1.编写一个日期时间类Datetime，能够输入、显示和修改日期（年、月、日）和时间（小时、分、秒）。程序文件命名为Exe3_1.java。
3.设计一个能够描述点（point）、线(line)、圆（circle）、矩形（rectangle）和三角形（triangle）等几何形状的类体系，能够计算不同形状的周长和面积（如果能够定义的话）。程序文件命名为Exe3_3.java。(1. Write a Datetime class, which can input, display and modify dates (year, month, day) and time (hours, minutes, seconds). The program file is named Exe3_1. java.
3. Designing a class system that can describe geometric shapes such as point, line, circle, rectangle and triangle. It can calculate the circumference and area of different shapes (if it can be defined). The program file is named Exe3_3. java.)