1.在Java中獲取當前日期時間

以下是一些代碼片段，用于在 Java 中顯示當前日期時間。

對于java.time.LocalDate, 使用LocalDate.now()

  DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd");
  LocalDate localDate = LocalDate.now();
  System.out.println(dtf.format(localDate));            // 2021/03/22

對于java.time.localTime, 使用LocalTime.now()

  DateTimeFormatter dtf = DateTimeFormatter.ofPattern("HH:mm:ss");
  LocalTime localTime = LocalTime.now();
  System.out.println(dtf.format(localTime));            // 16:37:15

對于java.time.LocalDateTime, 使用LocalDateTime.now()

  DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
  LocalDateTime now = LocalDateTime.now();
  System.out.println(dtf.format(now));                  //  2021/03/22 16:37:15

對于java.time.ZonedDateTime, 使用ZonedDateTime.now()

  // get current date-time, with system default time zone
  DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
  ZonedDateTime now = ZonedDateTime.now();
  System.out.println(dtf.format(now));                  // 2021/03/22 16:37:15
  System.out.println(now.getOffset());                  // +08:00
  // get current date-time, with a specified time zone
  ZonedDateTime japanDateTime = now.withZoneSameInstant(ZoneId.of("Asia/Tokyo"));
  System.out.println(dtf.format(japanDateTime));        // 2021/03/22 17:37:15
  System.out.println(japanDateTime.getOffset());        // +09:00

對于java.time.Instant, 使用Instant.now()

  Instant now = Instant.now();
  // convert Instant to ZonedDateTime
  DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
  ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(now, ZoneId.systemDefault());
  System.out.println(dtfDateTime.format(zonedDateTime));

對于java.util.Date, 使用new Date()

  DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
  Date date = new Date();
  System.out.println(dateFormat.format(date));           // 2021/03/22 16:37:15

對于java.util.Calendar, 使用Calendar.getInstance()

  DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
  Calendar cal = Calendar.getInstance();
  System.out.println(dateFormat.format(cal.getTime()));  // 2021/03/22 16:37:15

2.java.time.LocalDate

對于java.time.LocalDate, 用于LocalDate.now()獲取沒有時區的當前日期，并使用DateTimeFormatter.

package com.mkyong.app;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
public class LocalDateExample {
  public static void main(String[] args) {
      DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd");
      LocalDate localDate = LocalDate.now();
      System.out.println(dtf.format(localDate));    // 2021/03/22
  }
}

輸出

  2021/03/22

3.java.time.LocalTime

對于java.time.LocalTime, 用于LocalDate.now()獲取沒有時區的當前時間，并使用DateTimeFormatter.

package com.mkyong.app;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;
public class LocalTimeExample {
    public static void main(String[] args) {
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("HH:mm:ss");
        LocalTime localTime = LocalTime.now();
        System.out.println(dtf.format(localTime));    // 16:37:15
    }
}

輸出

  16:37:15

4. java.time.LocalDateTime

對于java.time.LocalDateTime, 用于LocalDateTime.now()獲取沒有時區的當前日期時間，并使用DateTimeFormatter.

package com.mkyong.app;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
public class LocalDateTimeExample {
    public static void main(String[] args) {
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
        LocalDateTime now = LocalDateTime.now();
        System.out.println(dtf.format(now));        //  2021/03/22 16:37:15
    }
}

輸出

  2021/03/22 16:37:15

5.java.time.ZonedDateTime

對于java.time.ZonedDateTime, 用于ZonedDateTime.now()獲取系統默認時區或指定時區的當前日期時間。

package com.mkyong.app;
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
public class ZonedDateTimeExample {
  public static void main(String[] args) {
      DateTimeFormatter dtf = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
      // Get default time zone
      System.out.println(ZoneOffset.systemDefault());         // Asia/Kuala_Lumpur
      System.out.println(OffsetDateTime.now().getOffset());   // +08:00
      // get current date time, with +08:00
      ZonedDateTime now = ZonedDateTime.now();
      System.out.println(dtf.format(now));                    // 2021/03/22 16:37:15
      System.out.println(now.getOffset());                    // +08:00
      // get get current date time, with +09:00
      ZonedDateTime japanDateTime = now.withZoneSameInstant(ZoneId.of("Asia/Tokyo"));
      System.out.println(dtf.format(japanDateTime));          // 2021/03/22 17:37:15
      System.out.println(japanDateTime.getOffset());          // +09:00
  }
}

輸出

Asia/Kuala_Lumpur
+08:00
2021/03/22 16:37:15
+08:00
2021/03/22 17:37:15
+09:00

6.java.time.Instant

對于java.time.Instant, 用于Instant.now()獲取自Unix 紀元時間(UTC 時間 1970 年 1 月 1 日午夜)以來經過的秒數，然后轉換為其他java.time.*日期時間類，如LocalDate,LocalDateTime和ZonedDateTime.

package com.mkyong.app;
import java.time.*;
import java.time.format.DateTimeFormatter;
public class InstantExample {
  private static final DateTimeFormatter dtfDate = DateTimeFormatter.ofPattern("uuuu/MM/dd");
  private static final DateTimeFormatter dtfTime = DateTimeFormatter.ofPattern("HH:mm:ss");
  private static final DateTimeFormatter dtfDateTime = DateTimeFormatter.ofPattern("uuuu/MM/dd HH:mm:ss");
  public static void main(String[] args) {
      // seconds passed since the Unix epoch time (midnight of January 1, 1970 UTC)
      Instant now = Instant.now();
      // convert Instant to LocalDate
      LocalDate localDate = LocalDate.ofInstant(now, ZoneId.systemDefault());
      System.out.println(dtfDate.format(localDate));
      // convert Instant to localTime
      LocalTime localTime = LocalTime.ofInstant(now, ZoneId.systemDefault());
      System.out.println(dtfTime.format(localTime));
      // convert Instant to LocalDateTime
      LocalDateTime localDateTime = LocalDateTime.ofInstant(now, ZoneId.systemDefault());
      System.out.println(dtfDateTime.format(localDateTime));
      // convert Instant to ZonedDateTime
      ZonedDateTime zonedDateTime = ZonedDateTime.ofInstant(now, ZoneId.systemDefault());
      System.out.println(dtfDateTime.format(zonedDateTime));
  }
}

輸出

2021/03/22
16:37:15
2021/03/22 16:37:15
2021/03/22 16:37:15

7. java.util.Date(舊版)

對于 legacy java.util.Date，使用new Date()ornew Date(System.currentTimeMillis()獲取當前日期時間，并使用SimpleDateFormat.

package com.mkyong.app;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
public class DateExample {
  public static void main(String[] args) {
      DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
      Date date = new Date();
      System.out.println(dateFormat.format(date));    // 2021/03/22 16:37:15
      // new Date() actually calls this new Date(long date)
      Date date2 = new Date(System.currentTimeMillis());
      System.out.println(dateFormat.format(date));    // 2021/03/22 16:37:15
  }
}

輸出

  2021/03/22 16:37:15
  2021/03/22 16:37:15

8. java.util.Calendar(舊版)

對于 legacy java.util.Calendar，用于Calendar.getInstance()獲取當前日期時間，并使用SimpleDateFormat.

package com.mkyong.app;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
public class CalendarExample {
  public static void main(String[] args) {
      DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
      Calendar cal = Calendar.getInstance();
      System.out.println(dateFormat.format(cal.getTime()));   // 2021/03/22 16:37:15
  }
}

輸出

 2021/03/22 16:37:15