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 for the next step calculation. Box indicated by the presence decimal point after "." Keys were invalid.