程序运行时，显示一个窗口，等待用户输入，用户可以从鼠标或键盘输入要计算的表达式，输入的表达式显示在窗口中，用户键入’=’ 符号后，窗口显示出结果。-Program runs, showing a window, waiting for user input, the user can input from the mouse or keyboard to calculate the expression, type of expression displayed in the window, the user type ' ='　symbol, the window displays the result.