常用日期方法

public class TestCalendar {
    // 日期也就是这了
    public static void main(String[] args) {
        // 获取当前年份、月份、日期
        Calendar cale = null;
        cale = Calendar.getInstance();
        int year = cale.get(Calendar.YEAR);
        int month = cale.get(Calendar.MONTH) + 1;
        int day = cale.get(Calendar.DATE);
        int hour = cale.get(Calendar.HOUR_OF_DAY);
        int minute = cale.get(Calendar.MINUTE);
        int second = cale.get(Calendar.SECOND);
        int dow = cale.get(Calendar.DAY_OF_WEEK);
        int dom = cale.get(Calendar.DAY_OF_MONTH);
        int doy = cale.get(Calendar.DAY_OF_YEAR);

        System.out.println("Current Date: " + cale.getTime());
        System.out.println("Year: " + year);
        System.out.println("Month: " + month);
        System.out.println("Day: " + day);
        System.out.println("Hour: " + hour);
        System.out.println("Minute: " + minute);
        System.out.println("Second: " + second);
        System.out.println("Day of Week: " + dow);
        System.out.println("Day of Month: " + dom);
        System.out.println("Day of Year: " + doy);

        // 获取当月第一天和最后一天
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        String firstday, lastday;
        // 获取前月的第一天
        cale = Calendar.getInstance();
        cale.add(Calendar.MONTH, 0);
        cale.set(Calendar.DAY_OF_MONTH, 1);
        Date time = cale.getTime();
        firstday = format.format(cale.getTime());
        // 获取前月的最后一天
        cale = Calendar.getInstance();
        cale.add(Calendar.MONTH, 1);
        cale.set(Calendar.DAY_OF_MONTH, 0);
        lastday = format.format(cale.getTime());
        Date time2 = cale.getTime();
        System.out.println(time);
        System.out.println(time2);
        System.out.println("本月第一天和最后一天分别是 ： " + firstday + " and " + lastday);

        // 获取当前日期字符串
        Date d = new Date();
        System.out.println("当前日期字符串1：" + format.format(d));
        System.out.println("当前日期字符串2：" + year + "/" + month + "/" + day + " " + hour + ":" + minute + ":" + second);
        SimpleDateFormat format2 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        System.out.println(format2.format(d));
    }
}

 解释：

Calendar中 DAY_OF_MONTH,  DAY_OF_YEAR, DAY_OF_WEEK， DATE 的区别在于
如果只是的add方法执行，结果都一样，因为都是将日期+1就没有区别，在月的日期中加1，还是年的日期中加1，还是日期本身加1效果都一样
但是Calendar设置DAY_OF_MONTH和DAY_OF_YEAR的目的不是用来+1的
将日期加1，这通过cal1.add(Calendar.DATE,1)就可以实现了

DAY_OF_WEEK，用来获得当前日期是一周的第几天
DAY_OF_MONTH的主要作用是cal.get(DAY_OF_MONTH)，用来获得这一天在是这个月的第多少天
DAY_OF_YEAR的主要作用是cal.get(DAY_OF_YEAR)，用来获得这一天在是这个年的第多少天