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Chapter 11

Chapter 11. Exceptions and Input/Output Operations. Topics. Exception Handling Using try and catch Blocks Catching Multiple Exceptions User-Defined Exceptions The java.io Package Reading from the Java Console Reading and Writing Text Files

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Chapter 11

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  1. Chapter 11 Exceptions and Input/Output Operations

  2. Topics • Exception Handling • Using try and catch Blocks • Catching Multiple Exceptions • User-Defined Exceptions • The java.io Package • Reading from the Java Console • Reading and Writing Text Files • Reading Structured Text Files Using StringTokenizer • Reading and Writing Objects to a File

  3. Exceptions • Illegal operations at run time can generate an exception, for example: • ArrayIndexOutOfBoundsException • ArithmeticException • NullPointerException • InputMismatchException • NumberFormatException

  4. Exceptions • An exception is an object that describes an unusual or erroneous situation • Exceptions are thrown by a program, and may be caught and handled by another part of the program • A program can be separated into a normal execution flow and an exception execution flow • An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

  5. Handling Exceptions • In a program without a Graphical User Interface, exceptions cause the program to terminate. • With this code: 12 String s = JOptionPane.showInputDialog( null, 13 "Enter an integer" ); … 17 int n = Integer.parseInt( s ); • If the user enters "a", we get this exception: • See Example 11.1 DialogBoxInput.java

  6. Exception Handling • Java has a predefined set of exceptions and errors that can occur during execution • A program can deal with an exception in one of three ways: • ignore it • handle it where it occurs • handle it an another place in the program • The manner in which an exception is processed is an important design consideration

  7. Exception Handling • If an exception is ignored by the program, the program will terminate abnormally and produce an appropriate message • The message includes a call stack trace that: • indicates the line on which the exception occurred • shows the method call trail that lead to the attempted execution of the offending line

  8. Handling Exceptions • We don't want invalid user input to terminate the program! • It is better to detect the problem and reprompt the user for the input. • We can intercept and handle some of these exceptions using try and catch blocks. • Inside the try block, we put the code that might generate an exception. • Inside catch blocks, we put the code to handle any exceptions that could be generated.

  9. The try Statement • 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

  10. Minimum try/catch Syntax try { // code that might generate an exception } catch( ExceptionClass exceptionObjRef ) { // code to recover from the exception } • If an exception occurs in the try block, the try block terminates and control jumps immediately to the catch block. • If no exceptions are generated in the try block, the catch block is not executed.

  11. The Exception Class Hierarchy • 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

  12. Checked and Unchecked Exceptions • Java distinguishes between two types of exceptions: • Unchecked exceptions are those that are subclasses of Error or RuntimeException • It is not mandatory to use try and catch blocks to handle these exceptions. • Checked exceptions are any other exceptions. • Code that might generate a checked exception must be put inside a try block. Otherwise, the compiler will generate an error.

  13. Checked Exceptions • An exception is either checked or unchecked • A checked exception either must be caught by a method, or must be listed in the throws clause of any method that may throw or propagate it • A throws clause is appended to the method header • The compiler will issue an error if a checked exception is not caught or asserted in a throws clause

  14. Unchecked Exceptions • An unchecked exception does not require explicit handling, though it could be processed that way • The only unchecked exceptions in Java are objects of type RuntimeException or any of its descendants • Errors are similar to RuntimeException and its descendants in that: • Errors should not be caught • Errors do not require a throws clause

  15. Exception Class Methods • Inside the catch block, you can call any of these methods of the Exception class:

  16. Catching a NumberFormatException int n = 0; // declare and initialize variable String s = JOptionPane.showInputDialog( null, "Enter an integer" ); try { n = Integer.parseInt( s ); System.out.println( "You entered " + n ); } catch ( NumberFormatException nfe ) { System.out.println( "Incompatible data." ); } • See Example 11.2 DialogBoxInput.java

  17. Initializing Variables for try/catch Blocks • Notice that we declare and initialize the input variable before we enter the try block. If we do not initialize the variable and then try to accessit after the try/catch blocks, we will receive the following compiler error: variable n might not have been initialized The error indicates that the only place where n is assigned a value is in the try block. If an exception occurs, the try block will be interrupted and we might not ever assign n a value. • Initializing the value before entering the try block solves this problem.

