GrepReader.javaAPI DocExample1929Sat Jun 02 02:41:40 BST 2001None

// This example is from _Java Examples in a Nutshell_. (
// Copyright (c) 1997 by David Flanagan
// This example is provided WITHOUT ANY WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for non-commercial purposes.
// For any commercial use, see


 * This class is a BufferedReader that filters out all lines that
 * do not contain the specified pattern.
public class GrepReader extends BufferedReader {
  String pattern;  // The string we are going to be matching.
  /** Pass the stream to our superclass, and remember the pattern ourself */
  public GrepReader(Reader in, String pattern) {
    this.pattern = pattern;
   * This is the filter: call our superclass's readLine() to get the
   * actual lines, but only return lines that contain the pattern.
   * When the superclass readLine() returns null (EOF), we return null.
  public final String readLine() throws IOException {
    String line;
    do { line = super.readLine(); }
    while ((line != null) && line.indexOf(pattern) == -1);
    return line;

   * This class demonstrates the use of the GrepReader class.
   * It prints the lines of a file that contain a specified substring.
  public static class Test {
    public static void main(String args[]) {
      try {
        if (args.length != 2) 
          throw new IllegalArgumentException("Wrong number of arguments");
        GrepReader in = new GrepReader(new FileReader(args[1]), args[0]);
        String line;
        while((line = in.readLine()) != null) System.out.println(line);
      catch (Exception e) { 
        System.out.println("Usage: java GrepReader$Test <pattern> <file>");