说明：  此java程序可以有三种用处：1、计算一次性投资本息和。2.计算periiodically投资的本息和。3、计算loan（贷款）一定数目每月应还多少
(This Java program can have three kinds of usefulness: 1 to calculate a one-time investment of principal and interest and (2) calculate periiodically investment of principal and interest. 3 calculation of the loan (loan) a certain number per month should also how much)