汇率转换

说明：  实现了一个用于多币种之间汇率转换的程序。用户可在左侧的下拉框中选择需要转换的两种币种，在上方输入框内输入需要转换的金额，点击“汇率转换”按钮后，下方输出框中显示转换出的对应金额并保留两位小数；点击“清空”按钮后两个文本框内容清空。且输出框内容不可修改。(A program for exchange rate conversion between multiple currencies is implemented. Users can select two currencies to be converted in the drop-down box on the left, enter the amount to be converted in the input box above, click the "exchange rate conversion" button, and display the corresponding amount in the output box below with two decimal digits retained; after clicking the "empty" button, the contents of the two text boxes are empty. And the contents of the output box can not be modified.)