Changing one character in a string...how hard can it be...???

Question

Hey guys,
I posted something the other day about changing a binary string to one complement. Well, now I want to change that again to the two's complement of the same number.
The (messy) code I got so far is as follows:
import java.util.*;
class twosComplement {
  public static void main(String[] args) {
      Scanner reader = new Scanner(System.in);
      String binary;
      String i = "0";
      String j = "1";
      System.out.println("Enter a binary String:");
      binary = reader.nextLine();
   //   int x = 15;
   // String binary = Integer.toBinaryString(x);
   // binary = ("00000000"+binary).substring(8-binary.length());
    System.out.println("Your binary number is " +binary);
    String complement = binary.replaceAll("0","x").replaceAll("1","0").replaceAll("x","1");
    System.out.println("One's complement of your binary number is " +complement);
    int n = 0;
    int k = binary.length()-n;
    String m = charAt(k);
    if (charAt(k).equals(i)) binary.replace(charAt(k), j);
    else n++;
    System.out.println(binary);
//    do {k++; }
//    while (charAt(k).equals(i)) 
    private static String charAt(int k) {
        throw new UnsupportedOperationException("Not yet implemented");
    }the bit at the bottom is something netbeans kindly offered to create for me, it cut out a few syntax errors but didn't quite get the program to work.
I'm pretty sure the root of my troubles is coming from the use of the "charAt" method, which as far as i know, would be better used with arrays, but I don't know of another way to sort this stuff...
any help would be muchly appreciated.
Aaron.

Answer 1

ok. The following is a section from the original code (see above). I was hoping this code would take a binary string (ie, '1010101') and change the least significant bit which is a '0' to a '1'. The following section is my attempt at solving this problem, but as I run it, I encounter errors. I've been studying java for 2 months. I would like to know if anybody knows WHY this isn't working, and could offer an alternative method (as I've already stated, I think the use of the method 'charAt' is the main problem)
int n = 0;
    int k = binary.length()-n;
    String m = charAt(k);
    if (charAt(k).equals(i)) binary.replace(charAt(k), j);
    else n++;
    }Is that better?