Converting tabs to spaces

Question

Just so you all know, this is an assignment for school that I'm having trouble with.
One of the parts of this assignment is to convert spaces to tabs in a text file. However, we have to maintain the original spacing (otherwise it would be simple). I'm honestly baffled as to how this can be accomplished. I hope I'm along the right lines in trying to use StringBuilder but I can't quite get the spacing to come out right. So my question is, how do I replace the spaces with tabs, but maintain the spacing of the original file.
Here is my SSCCE
import java.util.Scanner;
import java.io.*;
public class SpacesToTabs
     public static void main(String[] args)
          String line, fileName = null;
          StringBuilder sb = null;
          Scanner stdIn = new Scanner(System.in), in = null;
          BufferedWriter out = null;
          System.out.print("Enter filename[test.txt]: ");
          fileName = stdIn.nextLine();
          try
               //In and Out objects
               in = new Scanner(new BufferedReader(new FileReader(fileName)));
               out = new BufferedWriter(new FileWriter(fileName.substring(0, fileName.indexOf('.')) + "_new" + fileName.substring(fileName.indexOf('.'), fileName.length())));
               while(in.hasNext())
                    //Read a line
                    line = in.nextLine();
                    //Make a new StringBuilder
                    sb = new StringBuilder(line.length());
                    //Loop through that line
                    for(int i =0; i<line.length(); i++)
                         //If it's a space, replace it with a tab
                         //this is where I'm confused as to how I can maintain the spacing
                         if(line.charAt(i) == ' ')
                              sb.insert(i, '\t');
                         else
                         //If not a space, just copy it
                              sb.append(line.charAt(i));
                    //Write to the buffer
                    out.write(sb.toString());
                    out.newLine();
          catch(FileNotFoundException e)
               System.err.println(e.getMessage());
          catch(IOException e)
               System.err.println(e.getMessage());
          finally
               try
                    //flush and close all streams
                    out.flush();
                    in.close();
                    out.close();
               catch(IOException e)
                    System.err.println(e.getMessage());
}And here is a test file (test.txt)
blah blah blah line 1   tab
blah blah blah line 2   tab
blah blah blah line 3   tab
blah blah blah line 4   tab
t       tab     tab     tab

Answer 1

Teacher wrote:
My standard answer: You decide how many spaces = 1 tab for conversion purposes. If you want to substitute 4 space characters for one '\t', make it so (and vice versa, as well).Maybe I'm over thinking it. I wrote my teacher and this is how he replied. If you take that exactly at face value, then just replacing x amount of spaces with a tab is trivial. However, I'd like to keep working on my interpretation of the question as I think preservation of the spacing of the original document is important. Here is what I've managed to put together so far and the issues I'm having. The biggest problem I'm having at this stage is that the first character AFTER the spaces is getting eaten by the sb.replace call. I can't figure out why this is happening. If I reduce count by one (which should move back the substring being replaced by one character) then it STILL gets eaten and now there is a space after the tab. I've spent too long looking at this and I'm sure it's something simple I'm missing. I'll cool my brain for awhile and come back.
P.S. Ignore the comments, they are diagnostic for me but I left them in in case anyone wanted to use them.
//sb is a stringbuffer holding the current line read in from the file
                                       //We have to ignore spaces at the beginning of the line because 0 -1 causes index out of bounds error
                         if(i%8 == 0 && !(i==0) && line.charAt(i-1) == ' ')
                              //System.out.println("Character at index " + (i-1) + " is a(n) |" + line.charAt(i-1) + "|");
                                               //Start the index at the space character
                              int index = i-1;
                              int count = 0;
                              //System.out.println("i="+i+"index="+index);
                                               //go backwards, counting spaces, until we find how many
                              while(line.charAt(index) == ' ')
                                   index--;
                                   count++;
                                   //System.out.println("Counting spaces! Index is now " + index + ", count is now " +count);
                                               //increase index by one (If we don't do this, the last character before all the spaces gets eaten
                              index++;
                              //System.out.println("Replacing substring: |" +line.substring(index, index + count)+ "|\nStarting index: "+index+"\nEnding index: "+(index + count));
                              sb.replace(index, index + count, "\t");
                         else
                         //If not a space, just copy it
                              sb.append(line.charAt(i));