【深度思考】JDK8 中日期类型该如何使用，java 面试题百度网盘

2020 is leap year:true

3. LocalTime

3.1 获取时分秒

如果使用java.util.Date，那代码是下面这样的：

Date date = new Date();

int hour = date.getHours();int minute = date.getMinutes();int second = date.getSeconds();

System.out.println("hour: " + hour);System.out.println("minute: " + minute);System.out.println("second: " + second);

输出结果：

注意事项：这几个方法已经过期了，因此强烈不建议在项目中使用：

如果使用java.util.Calendar，那代码是下面这样的：

Calendar calendar = Calendar.getInstance();

// 12 小时制 int hourOf12 = calendar.get(Calendar.HOUR);// 24 小时制 int hourOf24 = calendar.get(Calendar.HOUR_OF_DAY);int minute = calendar.get(Calendar.MINUTE);int second = calendar.get(Calendar.SECOND);int milliSecond = calendar.get(Calendar.MILLISECOND);

System.out.println("hourOf12: " + hourOf12);System.out.println("hourOf24: " + hourOf24);System.out.println("minute: " + minute);System.out.println("second: " + second);System.out.println("milliSecond: " + milliSecond);

输出结果：

**注意事项：**获取小时时，有 2 个选项，1 个返回 12 小时制的小时数，1 个返回 24 小时制的小时数，因为现在是晚上 8 点，所以`cale

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ndar.get(Calendar.HOUR)返回8，而calendar.get(Calendar.HOUR_OF_DAY)`返回 20。

如果使用java.time.LocalTime，那代码是下面这样的：

LocalTime localTime = LocalTime.now();System.out.println("localTime:" + localTime);

int hour = localTime.getHour();int minute = localTime.getMinute();int second = localTime.getSecond();

System.out.println("hour: " + hour);System.out.println("minute: " + minute);System.out.println("second: " + second);

输出结果：

可以看出，LocalTime 只有时间没有日期。

4. LocalDateTime

4.1 获取当前时间

LocalDateTime localDateTime = LocalDateTime.now();System.out.println("localDateTime:" + localDateTime);

输出结果：

localDateTime: 2020-06-11T11:03:21.376

4.2 获取年月日时分秒

LocalDateTime localDateTime = LocalDateTime.now();System.out.println("localDateTime: " + localDateTime);

System.out.println("year: " + localDateTime.getYear());System.out.println("month: " + localDateTime.getMonthValue());System.out.println("day: " + localDateTime.getDayOfMonth());System.out.println("hour: " + localDateTime.getHour());System.out.println("minute: " + localDateTime.getMinute());System.out.println("second: " + localDateTime.getSecond());

输出结果：

4.3 增加天数/小时

LocalDateTime localDateTime = LocalDateTime.now();System.out.println("localDateTime: " + localDateTime);

LocalDateTime tomorrow = localDateTime.plusDays(1);System.out.println("tomorrow: " + tomorrow);

LocalDateTime nextHour = localDateTime.plusHours(1);System.out.println("nextHour: " + nextHour);

输出结果：

localDateTime: 2020-06-11T11:13:44.979

tomorrow: 2020-06-12T11:13:44.979

nextHour: 2020-06-11T12:13:44.979

LocalDateTime还提供了添加年、周、分钟、秒这些方法，这里就不一一列举了：

4.4 减少天数/小时

LocalDateTime localDateTime = LocalDateTime.now();System.out.println("localDateTime: " + localDateTime);

LocalDateTime yesterday = localDateTime.minusDays(1);System.out.println("yesterday: " + yesterday);

LocalDateTime lastHour = localDateTime.minusHours(1);System.out.println("lastHour: " + lastHour);

输出结果：

localDateTime: 2020-06-11T11:20:38.896

yesterday: 2020-06-10T11:20:38.896

lastHour: 2020-06-11T10:20:38.896

类似的，LocalDateTime还提供了减少年、周、分钟、秒这些方法，这里就不一一列举了：

4.5 获取时间是本周/本年的第几天

LocalDateTime localDateTime = LocalDateTime.now();System.out.println("localDateTime: " + localDateTime);

System.out.println("DayOfWeek: " + localDateTime.getDayOfWeek().getValue());System.out.println("DayOfYear: " + localDateTime.getDayOfYear());

