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);

 }

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

 /**
 * 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.
 *
 * <p> See the description of the class <code>Date</code> 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();

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

 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 <code>Calendar.set(year + 1900, month, date,
  * hrs, min, sec)</code> or <code>GregorianCalendar(year + 1900,
  * month, date, hrs, min, sec)</code>.
  */
 @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;
 }

 //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);

/**
 * 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.
 */

 //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);

 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];
 }

//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);

/**
 * 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.
 */