实现最基本的计算器界面，包括：
0~9的10个数字按钮，加、减、乘、除、等于5个运算符按钮，一个结果存放的文本区域。实现最基本的加、减、乘、除运算，并能得到正确结果。实现连续的运算，小数点的使用，并考虑各种可能导致异常的情况，将程序作完善。(To achieve the most basic calculator interface, including: 0~9 10 digital buttons, plus, subtract, multiply, divide, equal to 5 operator buttons, a result of the storage of the text area. Achieve the most basic addition, subtraction, multiplication and division operations, and get the correct results. The implementation of continuous operations, the use of decimal points, and consider a variety of may lead to abnormal circumstances, the procedures for perfection.)