Learning Java - Chapter 9 : Java

The following example, TextFileReadApp, illustrates how to use the FileReader stream to read strings from a text file. The goal is to read a file and count the number of lines in which a particular string occurs at least once.

We wrap the FileReader stream with a BufferedReader class and take advantage of its readLine() method to read a whole line at a time. We use the indexOf() method in the String class to search for the string of interest.

As usual, we enclose the stream reading within a try-catch statement to catch the IOException or one of its subclass exceptions that can be thrown by the stream constructors and the read methods.

TextFileReadApp.java

import java.io.*;
import java.util.*;

/** Demonstrate reading text from a file.**/
public class TextFileReadApp
{
  public static void main (String arg[]) {
    // Count the number of lines in which the string occurs
    String string_to_find = "new";

    File file = null;
    // Get the file from the argument line.
    if (arg.length > 0) file = new File (arg[0]);
    if (file == null || !file.exists ()) {
      System.out.println ("Default: TextFileReadApp.java");
      file = new File ("TextFileReadApp.java");
    }

    // Count the number of lines with the string of interest.
    int num_lines = 0;
    try {
      // Create a FileReader and then wrap it with BufferedReader.
      FileReader file_reader = new FileReader (file);
      BufferedReader buf_reader = new BufferedReader (file_reader);

      // Read each line of the file and look for the string of interest.
      do {
         String line = buf_reader.readLine ();
         if (line == null) break;
         if (line.indexOf(string_to_find) != -1) num_lines++;
      } while (true);
      buf_reader.close ();
    }
    catch (IOException e) {
        System.out.println ("IO exception =" + e );
    }
    System.out.printf ("Number of lines containing \"%s\"  = %3d %n",
                         string_to_find, num_lines);
  } // main

} //class TextFileReadApp

If we let the program examine the default input file, then the output will go as

Note that we close the stream explicitly. For this short program, which stops soon after finishing the read, closing the input stream is not a big deal, but in general, it is good practice to always close input and output streams when I/O transfers are completed.

References & Web Resources

Specifications:

Latest update: Dec. 9, 2004