利用Swing组件设计一个计算器，实现简单的加、减、乘、除功能。计算完后需要按“C”键清空再进行下步计算。显示框中存在小数点后再按“.”键则无效。-use Swing components design a calculator, a simple addition, subtraction, multiplication and functional. After the calculation required by the "C" key emptied again f