BigDecimal compare size method:
use compareTo method.
int flag = bigdemical1.compareTo(bigdemical2)
flag = -1, indicating that bigdemical1 is less than bigdemical2;
flag = 0, indicating that bigdemical1 is equal to bigdemical2;
flag = 1, indicating that bigdemical1 is greater than bigdemical2;
In actual applications, it can be directly compared with 0, if it is
greater than 0, bigdemical is greater than bigdemical1, and
equal to 0 is bigdemical is equal to bigdemical1 and
less than 0 is bigdemical is less than bigdemical1