Chapter 11 Handling Exceptions and Events. Chapter Objectives
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1 Chapter 11 Handling Exceptions and Events Chapter Objectives Learn what an exception is See how a try/catch block is used to handle exceptions Become aware of the hierarchy of exception classes Learn about checked and unchecked exceptions Java Programming: From Problem Analysis to Program Design, 5e 2 1
2 Chapter Objectives (continued) Learn how to handle exceptions within a program Discover how to throw and rethrow an exception Learn how to handle events in a program Java Programming: From Problem Analysis to Program Design, 5e 3 Exception Definition: an occurrence of an undesirable situation that can be detected during program execution Examples Division by zero Trying to open an input file that does not exist An array index that goes out of bounds Java Programming: From Problem Analysis to Program Design, 5e 4 2
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, 5e 5 Java s Mechanism of Exception Handling When an exception occurs, an object of a particular exception class is created Java provides a number of exception classes to effectively handle certain common exceptions such as division by zero, invalid input, and file not found Division by zero is an arithmetic error and is handled by the class ArithmeticException Java Programming: From Problem Analysis to Program Design, 5e 6 3
4 Java s Mechanism of Exception Handling (continued) 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 classexception (directly or indirectly) is the superclass of all the exception classes in Java Java Programming: From Problem Analysis to Program Design, 5e 7 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, 5e 8 4
5 try/catch/finally Block (continued) 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, 5e 9 try/catch/finally Block (continued) Java Programming: From Problem Analysis to Program Design, 5e 10 5
6 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, 5e 11 try/catch/finally Block (continued) 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, 5e 12 6
7 Order of catch Blocks The heading of a catch block specifies the type of exception it handles A catch block can catch either all exceptions of a specific type or all types of exceptions A reference variable of a superclass type can point to an object of its subclass Java Programming: From Problem Analysis to Program Design, 5e 13 Order of catch Blocks (continued) If in the heading of a catch block you declare an exception using the classexception, then that catch block can catch all types of exceptions because the classexception is the superclass of all exception classes In a sequence of catch blocks following a try block, a catch block declaring an exception of a subclass type should be placed before catch blocks declaring exceptions of a superclass type Java Programming: From Problem Analysis to Program Design, 5e 14 7
8 Java Exception Hierarchy Java Programming: From Problem Analysis to Program Design, 5e 15 Java Exception Hierarchy (continued) Java Programming: From Problem Analysis to Program Design, 5e 16 8
9 Java Exception Hierarchy (continued) Java Programming: From Problem Analysis to Program Design, 5e 17 Java Exception Hierarchy (continued) Java Programming: From Problem Analysis to Program Design, 5e 18 9
10 Java Exception Hierarchy (continued) Java Programming: From Problem Analysis to Program Design, 5e 19 Java s Exception Class class Exception Subclass of class Throwable Superclass of classes designed to handle exceptions Various types of exceptions I/O exceptions Number format exceptions File not found exceptions Array index out of bounds exceptions Various exceptions categorized into separate classes and contained in various packages Java Programming: From Problem Analysis to Program Design, 5e 20 10
11 The class Exception and its Constructors Java Programming: From Problem Analysis to Program Design, 5e 21 Java Exception Classes Java Programming: From Problem Analysis to Program Design, 5e 22 11
12 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 23 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 24 12
13 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 25 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 26 13
14 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 27 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 28 14
15 Java Exception Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e 29 Checked Exceptions Definition: any exception that can be recognized by the compiler Examples FileNotFoundExceptions Java Programming: From Problem Analysis to Program Design, 5e 30 15
16 Unchecked Exceptions Definition: exceptions that cannot be recognized when the program compiles (must be checked for by programmer) Examples Division by zero Array index out of bounds Syntax ExceptionType1, ExceptionType2, and so on are names of exception classes Java Programming: From Problem Analysis to Program Design, 5e 31 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, 5e 32 16
17 The classexception and the Operator instanceof A reference of a superclass type can point to objects of its subclass You can determine if a reference variable points to an object using the operator instanceof You can combine catch blocks using this facility Java Programming: From Problem Analysis to Program Design, 5e 33 The classexception and the Operator instanceof (continued) try System.out.print("Line 4: Enter the " + "dividend: "); dividend = console.nextint(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextint(); System.out.println(); quotient = dividend / divisor; System.out.println("Line 11: Quotient = " + quotient); catch (Exception eref) if (eref instanceof ArithmeticException) System.out.println("Line 14: Exception " + eref.tostring()); else if (eref instanceof InputMismatchException) System.out.println("Line 16: Exception " + eref.tostring()); Java Programming: From Problem Analysis to Program Design, 5e 34 17
