Files from Ch4

File Input and Output  Reentering data all the time could get tedious for the user.  The data can be saved to a file. Files can be input files or output files.  Files: Files have to be opened. Data is then written to the file. The file must be closed prior to program termination.  In general, there are two types of files: binary text

Writing Data To a File  Objects from the following classes are used to write data to files: FileWriter – This class allows basic file writing functionality. PrintWriter – This class allows the programmer to write files using the same style that is used to write to the screen (i.e. print and println).

The FileWriter Class  The FileWriter clas provides other classes with the basic file writing functionality. System.out.println(“Enter the filename.”); filename = Keyboard.readString(); FileWriter fwriter = new FileWriter(filename);  This will create an object that can access the file filename.  Warning: if the file already exists, it will be erased and replaced with the new file.

The PrintWriter Class  The PrintWriter class adds to the functionality of the FileWriter class.  The PrintWriter cannot directly access the file but must work through the FileWriter class.  The PrintWriter needs a FileWriter object in order to work: FileWriter fwriter = new FileWriter("StudentData.txt"); PrintWriter outputFile = new PrintWriter(fwriter);

The PrintWriter Class  Once linked to the fwriter object, the outputFile object can talk to the file. outputFile.open(); outputFile.println(“Jim”); outputFile.close();  Just as with the System.out object, the println method of the PrintWriter class will place a newline character after the written data.  The print method can be used to avoid writing the newline character.

Exceptions  When something unexpected happens in a Java program, an exception is thrown.  The method currently executing when the exception is thrown must either handle the exception or pass it up the line.  Handling the exception will be discussed later.  To pass it up the line, the method needs a throws clause in the method header.

Exceptions  To insert a throws clause in a method header, simply add the word throws and the name of the expected exception.  The class Exception can be used to catch all exceptions. public static void main(String[] args) throws IOException{…}  File I/O is a checked exception (meaning the exception must be handled or passed up).  A program with file I/O will generate a compile-time error if the exception is not handled or passed up.  Example:

FileWriteDemo.java  import java.util.Scanner; // Needed for Scanner class  import java.io.*; // Needed for file classes  public class FileWriteDemo  {  public static void main(String[] args) throws IOException  {  String filename; // File name  String friendName; // Friend's name  int numFriends; // Number of friends Scanner keyboard = new Scanner(System.in);   System.out.print("How many friends do you have? ");  numFriends = keyboard.nextInt();  keyboard.nextLine();   System.out.print("Enter the filename: ");  filename = keyboard.nextLine();

FileWriteDemo.java  FileWriter fwriter = new FileWriter(filename);  PrintWriter outputFile = new PrintWriter(fwriter);  for (int i = 1; i <= numFriends; i++)  {  // Get the name of a friend.  System.out.print("Enter the name of friend " +  "number " + i + ": ");  friendName = keyboard.nextLine();  outputFile.println(friendName);  }  outputFile.close();  System.out.println("Data written to the file.");  }

Appending Text to a File  To avoid erasing a file that already exists: Create a FileWriter object using an optional boolean argument that tells the object to append data to the file. FileWriter fwriter = new FileWriter(“filename”, true);  Data written to a file created in such a manner will be appended to the end of the current file.

Specifying a File Location Windows’ Crazy Backslash  Windows evolved from DOS.  Since DOS was simply a hacked version of CP/M, it maintained the backslash (\) as a directory separator.  Remember, if the backslash is used in a String literal, it is the escape character so there must be two of them. FileWriter fwriter = new FileWriter("A:\\PriceList.txt"); PrintWriter outputFile = new PrintWriter(fwriter);

Specifying a File Location  This is only necessary if the backslash is in a String literal.  If the backslash is in a String object then it will be handled properly.  Fortunately, Java allows Unix style filenames using the forward slash (/) to separate directories. FileWriter fwriter = new FileWriter("/home/rharrison/names.txt"); PrintWriter outputFile = new PrintWriter(fwriter);

Reading Data From a File  Java provides several classes to read data from a file. FileReader  Open an existing file for reading and establish a connection with it. BufferedReader  Uses a buffer to allow the reading of full lines of text at a time rather than one byte at a time.

The FileReader and BufferedReader Classes System.out.print("Enter the filename: "); filename = Keyboard.readString(); FileReader freader = new FileReader(filename); BufferedReader inputFile = new BufferedReader(freader);  The lines above: Prompt the user for a filename. Get the filename from the user. Create an instance of the FileReader class that is associated with the filename. Create an instance of the BufferedReader class that buffers the instance of the FileReader class.

The FileReader and BufferedReader Classes  Once an instance of BufferedReader is created, lines of text can be read in. customerName = inputFile.readLine();  A file pointer is created when the file is first opened.  As the file is read, the pointer moves to indicate the text that is to be read next.

Exceptions  The FileReader and BufferedReader classes can throw exceptions.  A throws IOException clause needs to be placed on the method header of the method that instantiates a FileReader or BufferedReader object.  Also, any method that uses a FileReader or BufferedReader needs a throws IOException clause.

Detecting The End of a File  The readLine() method of the BufferedReader class will return null if the end of the file has been reached. FileReader freader = new FileReader(filename); BufferedReader inputFile = new BufferedReader(freader); // Read the first item. String str = inputFile.readLine(); // If an item was read, display it // and read the remaining items. while (str != null) { System.out.println(str); str = inputFile.readLine(); } inputFile.close();// close the file when done.

Reading a File Flowchart Read the first item (priming read) Process the item Close the file Read next item Open the file Did readLine() return null?

FileReadDemo.java  import java.util.Scanner; // Needed for the Scanner class  import java.io.*; // Needed for file classes  public class FileReadDemo  {  public static void main(String[] args) throws IOException  {  String filename; // File name  String friendName; // Friend's name  Scanner keyboard = new Scanner(System.in);  System.out.print("Enter the filename: ");  filename = keyboard.nextLine();

FileReadDemo.java  FileReader freader = new FileReader(filename);  BufferedReader inputFile = new BufferedReader(freader);  friendName = inputFile.readLine();  while (friendName != null)  { System.out.println(friendName);  friendName = inputFile.readLine();  }  inputFile.close();  }