以下代码实现了在 Java 中获取当天的开始时间 （2018-1-1 00:00:00）到当天结束时间（2018-1-1 23:59:59）

    public static Date startOfDay() {
        Calendar todayStart = Calendar.getInstance();
        todayStart.set(Calendar.HOUR, 0);
        todayStart.set(Calendar.MINUTE, 0);
        todayStart.set(Calendar.SECOND, 0);
        todayStart.set(Calendar.MILLISECOND, 0);
        return todayStart.getTime();
    }

    public static Date endOfDay() {
        Calendar todayEnd = Calendar.getInstance();
        todayEnd.set(Calendar.HOUR, 23);
        todayEnd.set(Calendar.MINUTE, 59);
        todayEnd.set(Calendar.SECOND, 59);
        todayEnd.set(Calendar.MILLISECOND, 999);
        return todayEnd.getTime();
    }