  18. Recovering From an Exception • The previous code just printed a message when the exception occurred. • To continue processing and reprompt the user for good input, we can put the try and catch blocks inside a do/while loop.

  19. int n = 0; boolean goodInput = false; // flag variable String s = JOptionPane.showInputDialog( null, "Enter an integer" ); do { try { n = Integer.parseInt( s ); goodInput = true; // executed if no exception } catch ( NumberFormatException nfe ) { s = JOptionPane.showInputDialog( null, s + " is not an integer. " + "Enter an integer" ); } } while ( ! goodInput );

  20. Software Engineering Tip Write code to catch and handle exceptions generated by invalid user input. Although the methods of the Exception class are good debugging tools, they are not necessarily appropriate to use in the final version of a program. Always try to write code that is user-friendly.

  21. Catching Multiple Exceptions • If the code in the try block might generate multiple, different exceptions, we can provide multiple catch blocks, one for each possible exception. • When an exception is generated,the JVM searches the catch blocks in order.The first catch block with a parameter that matches the exception thrownwill execute; any remaining catch blocks will be skipped.

  22. catch Block Order • An exception will match any catch block with a parameter that names any of its superclasses. • For example, a NumberFormatException will match a catch block with a RuntimeException parameter. • All exceptions will match a catch block with an Exception parameter. • Thus, when coding several catch blocks, arrange the catch blocks with the specialized exceptions first, followed by more general exceptions.

  23. Exception Propagation • An exception can be handled at a higher level if it is not appropriate to handle it where it occurs • Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method • A try block that contains a call to a method in which an exception is thrown can be used to catch that exception

  24. The finally Block • Optionally, you can follow the catch blocks with a finally block. • The statements in the finally clause always are executed • The finally block will be executed whether or not an exception occurs. Thus: • if an exception occurs, the finally block will be executed when the appropriate catch block finishes executing • if no exception occurs, the finally block will be executed when the try block finishes • For example, a finally block might be used to close an open file. We demonstrate this later.

  25. Full try/catch/finally Syntax try { //code that might generate an exception } catch( Exception1Class e1 ) { //code to handle an Exception1Class exception } … catch( ExceptionNClass eN ) { //code to handle an ExceptionNClass exception } finally { //code to execute in any case }

  26. Catching Multiple Exceptions • We can write a program that catches several exceptions. • For example, we can prompt the user for a divisor. • If the input is not an integer, we catch the NumberFormatException and reprompt the user with an appropriate message. • If the input is 0, we catch an ArithmeticException when we attempt to divide by 0, and reprompt the user with an appropriate message.

  27. Example 11.4 // declare and initialize variables int divisor = 0; int quotient = 0; int dividend = 100; // initialize flag variable boolean goodInput = false; // prompt for input String s = JOptionPane.showInputDialog( null, "Enter an integer divisor" ); do { try { // attempt to convert the String to an int divisor = Integer.parseInt( s ); // attempt the division quotient = dividend / divisor; goodInput = true; } catch ( NumberFormatException nfe ) { s = JOptionPane.showInputDialog( null, s + " is not an integer. “ + "Enter an integer divisor" ); } catch ( ArithmeticException ae ) { s = JOptionPane.showInputDialog( null, "Divisor cannot be 0. “ + "Enter an integer divisor" ); } } while ( !goodInput ); JOptionPane.showMessageDialog( null, "The result is " + quotient );

  28. User-Defined Exceptions • We can design our own exception class. • Suppose we want to design a class encapsulating email addresses (EmailAddress class). • For simplicity, we say that a legal email address is a String containing the @ character. • Our EmailAddress constructor will throw an exception if its email address argument is illegal.   • To do this, we design an exception class named IllegalEmailException.

  29. User-Defined Exception • Java has an IllegalArgumentException class, so our IllegalEmailException class can be a subclass of the IllegalArgumentException class. • By extending the IllegalArgumentException class: • we inherit the functionality of an exception class, which simplifies our coding of the exception • we can associate a specific error message with the exception

  30. Extending an Existing Exception • We need to code only the constructor, which accepts the error message as a String. • General pattern: public class ExceptionName extends ExistingExceptionClassName { public ExceptionName( String message ) { super( message ); } } • See Example 11.5 IllegalEmailException.java

  31. Example 11.5 public class IllegalEmailException extends IllegalArgumentException { public IllegalEmailException( String message ) { super( message ); } }

  32. The throw Statement • Exceptions are thrown using the throw statement • Usually a throw statement is executed inside an if statement that evaluates a condition to see if the exception should be thrown

  33. Throwing an Exception • The pattern for a method that throws a user-defined exception is: accessModifier returnType methodName( parameters ) throws ExceptionName { if( parameter list is legal ) process the parameter list else throw new ExceptionName( "Message here" ); } • The message passed to the constructor identifies the error we detected. In a client's catch block, the getMessage method will retrieve that message.

