I/O Streams. program. Standard I/O. File I/O: Setting up streams from files. program. File I/O and Exceptions. Dr. Papalaskari 1

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1 File I/O and Exceptions CSC 1051 Data Structures and Algorithms I I/O Streams Programs read information from input streams and write information to output streams Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: source program destination Some slides in this presentation are adapted from Barbara Zimmerman s CSC 1051 slides and from the slides accompanying Java Software Solutions by Lewis & Loftus Standard I/O We use System.in with Scanner We use System.out with println statements System.err also goes to the console, by default standard input System.in program standard output System.out File I/O: Setting up streams from files Input: Scanner Output: PrintWriter standard input System.in input file Scanner program PrintWriter Scanner PrintWriter standard output System.out output file output file standard Error System.err input file Scanner standard Error System.err Dr. Papalaskari 1

2 TestData.java Author: Lewis/Loftus Demonstrates I/O exceptions and the use of a character file output stream. import java.util.random; import java.io.*; public class TestData Creates a file of test data that consists of ten lines each containing ten integer values in the range 10 to 99. public static void main(string[] args) throws IOException final int MAX = 10; int value; String filename = "test.txt"; PrintWriter outfile = new PrintWriter(fileName); text output file output streams should be closed explicitly Random rand = new Random(); for (int line=1; line <= MAX; line++) for (int num=1; num <= MAX; num++) value = rand.nextint(90) + 10; outfile.print(value + " "); outfile.println(); use just like System.out outfile.close(); System.out.println("Output file has been created: " + filename); System.out can still be used for other messages output streams should be closed explicitly Random rand = new Random(); for (int line=1; 60 line 85 <= MAX; 18 line++) for (int num=1; 33 num 25 <= MAX; 48 num++) value = rand.nextint(90) ; 68 outfile.print(value " "); outfile.println(); use just like System.out outfile.close(); System.out.println("Output file has been created: " + filename); Output Sample test.txt File created System.out can still be used for other messages Output file has been created: test.txt 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 all methods that do file I/O must list this exception: throws IOException unless they contain code to handle it (more on this later) Dr. Papalaskari 2

3 Exception Handling Three ways to deal an exception: Ignore Let the java runtime environment deal with it the program will terminate and produce an appropriate message Handle it where it occurs use try/catch block Handle it at another place in the program use try/catch block elsewhere The Java API has a predefined set of exceptions that can occur during execution Uncaught Exception Zero.java Author: Lewis/Loftus 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(numerator / denominator); System.out.println("This text will not be printed."); Uncaught Exception Output (when program terminates) Zero.java Author: Lewis/Loftus Exception in thread "main" java.lang.arithmeticexception: / by zero Demonstrates Zero.main(Zero.java:17) 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(numerator / denominator); where exception occurred call stack trace try / catch Create a try block surrounding code that we think may cause an exception catch clause has code to tell it what to do the exception handler Can have multiple catch clauses One for each type of exception thrown by try block System.out.println("This text will not be printed."); If no exception is thrown, processing s following the try statement (skips catch clauses) Dr. Papalaskari 3

4 Zero.java -- updated public class Zero public static void main(string[] args) int numerator = 10; int denominator = 0; try System.out.println (numerator/denominator); catch (ArithmeticException problem) System.out.println ( Bad division ); System.out.println ( this will not print ); catch clauses handle particular types of exceptions. Examples: StringIndexOutOfBoundsException Thrown by a charat method Thrown by a substring method NumberformatException Thrown by a parseint method If the string does not contain an integer ArithmeticException Thrown by division by zero IndexOutOfBoundsException bad index in array ProductCodes.java Author: Lewis/Loftus Demonstrates the use of a try-catch block. import java.util.scanner; public class ProductCodes Counts the number of product codes that are entered with a zone of R and and district greater than public static void main(string[] args) String code; char zone; int district, valid = 0, banned = 0; while (!code.equals("xxx")) 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); Scanner scan = new Scanner(System.in); System.out.print("Enter product code (XXX to quit): "); code = scan.nextline(); System.out.print("Enter product code (XXX to quit): "); code = scan.nextline(); System.out.println("# of valid codes entered: " + valid); System.out.println("# of banned codes entered: " + banned); Dr. Papalaskari 4

