BigDecimal 是比doubule更加精确的数，再算小数的时候由于二进制的原因double 和BigDecimal 假如结果是0.2，那么只能精确到0.199999999，解决这个方案

double 转String

class DoubleToString {

    public static String test2(double dou) {

        Double dou_obj = new Double(dou);

        NumberFormat nf = NumberFormat.getInstance();

        nf.setGroupingUsed(false);

        String dou_str = nf.format(dou_obj);

        return  dou_str;

    }

double c = 10.1;

    double b = 9.3;

    BigDecimal d = new BigDecimal(DoubleToString.test2(c));

    BigDecimal e = new BigDecimal(DoubleToString.test2(b));

      System.out.println(d.subtract(e).toString());

或者 

double c = 10.1;

    double b = 9.3;

    BigDecimal d = BigDecimal.valueOf(c);

    BigDecimal e = BigDecimal.valueOf(b);

    System.out.println(d.subtract(e));

BIgInteger是Integer 范围更大的integer 类