public static void main(String[] args) {

        System.out.println(3 * 0.1);

        System.out.println(4 * 0.1);

        System.out.println(3 * 0.1 == 0.3);

        System.out.println(13 * 0.1 == 1.3);

        System.out.println(9 * 0.1 == 0.9);

        System.out.println(3 * 0.1 / 3);

    }    

        结果是：

        0.30000000000000004

        0.4

        false

        true

        true

        0.10000000000000002