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 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 Rethrows the same exception for the calling environment to handle the exception Java Programming: From Problem Analysis to Program Design, 5e 35 Rethrowing and Throwing an Exception (continued) Useful when: Catch block catches exception but is unable to handle it Catch block decides exception should be handled by calling environment Allows programmer to provide exceptionhandling code in one place Syntax throw exceptionreference; Java Programming: From Problem Analysis to Program Design, 5e 36 18
19 Rethrowing and Throwing an Exception (continued) 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 imeref) System.out.println("Line 7: Exception " + imeref.tostring()); Java Programming: From Problem Analysis to Program Design, 5e 37 Rethrowing and Throwing an Exception (continued) 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 imeref) throw imeref; Java Programming: From Problem Analysis to Program Design, 5e 38 19
20 The Method printstacktrace Used to determine the order in which the methods were called and where the exception was handled Java Programming: From Problem Analysis to Program Design, 5e 39 The Method printstacktrace (continued) import java.io.*; public class PrintStackTraceExample1 public static void main(string[] args) try methoda(); catch (Exception e) System.out.println(e.toString() + " caught in main"); e.printstacktrace(); Java Programming: From Problem Analysis to Program Design, 5e 40 20
21 The Method printstacktrace (continued) public static void methoda() throws Exception methodb(); public static void methodb() throws Exception methodc(); public static void methodc() throws Exception throw new Exception("Exception generated " + "in method C"); Java Programming: From Problem Analysis to Program Design, 5e 41 The Method printstacktrace (continued) Sample Run java.lang.exception: Exception generated in method C caught in main java.lang.exception: Exception generated in method C at PrintStackTraceExample1.methodC (PrintStackTraceExample1.java:30) at PrintStackTraceExample1.methodB (PrintStackTraceExample1.java:25) at PrintStackTraceExample1.methodA (PrintStackTraceExample1.java:20) at PrintStackTraceExample1.main (PrintStackTraceExample1.java:9) Java Programming: From Problem Analysis to Program Design, 5e 42 21
22 Exception-Handling Techniques Terminate program Output appropriate error message upon termination Fix error and continue Repeatedly get user input Output appropriate error message until valid value is entered Log error and continue Write error messages to file and continue with program execution Java Programming: From Problem Analysis to Program Design, 5e 43 Creating Your Own Exception Classes Exception class you define extends class Exception or one of its subclasses Syntax to throw your own exception object: throw new ExceptionClassName(messageString); Java Programming: From Problem Analysis to Program Design, 5e 44 22
23 Creating Your Own Exception Classes (continued) public class MyDivisionByZeroException extends Exception public MyDivisionByZeroException() super("cannot divide by zero"); public MyDivisionByZeroException(String strmessage) super(strmessage); Java Programming: From Problem Analysis to Program Design, 5e 45 Event Handling Action events Handled by implementing interface ActionListener Window events Handled by implementing interface WindowListener Mouse events Handled by implementing interface MouseListener Key events Handled by implementing interface KeyListener Java Programming: From Problem Analysis to Program Design, 5e 46 23
24 Event Handling (continued) class WindowAdapter Implements interface WindowListener with empty bodies to methods class MouseAdapter Implements interface MouseListener with empty bodies to methods Java Programming: From Problem Analysis to Program Design, 5e 47 Registering Listeners Registering window listener object to GUI component Use method addwindowlistener Window listener object being registered is passed as parameter to method addwindowlistener Registering mouse listener object to GUI component Use method addmouselistener Mouse listener object being registered is passed as parameter to method addmouselistener Java Programming: From Problem Analysis to Program Design, 5e 48 24
25 Event Handling (continued) Java Programming: From Problem Analysis to Program Design, 5e 49 Event Handling (continued) Java Programming: From Problem Analysis to Program Design, 5e 50 25
26 Event Handling (continued) Java Programming: From Problem Analysis to Program Design, 5e 51 Programming Example: Calculator Java Programming: From Problem Analysis to Program Design, 5e 52 26
27 Chapter Summary Exception Definition Handling exceptions within a program try/catch/finally block Order of catch blocks Using try/catch blocks in a program The classexception and the operator instanceof Rethrowing and throwing an exception Exception Hierarchy Classes Java Programming: From Problem Analysis to Program Design, 5e 53 Chapter Summary (continued) Checked and unchecked exceptions The method printstacktrace Exception-handling techniques Terminate program Fix error and continue Log error and continue Creating your own exception classes Event handling Java Programming: From Problem Analysis to Program Design, 5e 54 27
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