CSC System Development with Java. Exception Handling. Department of Statistics and Computer Science. Budditha Hettige

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1 CSC System Development with Java Exception Handling Department of Statistics and Computer Science 1

2 2

3 Errors Errors can be categorized as several ways; Syntax Errors Logical Errors Runtime Errors 3

4 Syntax Errors Error in the syntax of a sequence of characters or tokens that is intended to be written in a particular programming language All syntax errors can be reliably detected until run-time Many Syntax errors can be detected at compile-time 4

5 Common Java syntax errors Capitalization of key words Line xx: class or interface declaration expected Writing a string over a new line Line xx: ';' expected Missing brackets in a no-argument message Line xx: Invalid expression statement Forgetting to import a package Line nn: Method yyyy not found in class xxxx. 5

6 Syntax Error correction 1. Go to the error line 2. Identify the error 3. Correct the error Error description Error line You must have a good working knowledge of error messages to discover the cause of the error 6

7 Logical Errors Errors that indicate the logic used when coding the program failed to solve the problem You do not get error messages with logic errors Your only clue to the existence of logic errors is the production of wrong solutions 7

8 Common Logic Errors in Java Using a variable before it is given a value int x; x = x + 1; System.out.println("X = " + x); Misplaced Semi-colon (usually with a loop or if statement) if ( x > y) ; System.out.println("X is bigger"); Confusing the equivalence operator == with the assignment operator = 8

9 Logical error correction Debugging can be used to find what the logical errors in your program Debug line Results 9

10 Runtime Errors low-level errors dereference of a null pointer out-of-bounds array access divide by zero attempt to open a non-existent file for reading bad cast (e.g., casting an Object that is actually a Boolean to Integer) higher-level call to Stack's "pop" method for an empty stack call to "factorial" function with a negative number call to List's nextelement method when hasmoreelements is false 10

11 Runtime Errors contd.. Errors can arise due to User error providing a bad file name or a poorly formatted input file Programmer error These errors should be detected as early as possible to provide good feedback. 11

12 Exceptions can occur at many levels Hardware/operating system level. Arithmetic exceptions; divide by 0, under/overflow. Memory access violations; segfault, stack over/underflow. Language level. Type conversion; illegal values, improper casts. Bounds violations; illegal array indices. Bad references; null pointers. Program level. User defined exceptions. 12

13 Ways to handle errors Write an error message and quit. This doesn't provide any recovery Return a special value to indicate that an error occurred calling code check for an error. This can reduce the efficiency of the code Use a reference parameter or a global variable to hold an error code Use exceptions. This seems to be the method of choice for modern programming languages. 13

14 What Is an Exception? An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. Example Divide by zero errors Accessing the elements of an array beyond its range Invalid input Hard disk crash Opening a non-existent file Heap memory exhausted 14

15 Example class DivByZero public static void main(string args[]) System.out.println(3/0); System.out.println( Pls. print me. ); Error Message Exception in thread "main" Divide by zero java.lang.arithmeticexception: / by zero at DivByZero.main(DivByZero.java:3) Where the error is occured 15

16 Default exception handler 1. Provided by Java runtime 2. Prints out exception description 3. Prints the stack trace 4. Hierarchy of methods where the exception occurred 5. Causes the program to terminate Exception in thread "main" java.lang.arithmeticexception: / by zero at DivByZero.main(DivByZero.java:3 16

17 What Happens When an Exception Occurs? 17

18 When an Exception Occurs? When an exception occurs within a method, the method creates an exception object and hands it off to the runtime system Creating an exception object and handing it to the runtime system is called throwing an exception Exception object contains information about the error, including its type and the state of the program when the error occurred 18

19 When an Exception Occurs? (1) The runtime system searches the call stack for a method that contains an exception handler 19

20 When an Exception Occurs? (2) When an appropriate handler is found, the runtime system passes the exception to the handler An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler The exception handler chosen is said to catch the exception. If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, the runtime system (and, consequently, the program) terminates and uses the default exception handler 20

21 Searching the Call Stack for an Exception Handler 21

22 How exception handler works? Exception "thrown" here Exception handler Exception handler Thrown exception matched against first set of exception handlers If it fails to match, it is matched against next set of handlers, etc. If exception matches none of handlers, program is abandoned 22

