Files and Interaction

This section is about reading data from files, and reading data interactively from the keyboard. Along the way, we will develop two template programs:

Lines.java
Echo.java

The first of these counts the number of lines in a file. The classes we need for doing input from a file are in the java.io package. Since we may need several of them, we will import the whole package:

// Count the number of lines in a given file.

import java.io.*;

class Lines
{
   ...
}

The main method does the usual stuff, creating the program object, checking the number of command line arguments, and calling an initial method for counting the lines. It also prints out the result message.

   // Get the filename from the command line and print the line count
   public static void main (String[] args)
   {  Lines program = new Lines();
      if (args.length != 1)
      {  System.out.println ("Usage: java Lines filename");
      }
      else
      {  int n = program.count(args[0]);
         System.out.println ("File " + args[0] + " has " + n + " lines.");
      }
   }

The count method is the one which reads the file:

   // Count the lines in a file
   int count (String filename)
   {  BufferedReader input;
      String line;
      int n;

      n = 0;
      input = new BufferedReader (new FileReader (filename));
      line = input.readLine();
      while (line != null)
      {  n++;
         line = input.readLine();
      }
      input.close();
      return n;
   }

The FileReader object that is created opens the file and prepares to read characters from it one by one. The BufferedReader object called input is wrapped round it, and adds buffering so that text can be read in a line at a time. The readLine method reads the next line from the file as a string. When it gets to the end of the file and there are no more lines, it returns null instead. After the loop, close is called to close the connection to the file. It is not strictly necessary to do this, because it is done automatically when the program finishes, but it is 'polite' to call it as soon as possible to allow other programs access to the file, and it is good practice so you don't forget in longer-running programs where it becomes important. For the curious, here is a longer discussion on these I/O facilites.

Discussion

Copy the program we have developed so far by cut-and-paste or by downloading Lines.java. How many error messages are produced when you compile it? <% a.pl("

" + guess + "

The right answer is"); a.pl("

" + answer + "

Go back and have another look"); a.pl("
Go on to the next page"); } %>