5 while Enter (!code.equals product code ("XXX")) (XXX to quit): TRV2475A5R-14 Enter product code (XXX to quit): TRD1704A7R-12 try Enter product code (XXX to quit): TRL2k74A5R-11 District zone = code.charat(9); is not numeric: TRL2k74A5R-11 Enter district product = Integer.parseInt(code.substring(3, (XXX to quit): TRQ2949A6M-04 7)); Enter valid++; product code (XXX to quit): TRV2105A2 Improper if (zone code == 'R' length: && district TRV2105A2 > 2000) Enter banned++; product code (XXX to quit): TRQ2778A7R-19 Enter product code (XXX to quit): XXX catch (StringIndexOutOfBoundsException exception) # of valid codes entered: 4 # of System.out.println banned codes entered: ("Improper 2 code length: " + code); catch (NumberFormatException exception) System.out.println("District is not numeric: " + code); Sample Run System.out.print ("Enter product code (XXX to quit): "); code = scan.nextline(); System.out.println("# of valid codes entered: " + valid); System.out.println("# of banned codes entered: " + banned); finally clause (optional) Executes no matter how the try block is exited Executes if an exception has been thrown Executes if an exception has not been thrown Must be after all the catch clauses finally System.out.println( Finally clause ); Exception Propagation Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method Propagation.java Author: Lewis/Loftus Demonstrates exception propagation. public class Propagation Invokes the level1 method to begin the exception demonstration. static public void main(string[] args) ExceptionScope demo = new ExceptionScope(); System.out.println("Program beginning."); demo.level1(); System.out.println("Program ending."); See Propagation.java and ExceptionScope.java Exception Propagation Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the level of the main method Output Propagation.java Program beginning. Author: Lewis/Loftus Demonstrates Level exception 1 beginning. propagation. Level 2 beginning. Level 3 beginning. public class Propagation The exception message is: / by zero Invokes the level1 method to begin the exception demonstration. The call stack trace: static public java.lang.arithmeticexception: void main(string[] args) / by zero at ExceptionScope.level3(ExceptionScope.java:54) ExceptionScope at demo ExceptionScope.level2(ExceptionScope.java:41) = new ExceptionScope(); at ExceptionScope.level1(ExceptionScope.java:18) System.out.println("Program beginning."); demo.level1(); at Propagation.main(Propagation.java:17) System.out.println("Program ending."); Level 1 ending. Program ending. See Propagation.java and ExceptionScope.java Dr. Papalaskari 5

6 ExceptionScope.java Author: Lewis/Loftus Demonstrates exception propagation. public class ExceptionScope Catches and handles the exception that is thrown in level3. public void level1() System.out.println("Level 1 beginning."); try level2(); catch (ArithmeticException problem) System.out.println(); System.out.println("The exception message is: " + problem.getmessage()); System.out.println(); System.out.println("The call stack trace:"); problem.printstacktrace(); System.out.println(); System.out.println("Level 1 ending."); Serves as an intermediate level. The exception propagates through this method back to level1. public void level2() System.out.println("Level 2 beginning."); level3(); System.out.println("Level 2 ending."); The Exception Class Hierarchy Performs a calculation to produce an exception. It is not caught and handled at this level. public void level3() int numerator = 10, denominator = 0; System.out.println("Level 3 beginning."); int result = numerator / denominator; System.out.println("Level 3 ending."); Dr. Papalaskari 6

7 Checked vs. Unchecked Exceptions Checked exceptions: Java requires that certain exceptions (e.g., IOException) are handled in some way. They can be either caught or listed in the throws clause of any method that may throw or propagate them Unchecked exceptions and Errors: Can be either caught or ignored Generally, Errors should NOT be caught Unchecked Exceptions Checked exceptions Two options: 1) catch and handle - OR - 2) list in throws clause of any method that may throw or propagate it Another view which are unchecked? Exception class there is much, much more the only ones that are unchecked Image source: Dr. Papalaskari 7

8 Which are Checked? NullPointerException IndexOutOfBoundsException StackOverflowError NullPointerException FileNotFoundException OutOfMemoryError NoSuchMethodException ThreadDeath ArithmeticException ClassNotFoundException 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 See CreatingExceptions.java See OutOfRangeException.java CreatingExceptions.java Author: Lewis/Loftus Demonstrates the ability to define an exception via inheritance. 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."); OutOfRangeException.java Author: Lewis/Loftus 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); System.out.print("Enter an integer value between " + MIN + " and " + MAX + ", inclusive: "); int value = scan.nextint(); Sample Run Determine if the exception should be thrown Enter an integer value between 25 and 40, inclusive: 69 if (value < MIN value > MAX) Exception in thread "main" OutOfRangeException: throw problem; Input value is out of range. at CreatingExceptions.main(CreatingExceptions.java:20) System.out.println("End of main method."); may never reach Dr. Papalaskari 8