Getting an invalid output java

i developed a java program for my android , which will solve the one variable linear equation . String st1 = "4x-2+2x=8" ; if i give the above string as input or any one variable linear equation i got error , no result being got. equatin= Onevariable(st1);

tried a lot and didn't get the problem in this program.

   public static String Onevariable (String eqn)
 {
  float ans = 0;
  float coeffSum = 0;
  float constSum = 0;
  float coeffx[] = new float[100];
  float[] constant = new float[100];
  // System.out.println(eqn);
  for (int i = 0, j = 0, k = 0; i < eqn.length() - 1;)
  {
      if (eqn.charAt(i + 1) == 'x' && i < eqn.indexOf("="))
      {
          if (i != 0 && eqn.charAt(i - 1) == '-')
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = -(Integer.parseInt(x, 10));
                  coeffx[j++] = n;
     }
          } else

          {
              String x = eqn.substring(i, i + 1);

              if (x != "+" && x != "-")

              {
                  int n = Integer.parseInt(x, 10);

                  coeffx[j++] = n;
              }
          }

          i += 3;
      }
      if (eqn.charAt(i + 1) == 'x' && i > eqn.indexOf("="))
      {
          if (eqn.charAt(i - 1) == '-')
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = Integer.parseInt(x, 10);
                  coeffx[j++] = n;
              }
          } else
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = -(Integer.parseInt(x, 10));
                  coeffx[j++] = n;
              }
          }
          i += 3;
      }
      if (eqn.charAt(i + 1) != 'x' && i < eqn.indexOf("="))
      {
          if (eqn.charAt(i - 1) == '-')
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = -(Integer.parseInt(x, 10));
                  constant[k++] = n;
              }
          } else
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = Integer.parseInt(x, 10);
                  constant[k++] = n;
              }
          }
          i += 2;
      }
      if (eqn.charAt(i + 1) != 'x' && i > eqn.indexOf("="))
      {
          if (eqn.charAt(i - 1) == '-')
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = Integer.parseInt(x, 10);
                  constant[k++] = n;
              }
          } else
          {
              String x = eqn.substring(i, i + 1);
              if (x != "+" && x != "-")
              {
                  int n = -(Integer.parseInt(x, 10));
                  constant[k++] = n;
              }
          }
          i += 2;
      }
  }
  for (int i = 0; i < coeffx.length; i++)
      coeffSum += coeffx[i];
  for (int i = 0; i < constant.length; i++)
      constSum += constant[i];
  ans = constSum / coeffSum;
  //System.out.println("Value of x = " + (-ans));
  String s = Float.toString(-ans);
  return s;
  }