计算器的基本界面和基本功能，适用于初学者，有加减乘除，根号，平方，退格等功能，简单，使用与初学者，整个源码只有一个类。-Calculator basic interface and basic functions for beginners, there are addition, subtraction, multiplication, root, square, backspace and other functions, simple, and beginners, the entire source code only one category.