  34. Example 11.6 public class EmailAddress { public static final char AT_SIGN = '@'; private String email; public EmailAddress( String newEmail ) throws IllegalEmailException { if ( newEmail.indexOf( AT_SIGN ) != - 1 ) email = newEmail; else throw new IllegalEmailException ( "Email address does not contain " + AT_SIGN ); } public String getHost( ) { int index = email.indexOf( AT_SIGN ); return email.substring( index + 1, email.length( ) ); } }

  35. Example 11.7 public class EmailChecker { public static void main( String [ ] args ) { Scanner scan = new Scanner( System.in ); System.out.print( "Enter your email address > " ); String myEmail = scan.next( ); try { EmailAddress address = new EmailAddress( myEmail ); System.out.println( "Your host is " + address.getHost( ) ); } catch( IllegalEmailException iee ) { System.out.println( iee.getMessage( ) ); } } }

  36. I/O Exceptions • Let's examine issues related to exceptions and I/O • A stream is a sequence of bytes that flow from a source to a destination • In a program, we read information from an input stream and write information to an output stream • A program can manage multiple streams simultaneously

  37. Standard I/O • There are three standard I/O streams: • standard output – defined by System.out • standard input – defined by System.in • standard error – defined by System.err • We use System.out when we execute println statements • System.out and System.err typically represent a particular window on the monitor screen • System.in typically represents keyboard input, which we've used many times with Scanner objects

  38. The IOException Class • Operations performed by some I/O classes may throw an IOException • A file might not exist • Even if the file exists, a program may not be able to find it • The file might not contain the kind of data we expect • An IOException is a checked exception

  39. Selected Input Classes in the java.io Package

  40. Hierarchy for Input Classes

  41. Selected java.io Output Classes

  42. Hierarchy for Output Classes

  43. Reading from the Java Console • System.in is the default standard input device, which is tied to the Java Console. • We have read from the console by associating a Scanner object with the standard input device: Scanner scan = new Scanner( System.in ); • We can also read from the console using these subclasses of Reader: • InputStreamReader • BufferedReader, uses buffering (read-ahead) for efficient reading

  44. Opening an InputStream • When we construct an input stream or output stream object, the JVM associates the file name, standard input stream, or standard output stream with our object. This is opening the file. • When we are finished with a file, we optionally call the close method to release the resources associated with the file. • In contrast, the standard input stream (System.in), the standard output stream (System.out), and the standard error stream (System.err) are open when the program begins.They are intended to stay open and should not be closed.

  45. Software Engineering Tip Calling the close method is optional. When the program finishes executing, all the resources of any unclosed files are released. It is good practice to call the close method, especially if you will be opening a number of files (or opening the same file multiple times.) Do not close the standardinput, output, or error devices, however. They are intended to remain open.

  46. Console Input Class Constructors

  47. Methods of the BufferedReader Class • Because an IOException is a checked exception, we must call these methods within a try block.

  48. Example 11.8 import java.io.InputStreamReader; import java.io.BufferedReader; import java.io.IOException; public class ConsoleInput { public static void main( String [ ] args ) { String stringRead = ""; try { // set up for input InputStreamReader isr = new InputStreamReader( System.in ); BufferedReader br = new BufferedReader( isr ); // prompt and read input System.out.println( "Please enter a phrase or sentence > " ); stringRead = br.readLine( ); } catch( IOException ioe ) { System.out.println( ioe.getMessage( ) ); } // echo data read System.out.println( "The string read was " + stringRead ); } }

  49. Writing Text Files • In Chapter 5 we explored the use of the Scanner class to read input from a text file • Let's now examine other classes that let us write data to a text file • The FileWriter class represents a text output file, but with minimal support for manipulating data • Therefore, we also rely on PrintStream objects, which have print and println methods defined for them

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