Find and replace a line in a text file

Question

Hi All,
I wanted to read a text file , find a line and replace that line with out affecting any other lines and with out creating a new file.
If any one of you know any API to do this work ,please throw some light

Answer 1

This seems to be a question on optimal implementation of search and replace.
For replacing text in the file itself, the sought text and replaced text must be the same length,
as inserting a character somewhere in a file, is better done in a new file.
You could do this with a java.io.RandomAccessFile[b].
The new IO package [b]java.nio provides a memory mapped file which is faster.
The following code does such patching.
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
public class Patch {
    private static byte[] sought;
     private static byte[] replacement;
     private static boolean matches(MappedByteBuffer bb, int pos) {
          for (int j = 0; j < sought.length; ++j)
               if (sought[j] != bb.get(pos + j))
                    return false;
          return true;
     private static void replace(MappedByteBuffer bb, int pos) {
          for (int j = 0; j < sought.length; ++j)
               byte b = (j < replacement.length)? replacement[j] : (byte)' ';
               bb.put(pos + j, b);
     private static void searchAndReplace(MappedByteBuffer bb, int sz) {
          int replacementsCount = 0;
          for (int pos = 0; pos <= sz - sought.length; ++pos)
               if (matches(bb, pos)) {
                    replace(bb, pos);
                    pos += sought.length - 1;
                    ++replacementsCount;
          System.out.println("" + replacementsCount + " replacements done.");
    // Search for occurrences of the input pattern in the given file
    private static void patch(File f) throws IOException {
          // Open the file and then get a channel from the stream
          RandomAccessFile raf = new RandomAccessFile(f, "rw"); // "rws", "rwd"
          FileChannel fc = raf.getChannel();
          // Get the file's size and then map it into memory
          int sz = (int)fc.size();
          MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_WRITE, 0, sz);
          searchAndReplace(bb, sz);
          bb.force(); // Write back to file, like "flush()"
          // Close the channel and the stream
          raf.close();
    public static void main(String[] args) {
          if (args.length == 0)
               args = new String[] { "row ", "item", "2.xml" };
          if (args.length < 3) {
               System.err.println("Usage: java Patch sought replacement file...");
               return;
          sought = args[0].getBytes();
          replacement = args[1].getBytes();
          if (sought.length != replacement.length) {
               // Better build-in some support for padding with blanks.
               System.err.println("Usage: sought (" + args[0] + ") and replacement (" + args[1] + ") must have same length");
               return;
          for (int i = 2; i < args.length; i++) {
               File f = new File(args);
               try {
                    patch(f);
               } catch (IOException x) {
                    System.err.println(f + ": " + x);