摘要：使用時(shí)調(diào)用類(lèi)的方法，該方法的描述是可以看出，該方法返回的是類(lèi)型的結(jié)果，結(jié)果記錄的是至今經(jīng)過(guò)的毫秒數(shù)。關(guān)于類(lèi)，可以很自由的定制表現(xiàn)形式，年月日時(shí)分秒，時(shí)間格式，。。。

初涉江湖，還望海涵！
寫(xiě)點(diǎn)東西，純粹是因?yàn)閭€(gè)人的記憶能力較弱，寫(xiě)些筆記罷了，若有錯(cuò)誤還望雅正！

代碼粘貼

public class MainActivity extends AppCompatActivity {
    public static final String TAG = "MainActivity";
    
    @RequiresApi(api = Build.VERSION_CODES.CUPCAKE)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //timestamp
        TextView timestamp = findViewById(R.id.timestamp_show);
        timestamp.setText("timestamp:" + System.currentTimeMillis());
        //date
        Date date = new Date();
        TextView date_show = findViewById(R.id.date_show);
        date_show.setText("Date:" + date.toString());
        //Calendar
        TextView calendar_show = findViewById(R.id.calendar_show);
        Calendar calendar = Calendar.getInstance();
        int year = calendar.get(Calendar.YEAR);
        int month = calendar.get(Calendar.MONTH);
        int day = calendar.get(Calendar.DATE);
        int hour = calendar.get(Calendar.HOUR);
        int minute = calendar.get(Calendar.MINUTE);
        int second = calendar.get(Calendar.SECOND);
        String calendar_show_string = "Calendar:" + year + "-" + month + "-" + day
                + "  " + hour + ":" + minute + ":" + second;
        calendar_show.setText(calendar_show_string);
        //Time
        TextView time_show = findViewById(R.id.time_show);
        Time time = new Time();
        time.setToNow();
        int time_year = time.year;
        int time_month = time.month;
        int time_day = time.monthDay;
        int time_hour = time.hour;
        int time_minute = time.minute;
        int time_second = time.second;
        String time_show_string = "Time:" + time_year + "-" + time_month + "-"
                + time_day + "  " + time_hour + ":" + time_minute + ":" + time_second;
        time_show.setText(time_show_string);
        //SimpleDateFormat
        TextView simpleDateFormat_show = findViewById(R.id.simpleDateFormat_show);
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String simpleDateFormat_tring = "SimpleDateFormat:" + format.format(new Date());
        simpleDateFormat_show.setText(simpleDateFormat_tring);
        Log.d(TAG, "onCreate: Long的最大值：" + Long.MAX_VALUE);

    }

1 timestamp
2 date
3 SimpleDateFormat
4 Calendar
5 Time

  //timestamp
  TextView timestamp = findViewById(R.id.timestamp_show);
  timestamp.setText("timestamp:" + System.currentTimeMillis());

timestamp，時(shí)間戳。
使用時(shí)調(diào)用System類(lèi)的currentTimeMillis()方法，該方法的描述是：

  /**
   * Returns the current time in milliseconds.  Note that
   * while the unit of time of the return value is a millisecond,
   * the granularity of the value depends on the underlying
   * operating system and may be larger.  For example, many
   * operating systems measure time in units of tens of
   * milliseconds.
   *
   * 
 See the description of the class Date for
   * a discussion of slight discrepancies that may arise between
   * "computer time" and coordinated universal time (UTC).
   *
   * @return  the difference, measured in milliseconds, between
   *          the current time and midnight, January 1, 1970 UTC.
   * @see     java.util.Date
   */
  public static native long currentTimeMillis();

可以看出，該方法返回的是long類(lèi)型的結(jié)果，結(jié)果記錄的是midnight, January 1, 1970 UTC至今經(jīng)過(guò)的毫秒數(shù)(milliseconds)。

System.currentTimeMillis()是一個(gè)native方法，是一個(gè)C/C++方法，由系統(tǒng)測(cè)量時(shí)間戳并返回測(cè)量結(jié)果，根據(jù)注釋描述，測(cè)量結(jié)果可能偏大，因?yàn)橛行┎僮飨到y(tǒng)測(cè)量時(shí)間是以十毫秒為單位的，類(lèi)Date中討論了關(guān)于系統(tǒng)時(shí)間和UTC時(shí)間產(chǎn)生差異的原因，可自行觀看！

Note：

UTC(coordinated universal time)是民用時(shí)間的標(biāo)準(zhǔn)，眾所周知，地球圍繞太陽(yáng)公轉(zhuǎn)一周的時(shí)間定義為一年，地球自轉(zhuǎn)一周定義為一天。有科學(xué)報(bào)道說(shuō)，地球漫長(zhǎng)的公轉(zhuǎn)中其實(shí)是在緩慢的接近太陽(yáng)，不管是否屬實(shí)，自轉(zhuǎn)和公轉(zhuǎn)會(huì)產(chǎn)生一些變化也是不可避免的，UTC就是正確測(cè)量時(shí)間的規(guī)則，當(dāng)測(cè)量到需要校正時(shí)間時(shí)，會(huì)以毫秒為單位進(jìn)行調(diào)整，稱(chēng)之為閏秒(leap seconds)，后面Time會(huì)提到！

