Sheet 3 HANDLING EXCEPTIONS Advanced Programming using Java By Nora Alaqeel

Java Programming: From Problem Analysis to Program Design, 4e 2 Exception Definition: an occurrence of an undesirable situation that can be detected during program execution Examples  Division by zero  Invalid input errors  An array index that goes out of bounds

Java Programming: From Problem Analysis to Program Design, 4e 3 Handling Exception within a Program Can use an if statement to handle an exception However, suppose that division by zero occurs in more than one place within the same block  In this case, using if statements may not be the most effective way to handle the exception

Java Programming: From Problem Analysis to Program Design, 4e 4 Java’s Mechanism of Exception Handling Java provides a number of exception classes to effectively handle certain common exceptions such as division by zero, invalid input, and file not found When an exception occurs, an object of a particular exception class is created

Java Programming: From Problem Analysis to Program Design, 4e 5 When a division by zero exception occurs, the program creates an object of the class ArithmeticException When a Scanner object is used to input data into a program, any invalid input errors are handled using the class InputMismatchException The class Exception is the superclass of all the exception classes in Java Java’s Mechanism of Exception Handling

Java Programming: From Problem Analysis to Program Design, 4e 6 Java Exception Hierarchy

Java Programming: From Problem Analysis to Program Design, 4e 7 Java Exception Hierarchy

Java Programming: From Problem Analysis to Program Design, 4e 8 Java Exception Hierarchy

Java Programming: From Problem Analysis to Program Design, 4e 9 try / catch / finally Block Statements that might generate an exception are placed in a try block The try block might also contain statements that should not be executed if an exception occurs The try block is followed by zero or more catch blocks A catch block specifies the type of exception it can catch and contains an exception handler

Java Programming: From Problem Analysis to Program Design, 4e 10 try / catch / finally Block The last catch block may or may not be followed by a finally block Any code contained in a finally block always executes, regardless of whether an exception occurs, except when the program exits early from a try block by calling the method System.exit If a try block has no catch block, then it must have the finally block

Java Programming: From Problem Analysis to Program Design, 4e 11 try / catch / finally Block

Java Programming: From Problem Analysis to Program Design, 4e 12 try / catch / finally Block (continued) If no exception is thrown in a try block, all catch blocks associated with the try block are ignored and program execution resumes after the last catch block If an exception is thrown in a try block, the remaining statements in the try block are ignored - The program searches the catch blocks in the order in which they appear after the try block and looks for an appropriate exception handler

Java Programming: From Problem Analysis to Program Design, 4e 13 try / catch / finally Block If the type of the thrown exception matches the parameter type in one of the catch blocks, the code of that catch block executes and the remaining catch blocks after this catch block are ignored If there is a finally block after the last catch block, the finally block executes regardless of whether an exception occurs

Java Programming: From Problem Analysis to Program Design, 4e 14 Order of catch Blocks The heading of a catch block specifies the type of exception it handles. If in the heading of a catch block you declare an exception using the class Exception, then that catch block can catch all types of exceptions because the class Exception is the superclass of all exception classes

Java Programming: From Problem Analysis to Program Design, 4e 15 Exception-Handling Techniques Terminate program  Output appropriate error message upon termination. Log error and continue  Display error messages and continue with program execution Fix error and continue  Repeatedly get user input  Output appropriate error message until valid value is entered

Java Programming: From Problem Analysis to Program Design, 4e 16 Unchecked Exceptions Syntax: ExceptionType1, ExceptionType2, and so on are names of exception classes

Java Programming: From Problem Analysis to Program Design, 4e 17 Exceptions Example Code public static void exceptionMethod() throws InputMismatchException,FileNotFoundException { //statements } The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException

Java Programming: From Problem Analysis to Program Design, 4e 18 Rethrowing and Throwing an Exception When an exception occurs in a try block, control immediately passes to one of the catch blocks; typically, a catch block does one of the following:  Completely handles the exception.  Rethrows the same exception for the calling environment to handle the exception.  Partially processes the exception; in this case, the catch block either rethrows the same exception or throws another exception for the calling environment to handle the exception

Java Programming: From Problem Analysis to Program Design, 4e 19 Rethrowing and Throwing an Exception Syntax throw exceptionReference;

Java Programming: From Problem Analysis to Program Design, 4e 20 import java.util.*; public class RethrowExceptionExmp1 { static Scanner console = new Scanner(System.in); public static void main(String[] args) { int number; try { number = getNumber(); System.out.println("Line 5: number = ”+ number); } catch (InputMismatchException ex) { System.out.println("Line 7: Exception ” + ex.toString()); } Rethrowing and Throwing an Exception

Java Programming: From Problem Analysis to Program Design, 4e 21 public static int getNumber()throws InputMismatchException { int num; try { System.out.print(“Line 11: Enter an integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException e) { throw e; } Rethrowing and Throwing an Exception

Trace By Nora Alaqeel