: invalid thingy"); Some applications log error messages  file  logging service (syslog).

10. 交通大學資訊工程學系 蔡文能 7- 第 9 頁 JavaMore Java Examples Throwing Objects Why throw an object?  Additional data encapsulated  An object is created upon invoking the throw statement  Can be created with additional data, passed to Ctor  Object is destroyed after the catch clause ends

11. 交通大學資訊工程學系 蔡文能 7- 第 10 頁 JavaMore Java Examples Exception Handling: throw You can throw an exception (object) from an exception handler (a catch block). Allows you to change exception type and/or error message. You can also alter the base of the stack trace fillInStackTrace()

12. 交通大學資訊工程學系 蔡文能 7- 第 11 頁 JavaMore Java Examples Exception Handling: Re- throw You can throw an exception from an exception handler (a catch block) without changing anything: called rethrowing The caller needs to deal with the exception. This also happens if you don't catch the exception! sometimes you need to take some action and then rethrow the exception.

13. 交通大學資訊工程學系 蔡文能 7- 第 12 頁 JavaMore Java Examples Another way to re-throw You can allow selected types of exceptions to be propogated to the caller of your method: void blah() throws IOException { Within blah() you don't need to catch these exceptions (to be able to compile).

14. 交通大學資訊工程學系 蔡文能 7- 第 13 頁 JavaMore Java Examples Exception example public class ThrowUp { public static void main(String[ ] args) { String tmp; try { // generate an ArrayIndexOutOfBoundsExceptions.. //.. (on purpose!). for (int i=0 ; i<args.length+10 ; i++) { tmp = args[i]; } System.out.println("I made it!"); } catch (ArrayIndexOutOfBoundsException up) { throw new SecurityException("Trouble"); } } // main }

15. 交通大學資訊工程學系 蔡文能 7- 第 14 頁 JavaMore Java Examples Exception Handling: Fix the problem. You can't fix things and then resume execution automatically you can do this in C++. You can have a loop the retries the code again.

16. 交通大學資訊工程學系 蔡文能 7- 第 15 頁 JavaMore Java Examples Exception Handling: exiting Sometimes the error is fatal, and you want to stop the program immediately. System.exit();

17. 交通大學資訊工程學系 蔡文能 7- 第 16 頁 JavaMore Java Examples Wait.java public class Wait { public static void main(String[ ] args) { boolean done=false; int seconds=0; try { seconds = Integer.parseInt(args[0]); } catch (Exception e) { System.out.println("Please specify the number of seconds"); System.exit(1); } long initialtime = System.currentTimeMillis( ); while (!done) { try { if (System.currentTimeMillis()-initialtime < seconds*1000) throw new Exception("Not Yet - keep trying"); done=true; } catch (Exception e) { System.out.println(e); } System.out.println("Done!\n"); }

18. 交通大學資訊工程學系 蔡文能 7- 第 17 頁 JavaMore Java Examples How/when do you generate exceptions? Use throw: throw new Exception("broken!"); You can use throw anywhere.  you detect some error that means the following code should not be executed. In some cases, you can think of throw as a alternate return

19. 交通大學資訊工程學系 蔡文能 7- 第 18 頁 JavaMore Java Examples Exception Enforcement In general, you do the following:  specify what exceptions each method can generate.  write code to catch all exceptions that can be generated by a method call. The compiler (usually) enforces this  it is a compilation error to call a method without catching it's declared exception types.

20. 交通大學資訊工程學系 蔡文能 7- 第 19 頁 JavaMore Java Examples RunTime Exceptions There are exceptions that are generated by the system (that are usually caused by programming mistakes):  NullPointerException (null references)  ArrayIndexOutOfBoundsException If you don't catch these, a stack trace will be generated and the program will terminate. The compiler does not force you to catch these exceptions.

21. 交通大學資訊工程學系 蔡文能 7- 第 20 頁 JavaMore Java Examples Exception Types Exceptions are objects! Exception types are classes.  A (quite large!) hierarchy of classes. All exception types are derived from the class Exception  there are some methods defined in this base class.

22. 交通大學資訊工程學系 蔡文能 7- 第 21 頁 JavaMore Java Examples NullPointer.java import java.util.*; public class NullPointer { public static void main(String[ ] args) { Date f = new Date(); f=null; PrintSomething2(f); // comment out to test PrintSomething PrintSomething(f); } // no checks, if x is null runtime exception default behavior static void PrintSomething(Object x) { System.out.println(x.getClass( ).getName()); } // we explicitly check for runtime exception! static void PrintSomething2(Object x) { try { System.out.println(x.getClass().getName()); } catch (RuntimeException re) { System.out.println("Fatal Error!"); System.exit(1); } } // main }

23. 交通大學資訊工程學系 蔡文能 7- 第 22 頁 JavaMore Java Examples Throwable Exception Error RunTimeException NullPointerException ArithmeticException IOException Exception Type Hierarchy (partial) EOFException VirtualMachineError

24. 交通大學資訊工程學系 蔡文能 7- 第 23 頁 JavaMore Java Examples Some Exception Methods These are actually inherited from throwable printStackTrace() fillInStackTrack() getMessage()

25. 交通大學資訊工程學系 蔡文能 7- 第 24 頁 JavaMore Java Examples Creating Your Own Exception Types It is often useful to create your own type of exception.  generally all you create is a name.  you can get fancy and add new methods to your exception class(es).

26. 交通大學資訊工程學系 蔡文能 7- 第 25 頁 JavaMore Java Examples User-defined Exception Type class FooException extends Exception {} //.. class BlahException extends Exception { BlahException(){} BlahException(String s) { super(s); } } //.. throw new BlahException("Invalid blah");

27. 交通大學資訊工程學系 蔡文能 7- 第 26 頁 JavaMore Java Examples using finally try { statements... } catch (ExceptionType1 ename1) { error handling statements... } catch (ExceptionType2 ename2) { error handling statements... } finally { … this code always executed … }

28. 交通大學資訊工程學系 蔡文能 7- 第 27 頁 JavaMore Java Examples Why finally ? What is there to clean up?  No memory cleanup required in Java!  No destructors to call! Sometimes you need to set the state of things (fields) to some stable (acceptable) state.

29. 交通大學資訊工程學系 蔡文能 7- 第 28 頁 JavaMore Java Examples Exception Handling again 謝謝捧場 http://www.csie.nctu.edu.tw/~tsaiwn/java/ 蔡文能