数字字面量加下划线

Java7开始，数字字面量可以加下划线（方便人类区分）

public class HelloWorld {
    public static void main(String []args) {
       System.out.println(1_000_000);
        System.out.println(0b1111_0100_0010_0100_0000);
        System.out.println(0xf_42_40);
        System.out.println(03_641_100);
    }
}

    double x = 999_999.99;
    System.out.println(x);