System.currentTimeMillis()的返回結(jié)果是一個(gè)記錄從1970開(kāi)始的毫秒數(shù)的long型結(jié)果，最容易想到的是long是有范圍區(qū)間的，如果有一天記錄的毫秒數(shù)超出long的范圍怎么辦！所以我計(jì)算了以下，long的最大值為0x7fff,ffff,ffff,ffff，取整大約為922億億，一年算365天，不考慮閏年，一天246060*60毫秒一年取整大約18億毫秒，922億/18，大約為50億年，考慮到太陽(yáng)的壽命，貌似也有用盡的一天。。。。但是，那么長(zhǎng)的時(shí)間，鬼知道會(huì)發(fā)展成什么樣！

  //date
  Date date = new Date();
  TextView date_show = findViewById(R.id.date_show);
  date_show.setText("Date:" + date.toString());

通過(guò)實(shí)例化Date類(lèi)獲取date實(shí)例從而獲取時(shí)間，簡(jiǎn)單通過(guò)toString()打印結(jié)果

Date類(lèi)的注釋特別描述了

日歷記時(shí)中，一年定為365天，閏年多一天，這表明，時(shí)間并不總是一天246060*60毫秒，需要用閏年加一天來(lái)調(diào)整。在coordinated universal time (UTC)的時(shí)間定義中，是通過(guò)閏秒(leap second)來(lái)調(diào)整時(shí)間的，并且總是在6月30日或12月31日，具體表現(xiàn)為該類(lèi)對(duì)秒的限制在0 to 61，60和61發(fā)生在leap second時(shí)。

構(gòu)造函數(shù)

    public Date() {
        this(System.currentTimeMillis());
    }
    
    public Date(long date) {
        fastTime = date;
    }
    
    /**
     * @param   year    the year minus 1900.
     * @param   month   the month between 0-11.
     * @param   date    the day of the month between 1-31.
     * @param   hrs     the hours between 0-23.
     * @param   min     the minutes between 0-59.
     * @param   sec     the seconds between 0-59.
     * @see     java.util.Calendar
     * @deprecated As of JDK version 1.1,
     * replaced by Calendar.set(year + 1900, month, date,
     * hrs, min, sec) or GregorianCalendar(year + 1900,
     * month, date, hrs, min, sec).
     */
    @Deprecated
    public Date(int year, int month, int date, int hrs, int min, int sec) {
        int y = year + 1900;
        // month is 0-based. So we have to normalize month to support Long.MAX_VALUE.
        if (month >= 12) {
            y += month / 12;
            month %= 12;
        } else if (month < 0) {
            y += CalendarUtils.floorDivide(month, 12);
            month = CalendarUtils.mod(month, 12);
        }
        BaseCalendar cal = getCalendarSystem(y);
        cdate = (BaseCalendar.Date) cal.newCalendarDate(TimeZone.getDefaultRef());
        cdate.setNormalizedDate(y, month + 1, date).setTimeOfDay(hrs, min, sec, 0);
        getTimeImpl();
        cdate = null;
    }

無(wú)參大Date()直接把System.currentTimeMillis()的時(shí)間戳返回給fastTime，另一個(gè)就是設(shè)定好年月日時(shí)分秒來(lái)創(chuàng)建對(duì)象，其中的設(shè)定是年是1900+參數(shù)year并且也對(duì)月份超出范圍做出了處理，但是該構(gòu)造方法已是@Deprecated(棄用)了

Date類(lèi)中大部分的方法都已經(jīng)棄用，要特別是多帶帶獲取年或者月等信息的方法，基本上都已經(jīng)棄用，留下的有打印即toString()和一些比較等功能性的方法

  //SimpleDateFormat
  TextView simpleDateFormat_show = findViewById(R.id.simpleDateFormat_show);
  SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
  String simpleDateFormat_tring = "SimpleDateFormat:" + format.format(new Date());
  simpleDateFormat_show.setText(simpleDateFormat_tring);

SimpleDateFormat類(lèi)的核心是Text的formatting(格式化)和Time的parsing(解析)，SimpleDateFormat()通過(guò)傳入一個(gè)字符串來(lái)格式化需要的表現(xiàn)形式，樣例中通過(guò)調(diào)用format()傳入Date無(wú)參對(duì)象，實(shí)際上是調(diào)用System.currentTimeMillis()獲取最基本的時(shí)間，SimpleDateFormat類(lèi)的作用是把傳入的Date類(lèi)時(shí)間定制化封裝，從而得到需要的結(jié)果。

Note：
關(guān)于SimpleDateFormat類(lèi)，可以很自由的定制表現(xiàn)形式，年月日時(shí)分秒，時(shí)間格式，AD/BC。。。

