Java C5. Exception Handling doc

1 Chapter 5. Exception Handling ITSS Java Programming CAO Tuan-Dung, HUT 2 What is an exception?  Exceptional event  Error that occurs during runtime  Cause normal program flow to be disrupted  Examples  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 3 Example // divide by zero class DivByZero { public static void main(String args[]) { System.out.println(3/0); System.out.println(“Pls. print me.”); } } // array out of bound int array[] = new int[2]; array[0] = 1; array[1] = 2; array[2] = 3; // execute method of object that does not exist String s = null; s.length(); Exception Object ArithmeticException Exception Object ArrayIndexOutOfBoundException Exception Object NullPointerException 4 Default exception handling  Displays this error message Exception in thread "main" java.lang.ArithmeticException: / by zero at DivByZero.main(DivByZero.java:3)  Default exception handler  Provided by Java runtime  Prints out exception description  Prints the stack trace  Hierarchy of methods where the exception occurred  Causes the program to terminate 5 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  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 6 Lab: Exception  Run this program and observe the exceptions exceptions of the type that is different according to the specified value. prompt> java Exceptions <ExceptionNO> ExceptionNO:1→ArrayIndexOutOfBoundsEx ception ExceptionNO:2→NullPointerException ExceptionNO:3→ArithmeticException 7 Exception.java class Exceptions { public static void main(String args[])throws Exception { if(args.length != 1) { System.out.println("Usage : java Exceptions <ExceptionNO>"); System.out.println("ExceptionNO:1→ArrayIndexOutOfBoundsException"); System.out.println("ExceptionNO:2→NullPointerException"); System.out.println("ExceptionNO:3→ArithmeticException"); System.exit(1); } int exceptionNo = Integer.parseInt( args[0] ); // Generate exception that is different according to the input value switch(exceptionNo) { case 1: // ArrayIndexOutOfBoundsException int array[] = new int[3]; array[3] = 100; break; 8 Exception.java case 2: // NullPointerException String s = null; int length = s.length(); break; case 3: // ArithmeticException int x = 100; int y = 0; int z = x / y; break; default: System.out.println(“It doesn‘t match with ExceptionNO."); } } } 9 System exceptions  Exceptions provided by Java language 10 Exception handling  To handle an exception in a program, the line that throws the exception is executed within a try block  A try block is followed by one or more catch clauses  Each catch clause has an associated exception type and is called an exception handler  When an exception occurs, processing continues at the first catch clause that matches the exception type [...]... ExceptionNO:1→ArrayIndexOutOfBoundsException ExceptionNO:2→NullPointerException ExceptionNO:3→ArithmeticException 12 ExceptionsTryCatch .java class ExceptionsTryCatch { public static void main(String args[])throws Exception { if(args.length != 1) { System.out.println("Usage : java Exceptions "); System.out.println("ExceptionNO:1→ArrayIndexOutOfBoundsException"); System.out.println("ExceptionNO:2→NullPointerException");.. .Exception handling Syntax: try { } catch ( ) { } } catch ( ) { } 11 Example  This program shows how the exceptions in previous example are caught and processed  Usage: prompt> java ExceptionsTryCatch ExceptionNO:1→ArrayIndexOutOfBoundsException... LimitException extends Exception{ Throwable // Constructor public LimitException(String message){ super(message); } :// Addition of necessary attribute and method Exception } XXException XX 2Exception XX 1Exception 16 Error processing in User define exception  Flow of exception handling     Execute a processing that has the possibility to generate an error A systematic or operational error occurs An exception. .. (4) Exception object is notified and error processing is done (2) Error occurred Ex ce System.out.println(e.getMessage()) ; Exception Object (3) Exception object is generated 18 Method that throws exception [Modifier] Return value type Method name(Argument…) throws Exception Class, { throw Exception Object declare the exception type that generating Exception Object } the method might throw Exception handling. .. System.out.println("ExceptionNO:2→NullPointerException"); System.out.println("ExceptionNO:3→ArithmeticException"); System.exit(1); } int exceptionNo = Integer.parseInt( args[0] ); try { // Generate exception that is different according to the input value switch(exceptionNo) { case 1: // ArrayIndexOutOfBoundsException int array[] = new int[3]; array[3] = 100; break; 13 ExceptionsTryCatch .java case 2: // NullPointerException String s = null; int... definition Exception  Classes that define exceptions are related by inheritance, forming an exception class hierarchy  All error and exception classes are descendents of the Throwable class  A programmer can define an exception by extending the Exception class or one of its descendants  The parent class used depends on how the new exception will be used 15 User definition Exception public class LimitException... // ArithmeticException int x = 100; int y = 0; int z = x / y; break; default: System.out.println(“It doesn‘t match with ExceptionNO."); } }catch(ArrayIndexOutOfBoundsException e){ System.out.println(“ArrayIndexOutOfBoundsException is caught "); }catch(NullPointerException e){ System.out.println(“NullPointerException is caught."); catch(ArithmeticException e){ System.out.println(“ArithmeticException is... generated The exception object is notified,and the error processing according to the exception object is executed 17 Error processing in User define exception try { (1) Execute a processing that has the possibility to generate an error obj.withdraw(10000000); } catch (LimitException e){ public void withdraw(long money) throws LimitException, InsufficiencyException { if (money>=1000000) throw new LimitException... System.out.println("Balance : " + balance); } } 23 Solution: Bank .java // Exception of exceeded withdrawal amount limit class LimitException extends Exception { public LimitException(String message) { super(message); } } // Insufficient balance exception class InsufficiencyException extends Exception { public InsufficiencyException(String message) { super(message); } } 24 Wrapper Class  Some Facts:  Primitive data types are... LimitException, InsufficiencyException { if(money >= 1000000) { throw new LimitException(“Withdrawal amount limit is exceeded"); } if(money > balance) { throw new InsufficiencyException("Insufficient balance"); } balance -= money; } // Display of account information public void print() { System.out.println("ID : " + id); System.out.println("Balance : " + balance); } } 23 Solution: Bank .java // Exception . null; s.length(); Exception Object ArithmeticException Exception Object ArrayIndexOutOfBoundException Exception Object NullPointerException 4 Default exception handling  Displays this error message Exception. <ExceptionNO> ExceptionNO:1→ArrayIndexOutOfBoundsEx ception ExceptionNO:2→NullPointerException ExceptionNO:3→ArithmeticException 7 Exception .java class Exceptions { public static void main(String args[])throws Exception { if(args.length. match with ExceptionNO."); } } } 9 System exceptions  Exceptions provided by Java language 10 Exception handling  To handle an exception in a program, the line that throws the exception