1 Text File I/O Overview l I/O streams l Opening a text file for reading l Reading a text file l Closing a stream l Reading numbers from a text file l Writing or appending to a text file l Preview: StringTokenizer and StringBuffer classes.

2 I/O streams Binary and Text files l Binary file: A file whose contents must be handled as a sequence of binary digits. l Text file: A file whose contents are to be handled as a sequence of characters. Why use files for I/O? 1. Files provide permanent storage of data. 2. Files provide a convenient way to deal with large quantities of data. I/O streams l In Java, I/O is handled by streams. l An input stream is an object that takes data from a source and delivers it to a program. l An output stream is an object that takes data from a program and delivers data it to a destination. l Java has the following standard streams: System.in, System.out, and System.err. System.in is connected to the keyboard. System.out and System.err are connected to the screen.

3 Opening a text file for reading You create a stream of the class BufferedReader and connect it to a text file for reading as follows: BufferedReader streamName = new BufferedReader (new FileReader(filename)); Where filename is a File object or a constant string or a String variable containing the name or the full path of the file to be read. Example of valid filenames: 1. “myinput.txt” 2. “C:\\homework\\StudentTest.java” 3. “C:/homework/StudentTest.java” 4. fileObject = new File(“C:/homework/StudentTest.java”); BufferedReader streamName = new BufferedReader (new FileReader(fileObject));    The full path to a file can be read from the keyboard; in that case you must not type any of the backslashes twice.

4 Opening a text file for reading (Cont’d) l The class BufferedReader has no constructor that takes a file name or a File object as its argument. l The class FileReader will accept a file name (or an object of the class File) as a constructor argument and produce a stream that is a Reader. l The constructor for BufferedReader will accept a Reader as an argument. l Both BufferedReader and FileReader classes belong to the java.io package. l The FileReader constructor throws a FileNotFoundException, if the text file to be opened for reading does not exist: FileReader(String filename) throws FileNotFoundException l The FileNotFoundException is a subclass of the class IOException, so any catch-block that catches exceptions of the class IOException will also catch exceptions of the class FileNotFoundException.

5 Reading a text file After a text file has been opened for reading you can use the methods readLine( ) or read( ) of the stream to read from the file: 1. public String readLine( ) throws IOException This method reads a line of input from the input stream and returns that line as a string. If an attempt is made to read beyond the end of file, null is returned. 2.public int read( ) throws IOException This method reads a single character from the input stream and returns that character as an integer value. To obtain the character, you must perform a type cast on the value returned. For example: char next = (char) inputStream.read( ) If an attempt is made to read beyond the end of the file, -1 is returned. Note: The end of file may be detected by using the boolean method ready( ) of an input stream. The method returns false if the end of file is reached; otherwise it returns true:... String input = inputStream.readLine( ); while( inputStream.ready( ) ) {... input = inputStream.readLine( ); }

6 Closing a stream When your program has finished writing to or reading from a file, it should close the stream connected to that file by calling the close( ) method of the stream: streamName.close( ) The method close( ) is defined as: public void close( ) throws IOException »When you close a file, the system releases any resources used to connect the stream to the file. »If your program does not close a file before the program ends, then the system will close it for you.

7 Example1 (File display) l Example: The following program displays the contents of the file myinput.txt on the screen by reading one character at a time: import java.io.*; public class ShowFile { public static void main(String[ ] args) throws IOException { int input; BufferedReader fin = null; try { fin = new BufferedReader(new FileReader("myinput.txt")); } catch(FileNotFoundException e) { System.out.println("Error - File myinput.txt not found"); System.exit(1); } while( ( input = fin.read( )) != -1) System.out.print((char) input); fin.close( ); }

8 Reading numbers from a text file l The BufferedReader class has no methods that can read a number. l To read a number from a text file, use the parse method of an appropriate Wrapper class. l Example: The following program reads float numbers from a file. It displays the numbers on the screen, computes their average, and displays that average on the screen: import java.io.*; public class Average { public static void main(String[ ] args) { try{ BufferedReader fin = new BufferedReader(new FileReader("numbers.txt")); String inputLine; float grade, sum = 0.0F; int count = 0; while( ( inputLine = fin.readLine( )) != null) { grade = Float.parseFloat(inputLine); sum += grade; count++; System.out.println(grade); }

9 Reading numbers from a text file (Cont’d) if(count = = 0) System.err.println("Error - no grades were read"); else System.out.println("\nThe average is " + sum / count); fin.close( ); } catch(FileNotFoundException e) { System.err.println("Error - File myinput.txt not found"); } catch(IOException e) { System.err.println("Error - An I/O error occured"); } catch(NumberFormatException e) { System.err.println("Error - An invalid float number read"); }

10 Writing or appending to a text file A text file is opened for writing, either one character at a time or one line at a time, by a statement of the form: PrintWriter streamName = new PrintWriter(new FileWriter(filename)); Any preexisting file by the same name is destroyed. A text file is opened for appending, either one character at a time or one line at a time, by a statement of the form: PrintWriter streamName = new PrintWriter(new FileWriter(filename, true)); Any preexisting file by the same name is not destroyed. Both PrintWriter and FileWriter classes belong to java.io package.  The PrintWriter class has methods print( ) and println( ) that print either one character or one line at a time. Each constructor of the FileWriter can throw an IOException: FileWriter(String filename) throws IOException FileWriter(String filename, boolean appendFlag) throws IOException

11 Writing or appending to a text file (Cont’d) The class FileWriter has constructors that takes a filename or a File object as its argument; however this class does not have methods to write one line at a time. The class PrintWriter does not have a constructor that takes a filename or an object of the class File as its argument; however it has a constructor that takes a FileWriter object as argument. It also has a method to write one line at a time.  So we use the class FileWriter together with the class PrintWriter.

12 Example (Copying a text file) Example: The following program copies one file to another; but it converts every lowercase character to uppercase. import java.io.*; public class FileCopy{ public static void main(String[ ] args) { int input; BufferedReader fin = null; PrintWriter fout = null; try { fin = new BufferedReader(new FileReader("myinput.txt")); } catch(FileNotFoundException e){ System.out.println("Input File not found"); System.exit(1); } try{ fout = new PrintWriter(new FileWriter("myoutfile.txt")); } catch(IOException e){ System.out.println("Error opening output file"); System.exit(1); }

13 Example (Copying a text file) (Cont’d) try { while(( input = fin.read( )) != -1 ) { char ch = (char) input; if(Character.isLowerCase(ch)) ch = Character.toUpperCase(ch); fout.print(ch); } catch(IOException e) { System.out.println("Error in reading the file myinput.txt"); } try{ fin.close( ); fout.close( ); } catch(IOException e) { System.out.println("Error in closing a file"); } System.out.println("File copied successfully"); }