10.2 – Zero.java

1. 10.2 – Zero.java //******************************************************************** // Zero.java Java Foundations // Demonstrates an uncaught exception. //******************************************************************** public class Zero { //----------------------------------------------------------------- // Deliberately divides by zero to produce an exception. //----------------------------------------------------------------- public static void main (String[] args) { int numerator = 10; int denominator = 0; System.out.println ("Before the attempt to divide by zero."); System.out.println (numerator / denominator); System.out.println ("This text will not be printed."); } }

2. 10.1 – Exceptions vs Errors • An exceptionis an object describing unusual or erroneous situation • Division by 0 in computing expression (ArithmeticException) • Array index out of bounds (IndexOutOfBoundsException) • Null pointer cannot be followed (NullPointerException) • I/O problems (e.g., no space on disk to save file, file not found: IOException) • No permissions to do something (e.g., to save a file: FileNotFoundException) • Exceptions are “thrown”by a program, and may be “caught”and “handled”by another part of the program • (An erroris also an object, but it represents a unrecoverable situation and should not be caught)

3. 10.2 – ZeroPlus.java //******************************************************************** // ZeroPlus.java // Demonstrates an exception caught and HANDLED. //******************************************************************** public class ZeroPlus { //----------------------------------------------------------------- // Deliberately divides by zero to catch an exception. //----------------------------------------------------------------- public static void main (String[] args) { int numerator = 10; int denominator = 0; System.out.println ("Before the attempt to divide by zero."); try{ System.out.println (numerator / denominator); }catch (ArithmeticException arex) { System.out.println ("Attempt to divide by zero. Not good."); } System.out.println ("This text will NOW be printed."); } }

4. 10.3 – The try Statement • To handle an exception, the line that throws the exception is executed within a try block • A try block is followed by one or more catch clauses • When an exception occurs, processing continues at the first catch clause that matches the exception type // here is code that // should generate no exceptions try {// code to monitor // several possible things // that can go wrong // goes here}catch (ExceptionTypeA ex) {//handler for ExceptionTypeA}catch (ExceptionTypeB ex) {//handler for ExceptionTypeB}// after a catch, continue here

5. Using Exceptions in an “exceptional” way ;-) D I S T Z // Counts the number of product codes that are entered // with a zone of R and district greater than 2000. try { zone = code.charAt(9); district = Integer.parseInt(code.substring(3, 7)); valid++; if (zone == 'R' && district > 2000) banned++; } catch (StringIndexOutOfBoundsException exception) { System.out.println ("Improper code length: " + code); } catch (NumberFormatException exception) { System.out.println ("District is not numeric: " + code); }

6. 10.5 EC Hierarchy

7. Asserted in the throws clause //******************************************************************** // TestData.java Java Foundations // Demonstrates asserted clause. //******************************************************************** import java.util.Random; import java.io.*; public class TestData { //----------------------------------------------------------------- // Will read/write to a file and things can go bad. //----------------------------------------------------------------- public static void main (String[] args) throws IOException { String file = "test.dat"; // more on FileIO shortly (more…)

8. 10.5 – An exception is either checkedor unchecked • An unchecked exception does not require explicit handling • The only unchecked exceptions in Java are objects of type RuntimeExceptionor 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 • A checked exception requires explicit handling. It must • be caught by a method, (using try-catch block) or • be asserted in the throws clauseof any method that may throw or propagate it • The compiler will issue error if a checked exception is not caught or asserted in a throws clause

9. You can create your own Exceptions // Demonstrates the ability to define an exception. import java.util.Scanner; public class CreatingExceptions { // Creates an exception object and possibly throws it. public static void main (String[] args) throws OutOfRangeException { final int MIN = 25, MAX = 40; Scanner scan = new Scanner (System.in); OutOfRangeException problem = new OutOfRangeException ("Input value is out of range."); System.out.print ("Enter an integer b/w" + MIN + " & " + MAX ); int value = scan.nextInt(); // Determine if the exception should be thrown if (value < MIN || value > MAX) throw problem; System.out.println ("End of main method."); // may never reach } }

10. 10.5 – OutOfRangeException.java //******************************************************************** // OutOfRangeException.java Java Foundations // // Represents an exceptional condition in which a value is out of // some particular range. //******************************************************************** public class OutOfRangeException extends Exception { //----------------------------------------------------------------- // Sets up the exception object with a particular message. //----------------------------------------------------------------- OutOfRangeException (String message) { super (message); } }

11. Displaying the contents of a file /* Read in the contents of a file line by line, * and print out each line after it is read in. * Stop when the end of the file is reached. */ public staticvoid displayFile (String inFileName) { try { Scanner fileScan = new Scanner (new File(inFileName)); while (fileScan.hasNext()) { String line = fileScan.nextLine(); System.out.println (line); System.out.println(); } } } catch (IOException ex) { System.out.println(ex); } }

12. Displaying the contents of a web page /* Read in the contents of a web page line by line, * and print out each line after it is read in. * Stop when the end of the web page is reached. */ public staticvoid displayWebPage (String urlName) { try { URL u = new URL(urlName); Scanner urlScan = new Scanner( u.openStream() ); // will throw exception while (urlScan.hasNext()) { String line = urlScan.nextLine(); System.out.println (line); System.out.println(); } } catch (IOException ex) { System.out.println(ex); } }

13. Displaying contents read in from keyboard /* Read in lines of text from the keyboard, * and print out each line after it is read in. * Stop when the user enters “quit”. */ public staticvoid displayKeyboardInput () { try { Scanner keyboardScan = new Scanner (System.in); do { String line = keyboardScan.nextLine(); System.out.println (line); System.out.println(); } while (keyboardScan.hasNext()); } catch (IOException ex) { System.out.println(ex); } }

14. Copying a File /* Copies an input file to an output file. * Displays an error message if the output file cannot be created. */ public staticvoid copyFile (String inFileName, String outFileName) { try { Scanner reader=new Scanner (new File(inFileName)); PrintWriter writer=new PrintWriter (new File(outFileName)) while (reader.hasNext()) { // Read and write line to output file writer.println(reader.nextLine());} } catch (IOException ex) { System.out.println(ex); // Handle file-not-found } }