Correctness and Robustness

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1 Correctness and Robustness Advanced Computer Programming Asst. Prof. Dr. Kanda Runapongsa Saikaew Department of Computer Engineering Khon Kaen University 1

2 Agenda Introduction to Correctness and Robustness Exceptions Handling exceptions using try...catch Handling exceptions using throws Creating your own exception class Assertions 2

3 Correct & Robust Computer programs that fail are much too common A tiny error can cause a program to misbehave or crash A program is correct if it accomplishes the task that it was designed to perform It is robust if it can handle illegal inputs and other unexpected situations in a reasonable way 3

4 Example of Correctness & Robustness Consider a program that is designed to read some numbers from the user and then print the same numbers in sorted order The program is correct if it works for any set of input numbers It is robust if it can also deal with non numeric input Printing an error message and ignoring the bad input A non robust program might crash or give nonsensical output in the same circumstance 4

5 Exceptions and Exception Classes The word exception is meant to be more general than error It includes any circumstance that arises as the program is executed which is meant to be treated as an exception to the normal flow of control of the program An exception might be an error, or it might just be a special case that you would rather not have clutter up your elegant algorithm 5

6 Exception Thrown to be Caught When an exception occurs during the execution of a program, we say that the exception is thrown When this happens, the normal flow of the program is thrown off track, and the program is in danger of crashing However, the crash can be avoided if the exception is caught and handled in some way An exception that is not caught will generally 6 cause the program to crash

7 Exception Thrown When an exception occurs, the thing that is actually thrown is an object This object can carry information (in its instance variables) from the point where the exception occurs to the point where it is caught and handled This information always includes the subroutine call stack, which is a list of the subroutines that were being executed when the exception was thrown 7

8 Example of Unhandled Exception The thrown object indicates that The exception that occurs is java.lang.arrayindexoutofboundsexception The exception occurs in file ExceptionDemo1.java line number 5 8

9 Error & Exception Error : cannot be handled by programming VirtualMemoryError OutOfMemoryError... Exception : can be handled by better programming ArrayIndexOutOfBoundsException NullPointerException... 9

10 Throwable, Error, and Exception 10

11 Exceptions 1. RuntimeException Can be avoided exception with correct coding Exp. ArrayIndexOutOfBoundException Handle by testing with if statement 2. IOException Cannot be avoided. Need to be handled Exp. FileNotFoundException Cannot be handled by if statement but must be handled with try/catch 11

12 Popular Exceptions ArithmeticException ArrayIndexOutOfBoundException EOFException FileNotFoundException InterruptedException IOException NullPointerException NumberFormatException 12

13 Two Ways in Handling Exceptions We have two ways in handling exceptions Using try... catch try { normal code execution} catch (Exception e) { code to run when exception occurs} Using throws statement in method header and method definition 13

14 Catching Exceptions Syntax for try...catch try { statements; } catch (Exception1 e) { handler for exception1 } catch (Exception2 e) { handler for exception2 }... catch (ExceptionN e) { handler for exceptionn } 14

15 Claiming, Throwing, and Catching Exceptions 15

16 Catching Exceptions Flow 16

17 Claiming Exceptions Claiming exception at method header Using a comma to separate multiple exceptions public void mymethod() throws IOException public void mymethod() throws IOException, OtherException Exception must be caught somewhere along the path of claiming 17

18 Throwing Exception We can throw the object of the existing Exception class throw new ArithmeticException(); We can throw the object of our new defined Exception class 1) Create object without name throw new TheException(); 2) Create object that has name TheException e = new TheException(); throw e; 18

19 Rethrowing Exceptions We can also throw the exception for others to handle try { statements; } catch(theexception e) { // perform operations before // throwing the exception to others throw e; } 19

20 No Handling Exception 20

21 Not Handling Exception Properly 21

22 Handling Exception Properly 22

23 ArrayExceptionHandlingDemo Using try...catch to handle the case which the user does not provide the first program argument 23

24 MultiExceptionsHandlingDemo Have several catch clauses to catch multiple exceptions that may occur 24

25 Placing the Most Specific Exception First Need to specify the most specific exception first to avoid problem unreachable catch block of exception RuntimeException is a superclass of ArithmeticException and NumberFormatException 25

26 Finally Block The codes in the finally block must be executed no matter whether the exception occurs or not except that the program runs the statement System.exit(0) try { statements; } catch(theexception e) { handling e; } finally { finalstatements; } 26

27 FinallyDemo 27

28 FinallyDemo2 28

29 Inheritance and Exception Method with throws statement effects inheriting class Overridden method cannot use throws with more general exceptions than its superclass method More general exceptions are superclasses of more specific exceptions 29

30 InheritanceException (Incorrect) IOException is the superclass of FileNotFoundException while OverrideExceptionV2 is the subclass of ParentV2 30

31 InheritanceException (Correct) 31

32 Defining a New Exception Create a class that is a subclass of class Exception Implement the constructor of the new exception class since class Exception has these two constructors public Exception() public Exception(String s) 32

33 MyOwnException 33

34 Claiming and Throwing Exception Need to claim the exception at method header and throw it from the method body Claim the exception at method header in this syntax [return type] [method name]([method parameters]) throws [exception class name] Throw it from the method body in this syntax throw [the object in the exception class] 34

35 FileHandler with MyOwnException 35

36 Assertion Assertions are used to stop execution when "impossible" situations are detected assert statements provide an easy means to check for impossible conditions, with little or no runtime cost. The purpose of asserts is to detect programming errors, and they should not be used in the case of erroneous user input or actions 36

37 Check Possible Errors As Soon As Possible Discovering bugs as early as possible is good. Every program starts with bugs, and the debugging process is faster and simpler when the bugs are detected early. It's better to discover a problem at compile time than at run time, and it's better to discover a run time bug as early as possible 37

38 Two Forms of assert Statement 1. Usual form : an assert statement has two parts separated by a colon The first part is the boolean condition which must be true for execution to continue If it is false, an AssertionError is thrown, which terminates execution and display the message string The second part is the display message string when the assert statement is false Example: assert d!= 0: The divider must not be zero 38

39 Two Forms of assert Statement 2. Abbreviated form The simplest form the assert statement specifies only a boolean expression that must be true Example: assert (s!= null); if (s.getname().equals( Chanapat )) { System.out.println( Please pay attention in class ); } 39

40 Enabling Assertions at Runtime Assertion checking defaults to off at runtime JVM runtime enableassertions ( ea) option java ea MyProg 40

41 Asserting that the Object is Not Null 41

42 References David J. Eck, Introduction to Programming Using Java, Version 5.0, December con.com/itsg/virtualcd/java/archives/020 Thanisa Kruawaisayawan, Exception Handling, Kandaraj Piamrat, Exception and Handling and Assertion, learning.kku.ac.th 42

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