Download link: Java Exception Handling

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Cameron Scott
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1 What is an Exception? Java Exception Handling Error that occurs during runtime Exceptional event Cause normal program flow to be disrupted Java Exception Examples 1. Divide by zero errors 2. Accessing the elements of an array beyond its range 3. Invalid input 4. Hard disk crash 5. Opening a non-existent file 6. Heap memory exhausted /* Java Exception Example 1 */ /* Save with file name ExceptionExample1.java */ public class ExceptionExample1 The default exception handler provided by java runtime will prints out the exception description, stack trace (Hierarchy of methods where the exception occurred) and Causes the program to terminate. The Exception message looks like the following message. Exception in thread "main" java.lang.arithmeticexception: / by zero at ExceptionExample1.main(ExceptionExample1.java:8) What Happens 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. The runtime system searches the call stack for a method that contains an exception handler When an appropriate handler is found, the runtime system passes the exception to the handler 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 Benefits of Java Exception Handling Separating Error-Handling code from regular business logic code Propagating errors up the call stack Grouping and differentiating error types 1

2 Separating Error Handling Code from Regular Code Exceptions enable you to write the main flow of your code and to deal with the exceptional cases elsewhere. Note that exceptions don't spare you the effort of doing the work of detecting, reporting, and handling errors, but they do help you organize the work more effectively. Catching Exceptions in Java with -catch Syntax: < code to be monitored for exceptions >; catch(< ExceptionType1 > < ObjName >) < handler if ExceptionType1 occurs >;... catch (< ExceptionTypeN > < ObjName >) < handler if ExceptionTypeN occurs >; /* Java Exception Example 2 */ /* Save with file name ExceptionExample2.java */ public class ExceptionExample2 System.out.println(ae); Catching Exceptions in Java with multiple catch Multiple catches should be ordered from subclass to super class. /* Java Exception Example 3 */ /* Save with file name ExceptionExample3.java */ public class ExceptionExample3 int nos[] = new int[7]; //throw ArrayIndexOutOfBoundsException exception 2

3 nos[10] = 777; System.out.println("Arithmetic Exception : "+ae); catch(arrayindexoutofboundsexception arre) System.out.println("ArrayIndexOutOfBoundsException : "+arre); Catching Exceptions in Java with nested -catch /* Java Exception Example 4 */ /* Save with file name ExceptionExample4.java */ public class ExceptionExample4 int nos[] = new int[7]; //throw ArrayIndexOutOfBoundsException exception nos[10] = 777; catch(arrayindexoutofboundsexception arre) System.out.println("Arithmetic Exception : "+arre); System.out.println("No Exception Occurred in outer -block"); catch(exception ex) System.out.println("Some other Exception : "+ex); Catching Exceptions in java with finally The finally block code is always executed. /* Java Exception Example 5 */ /* Save with file name ExceptionExample5.java */ public class ExceptionExample5 System.out.println(" Block Executed"); 3

4 System.out.println("catch Block Executed"); finally System.out.println("finally block always Executed"); Throwing Exceptions with throw keyword Java allows you to throw exceptions (generate exceptions). An exception you throw is an object. /* Java Exception Example 6 */ /* Save with file name ExceptionExample6.java */ public class ExceptionExample6 int value = 0; if(value == 0) throw new Exception("Value Should not Zero!"); catch(exception ex) System.out.println(ex); Rules in Java 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 Checked exception Java compiler checks if the program either catches or lists the occurring checked exception If not, compiler error will occur Unchecked exceptions 4

5 1. Not subject to compile-time checking for exception handling 2. Built-in unchecked exception classes Error, RuntimeException and Their subclasses 3. Handling all these exceptions may make the program cluttered and may become a nuisance Creating Your Own Exception Class Steps to follow 1. Create a class that extends the RuntimeException or the Exception class 2. Customize the class i.e. Members and constructors may be added to the class How To Use Your Own Exceptions The following example shows how to use your own exceptions in java. /* Save with file name MyOwnException.java */ public class MyOwnException extends Exception String msg = null; public MyOwnException(String msg) super(msg); this.msg = msg; public String printwhathappened() if(msg == null) return ""; else return msg; /* Java Exception Example 7 */ /* Save with file name ExceptionExample7.java */ public class ExceptionExample7 int value = 0; if(value == 0) throw new MyOwnException("Value Should not Zero!"); catch(myownexception ex) System.out.println(ex.printWhatHappened()); 5

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

CSC System Development with Java. Exception Handling. Department of Statistics and Computer Science. Budditha Hettige CSC 308 2.0 System Development with Java Exception Handling Department of Statistics and Computer Science 1 2 Errors Errors can be categorized as several ways; Syntax Errors Logical Errors Runtime Errors