Java 日期时间获取和显示

Java正确获取星期Calendar.DAY_OF_WEEK
http://chamcon.iteye.com/blog/2144433

Java SimpleDateFormat 中英文时间格式化转换

https://blog.csdn.net/KingWTD/article/details/48089111

java-使用SimpleDateFormat格式化日期
http://zy-email1991.iteye.com/blog/2243021

编写一个java程序用以将AM/PM格式的时间转换为24小时格式
https://zhidao.baidu.com/question/1798461878119559747.html?qbl=relate_question_1&word=java%20am%20pm%20%D3%A2%CE%C4

个人写的一个Demo:

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

public class Main {

    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance(TimeZone.getDefault());
        // FAQ :
        // 1.不同地区国家时间格式显示和同步问题？ UTC,格林威治？夏威夷？国际化 ~ locale
        // 2.本地client和服务器时间同步问题，比如定时下发执行任务，本地预定时早于服务器当前时间，则无法执行
        System.out.println("\nTimezone: " + TimeZone.getDefault());
        
        long time = calendar.getTimeInMillis();
        Date date1 = calendar.getTime();
        Date date2 = new Date(); // get current date
        
        // method1: direct format fun call
//        a ~ AM/PM, 上午/下午
        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd HH:mm"); // HH ~ 24小时制
        System.out.printf("\nTime1 in 24h : " + sdf1.format(calendar.getTime()));

        SimpleDateFormat sdf11 = new SimpleDateFormat("yyyy-MM-dd hh:mm"); // hh ~ 12小时制
        System.out.printf("\nTime11 in 12h : " + sdf11.format(calendar.getTime()));
        
        SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd a HH:mm", Locale.ENGLISH);
        System.out.printf("\nTime2: " + sdf2.format(calendar.getTime()));
        
        SimpleDateFormat sdf3 = new SimpleDateFormat("yyyy年MM月dd日HH:mm");
        System.out.printf("\nTime3: " + sdf3.format(calendar.getTime()));
        
        SimpleDateFormat sdf4 = new SimpleDateFormat("yyyy年MM月dd日 a HH:mm");
        System.out.printf("\nTime4: " + sdf4.format(calendar.getTime()));

        // method2: use self define format func
        int year = calendar.get(Calendar.YEAR);
        int month = calendar.get(Calendar.MONTH);
        int day = calendar.get(Calendar.DAY_OF_MONTH);
        int hour = calendar.get(Calendar.HOUR_OF_DAY);
        int minute = calendar.get(Calendar.MINUTE);
        int second = calendar.get(Calendar.SECOND);

        // 欧美国家一周的第一天是星期天，这在java api代码中是写死的，因此这里要自己手动做下判断
        // Calendar中的星期一、二、三、四、五、六、日、DAY_OF_WEEK等都是以一个常量的形式存在的，
        // 我们设置setFirstDayOfWeek()并不能改变这些常量的值
        int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK); // 获取当天是星期几
        boolean isFirstSunday = (calendar.getFirstDayOfWeek() == Calendar.SUNDAY);
        if (isFirstSunday) {
            dayOfWeek -= 1;
            if (dayOfWeek == 0) {
                dayOfWeek = 7;
            }
        }

        int dayOfWeekInMonth = calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH); // 某月中第几周. 按这个月1号算,1号起就是第1周,8号起就是第2周
        int weekOfMonth = calendar.get(Calendar.WEEK_OF_MONTH); // 该月的第几周
        int dayOfYear = calendar.get(Calendar.DAY_OF_YEAR); // 一年的第几天，从1号算起
        System.out.println("\n" + getFormattedTime(year, month, day, hour, minute));
        System.out.println("\ndayOfWeek: " + dayOfWeek + "\ndayOfWeekInMonth: " + dayOfWeekInMonth 
                + "\ndayOfYear: " + dayOfYear);
    }

    /**
     * 格式化日期字符串，形如 xx年xx月xx日xx:xx:xx
     * @param year
     * @param month
     * @param day
     * @param hour
     * @param minute
     * @return String
     */
    public static String getFormattedTime(int year, int month, int day, int hour, int minute) {
        String yearStr = "";
        String monthStr = "";
        String dayStr = "";
        String hourStr = "";
        String minuteStr = "";
        // year
        if (year < 10) {
            yearStr = "0" + year;
        } else {
            yearStr = String.valueOf(year);
        }
        // month
        if (month < 10) {
            monthStr = "0" + month;
        } else {
            monthStr = Integer.toString(month);
        }
        // day
        if (day < 10) {
            dayStr = "0" + day;
        } else {
            dayStr = Integer.toString(day);
        }
        // hour
        if (hour < 10) {
            hourStr = "0" + hour;
        } else {
            hourStr = Integer.toString(hour);
        }
        // minute
        if (minute < 10) {
            minuteStr = "0" + minute;
        } else {
            minuteStr = Integer.toString(minute);
        }
        return yearStr + "年" + monthStr + "月" + dayStr + "日" + hourStr + ":" + minuteStr;
    }
}