code: public static String replace(String s, String sub, String with)
      {
          int c=0;
          int i=s.indexOf(sub,c);
          if (i == -1)
              return s;
      
          StringBuffer buf = new StringBuffer(s.length()+with.length());
  
          synchronized(buf)
          {
              do
              {
                  buf.append(s.substring(c,i));
                  buf.append(with);
                  c=i+sub.length();
              } while ((i=s.indexOf(sub,c))!=-1);
              
              if (c<s.length())
                  buf.append(s.substring(c,s.length()));
              
              return buf.toString();
          }
      }
docs: replace substrings within string.