将compareTo方法的返回值相乘。
如果值为中，compareTo方法的结果有不同的符号。
使乘法结果为零或具有负号。

Multiply return values of compareTo method. If the value is middle, results of compareTo method have different signs. do multiply result is zero or has negative sign.

String getMiddle(String a,String b,String c)
{
    String middle = "";
    if (a.compareTo(b)*a.compareTo(c) <= 0) middle = a;
    else if (b.compareTo(a)*b.compareTo(c) <= 0) middle = b;
    else if (c.compareTo(b)*c.compareTo(a) <= 0) middle = c;
    return middle;
}