23 Advantages Separating Error-Handling code from regular business logic code Propagating errors up the call stack Grouping and differentiating error types 23

24 Java Exception Handling A method can duck any exceptions thrown within it, thereby allowing a method farther up the call stack to catch it. Hence, only the methods that care about errors have to worry about detecting errors Any checked exceptions that can be thrown within a method must be specified in its throws clause 24

25 Grouping and Differentiating Error Types Because all exceptions thrown within a program are objects, the grouping or categorizing of exceptions is a natural outcome of the class hierarchy An example of a group of related exception classes in the Java platform are those defined in java.io IOException and its descendants IOException is the most general and represents any type of error that can occur when performing I/O Its descendants represent more specific errors. For example, FileNotFoundException means that a file could not be located on disk. 25

26 Catching Exceptions try <code to be monitored for exceptions> catch (<ExceptionType1> <ObjName>) <handler if ExceptionType1 occurs>... catch (<ExceptionTypeN> <ObjName>) <handler if ExceptionTypeN occurs> 26

27 Example 27

28 Multiple catch 28

29 How try and catch works? NO Match Try Catch Catch Statement after the last catch Leaves the method 29

30 Example 1. Execute This 2. Goto this catch() 3. Execute This 30

31 No any catch for matching??? 31

32 Catching Exceptions with finally Syntax: try <code to be monitored for exceptions> catch (<ExceptionType1> <ObjName>) <handler if ExceptionType1 occurs>... finally <code to be executed before the try block ends> 32

33 Catching Exceptions the finally Keyword Block of code is always executed despite of different scenarios: Forced exit occurs using a return, a continue or a break statement Normal completion Caught exception thrown Exception was thrown and caught in the method Uncaught exception thrown Exception thrown was not specified in any catch block in the method 33

34 How try and catch finaly works? Try Catch Catch NO Match Finally() Statement after the last catch 34

35 Sequence of Events for finally clause Preceding step try block throw statement matching catch finally next step 35

36 Throwing Exceptions Java allows you to throw exceptions (generate exceptions) throw <exception object>; An exception you throw is an object You have to create an exception object in the same way you create any other object Example: throw new ArithmeticException( testing... ); 36

37 Rules in Exception Handling A method is required to either catch or list all exceptions it might throw Except for Error or RuntimeException, or their subclasses If a method may cause an exception to occur but does not catch it, then it must say so using the throws keyword Applies to checked exceptions only Syntax: <type> <methodname> (<parameterlist>) throws <exceptionlist> <methodbody> 37

38 Sequence of Events for No throw Preceding step try block throw statement unmatched catch matching catch unmatched catch next step 38

39 Sequence of Events for throw Preceding step try block throw statement unmatched catch matching catch unmatched catch next step 39

40 The exception hierarchy 40

41 The Error and Exception Classes Throwable class Root class of exception classes Immediate subclasses Error Exception Exception class Conditions that user programs can reasonably deal with Usually the result of some flaws in the user program code Examples Division by zero error Array out-of-bounds error 41

42 The Error and Exception Classes Error class Used by the Java run-time system to handle errors occurring in the run-time environment Generally beyond the control of user programs Examples Out of memory errors Hard disk crash 42

43 User define Exception 43

44 Creating Your Own Exception Class Steps to follow Create a class that extends the RuntimeException or the Exception class Customize the class Members and constructors may be added to the class Example: class HateStringExp extends RuntimeException /* some code */ 44

45 Sample Code class TestHateString public static void main(string args[]) String input = "invalid input"; try if (input.equals("invalid input")) throw new HateStringExp(); System.out.println("Accept string."); catch (HateStringExp e) System.out.println("Hate string! ); class HateStringExp extends RuntimeException /* some code */ 45

46 Example public class CustomExceptionTest public static void main(string[] args) throws Exception int age = getage(); if (age < 0) throw new NegativeAgeException(age); Else System.out.println("Age entered is " + age); static int getage() return -10; 46

47 Exception class public class NegativeAgeException extends Exception private int age; public NegativeAgeException(int newage) age = newage; public String tostring() return "Age cannot be negative" + " " +age ; 47

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