File org = new File("test.txt");
File bak = new File("test.txt.bak");
BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(org)));
PrintStream ps = new PrintStream(new FileOutputStream(bak));
String buf = null;
while ((buf=br.readLine()) != null) {
    //add, modify, delete, so on,
    ps.println(buf);
}
br.close();
ps.close();
org.delete();
if (! org.exists()) {
    bak.renameTo(org);
}

------解决方案--------------------
读取指定行
http://blog.csdn.net/yaowenqian99/article/details/5403694

------解决方案--------------------
http://www.cnblogs.com/rollenholt/archive/2011/09/11/2173787.html

------解决方案--------------------
import java.io.*;
public class Lian {
   public static void main(String [] args) throws IOException {
   	File org = new File("c:/old.txt");
   	File bak = new File("c:/new.txt");
   	org.createNewFile();
   	bak.createNewFile();
   	InputStreamReader in= new InputStreamReader(new FileInputStream(org));  
   	BufferedReader br = new BufferedReader(in);
   	PrintStream ps = new PrintStream(new FileOutputStream(bak));
   	String buf = null;
   	while ((buf=br.readLine()) != null) {
   	    ps.println(buf);
   	}
   	br.close();
   	ps.close();
   	org.delete();
   	if (! org.exists()) {
   	    bak.renameTo(org);
   	}
   }
}


------解决方案--------------------
http://www.cnblogs.com/rollenholt/archive/2011/09/11/2173787.html这个很详细，希望能帮到楼主