定制化所用字母的含義：

G => AD/BC(年份為負(fù)數(shù)時(shí))，1+

y => Year，1+

Y => Week year，24+

M => Month in year，1+

w => Week in year，1+

W => Week in month，1+

D => Day in year，1+

d => Day in month，1+

F => Day of week in month，1+

E => Day name in week，1+

u => Day number of week(1 = Monday, ..., 7 = Sunday)，24+

a => Am/pm marker，1+

H => Hour in day (0-23)，1+

k => Hour in day (1-24)，1+

K => Hour in am/pm (0-11)，1+

h => Hour in am/pm (1-12)，1+

m => Minute in hour，1+

s => Second in minute，1+

S => Millisecond，1+

z => Time zone:General time zone,PST,GMT-08:00，1+

Z => Time zone:RFC 822 time zone，1+

X => Time zone:ISO 8601 time zone，1+

定制化使用"字符串"，在該字符串中使用"字符"表示在年月日等數(shù)據(jù)外的部分，如分隔符

SimpleDateFormat類(lèi)的時(shí)間格式定制包括年月日等數(shù)據(jù)的表現(xiàn)形式，連接符，日期格式的描述，如Time zone，AM/PM，AD/BC。。。

SimpleDateFormat類(lèi)中存在的問(wèn)題是線(xiàn)程同步

/**
 * Date formats are not synchronized.
 * It is recommended to create separate format instances for each thread.
 * If multiple threads access a format concurrently, it must be synchronized
 * externally.
 */

SimpleDateFormat是線(xiàn)程不同步的，要在多線(xiàn)程中使用則要在線(xiàn)程外同步.

  //Calendar
  TextView calendar_show = findViewById(R.id.calendar_show);
  Calendar calendar = Calendar.getInstance();
  int year = calendar.get(Calendar.YEAR);
  int month = calendar.get(Calendar.MONTH);
  int day = calendar.get(Calendar.DATE);
  int hour = calendar.get(Calendar.HOUR);
  int minute = calendar.get(Calendar.MINUTE);
  int second = calendar.get(Calendar.SECOND);
  String calendar_show_string = "Calendar:" + year + "-" + month + "-" + day + "  " + hour + ":" + minute + ":" + second;
  calendar_show.setText(calendar_show_string);

Calendar是一個(gè)抽象類(lèi)通過(guò)其內(nèi)定義的Calendar.getInstance()靜態(tài)方法實(shí)例化對(duì)象而該靜態(tài)方法最終是通過(guò)返回一個(gè)new GregorianCalendar(zone, aLocale)來(lái)實(shí)現(xiàn)初始化！

Calendar類(lèi)內(nèi)部定義了關(guān)于時(shí)間需要用到的索引并用一個(gè)int數(shù)組存儲(chǔ)相關(guān)數(shù)據(jù)

  public final static int ERA = 0;
  public final static int YEAR = 1;
  public final static int MONTH = 2;
  public final static int WEEK_OF_YEAR = 3;
  ...
  @SuppressWarnings("ProtectedField")
  protected int fields[];

    public int get(int field)
    {
        complete();
        return internalGet(field);
    }
    
        protected final int internalGet(int field)
    {
        return fields[field];
    }

Calendar類(lèi)的簡(jiǎn)單實(shí)用就是通過(guò)調(diào)用get方法從數(shù)組中獲取相應(yīng)的數(shù)據(jù)

  //Time
  TextView time_show = findViewById(R.id.time_show);
  Time time = new Time();
  time.setToNow();
  int time_year = time.year;
  int time_month = time.month;
  int time_day = time.monthDay;
  int time_hour = time.hour;
  int time_minute = time.minute;
  int time_second = time.second;
  String time_show_string = "Time:" + time_year + "-" + time_month + "-" + time_day + "  " + time_hour + ":" + time_minute + ":" + time_second;
  time_show.setText(time_show_string);

把這段代碼打入到剪輯器，你會(huì)看到Time這個(gè)類(lèi)是棄用了的

官方的注釋解釋是這樣的

/**
 * An alternative to the {@link java.util.Calendar} and
 * {@link java.util.GregorianCalendar} classes. An instance of the Time class represents
 * a moment in time, specified with second precision. It is modelled after
 * struct tm. This class is not thread-safe and does not consider leap seconds.
 */

可以看到，描述上說(shuō)，這是線(xiàn)程不安全的類(lèi)，同時(shí)也沒(méi)有處理leap seconds(閏秒)的能力，還舉出了幾個(gè)例子。

雖然是棄用的方法，但是還是可以看看怎么使用Time類(lèi)的，簡(jiǎn)單地說(shuō)，就是通過(guò)對(duì)象.變量的形式獲取，也就是說(shuō)，Time不像Calendar類(lèi)那樣使用數(shù)組存儲(chǔ)數(shù)據(jù)，Time就是通過(guò)創(chuàng)建public int 數(shù)據(jù) 的形式來(lái)保存數(shù)據(jù)，也就是這些數(shù)據(jù)都是public的

總的來(lái)說(shuō)，獲取數(shù)據(jù)的時(shí)候，通過(guò)Time的形式，如int time_hour = time.hour;這樣的寫(xiě)法，其實(shí)才是最舒服的(個(gè)人感覺(jué))，當(dāng)然，最重要的還是安全問(wèn)題