输出结果：

localDateTime: 2020-06-11T11:32:31.731

DayOfWeek: 4

DayOfYear: 163

5. DateTimeFormatter

JDK8 中推出了java.time.format.DateTimeFormatter来处理日期格式化问题，《阿里巴巴 Java 开发手册》中也是建议使用DateTimeFormatter代替SimpleDateFormat。

5.1 格式化 LocalDate

LocalDate localDate = LocalDate.now();

System.out.println("ISO_DATE: " + localDate.format(DateTimeFormatter.ISO_DATE));System.out.println("BASIC_ISO_DATE: " + localDate.format(DateTimeFormatter.BASIC_ISO_DATE));System.out.println("ISO_WEEK_DATE: " + localDate.format(DateTimeFormatter.ISO_WEEK_DATE));System.out.println("ISO_ORDINAL_DATE: " + localDate.format(DateTimeFormatter.ISO_ORDINAL_DATE));

输出结果：

如果提供的格式无法满足你的需求，你还可以像以前一样自定义格式：

LocalDate localDate = LocalDate.now();

System.out.println("yyyy/MM/dd: " + localDate.format(DateTimeFormatter.ofPattern("yyyy/MM/dd")));

输出结果：

yyyy/MM/dd: 2020/06/11

5.2 格式化 LocalTime

LocalTime localTime = LocalTime.now();System.out.println(localTime);System.out.println("ISO_TIME: " + localTime.format(DateTimeFormatter.ISO_TIME));System.out.println("HH:mm:ss: " + localTime.format(DateTimeFormatter.ofPattern("HH:mm:ss")));

输出结果：

14:28:35.230

ISO_TIME: 14:28:35.23

HH:mm:ss: 14:28:35

5.3 格式化 LocalDateTime

LocalDateTime localDateTime = LocalDateTime.now();System.out.println(localDateTime);System.out.println("ISO_DATE_TIME: " + localDateTime.format(DateTimeFormatter.ISO_DATE_TIME));System.out.println("ISO_DATE: " + localDateTime.format(DateTimeFormatter.ISO_DATE));

输出结果：

2020-06-11T14:33:18.303

ISO_DATE_TIME: 2020-06-11T14:33:18.303

ISO_DATE: 2020-06-11

6. 类型相互转换

6.1 Instant 转 Date

JDK8 中，Date新增了from()方法，将Instant转换为Date，代码如下所示：

Instant instant = Instant.now();System.out.println(instant);

Date dateFromInstant = Date.from(instant);System.out.println(dateFromInstant);

输出结果：

2020-06-11T06:39:34.979Z

Thu Jun 11 14:39:34 CST 2020

6.2 Date 转 Instant

JDK8 中，Date新增了toInstant方法，将Date转换为Instant，代码如下所示：

Date date = new Date();Instant dateToInstant = date.toInstant();System.out.println(date);System.out.println(dateToInstant);

输出结果：

Thu Jun 11 14:46:12 CST 2020

2020-06-11T06:46:12.112Z

6.3 Date 转 LocalDateTime

Date date = new Date();Instant instant = date.toInstant();LocalDateTime localDateTimeOfInstant = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());System.out.println(date);System.out.println(localDateTimeOfInstant);

输出结果：

Thu Jun 11 14:51:07 CST 2020

2020-06-11T14:51:07.904

6.4 Date 转 LocalDate

Date date = new Date();Instant instant = date.toInstant();LocalDateTime localDateTimeOfInstant = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());LocalDate localDate = localDateTimeOfInstant.toLocalDate();System.out.println(date);System.out.println(localDate);

输出结果：

Thu Jun 11 14:59:38 CST 2020

2020-06-11

可以看出，Date是先转换为Instant，再转换为LocalDateTime，然后通过LocalDateTime获取LocalDate。

6.5 Date 转 LocalTime

Date date = new Date();Instant instant = date.toInstant();LocalDateTime localDateTimeOfInstant = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());LocalTime toLocalTime = localDateTimeOfInstant.toLocalTime();System.out.println(date);System.out.println(toLocalTime);

输出结果：

Thu Jun 11 15:06:14 CST 2020

15:06:14.531

可以看出，Date是先转换为Instant，再转换为LocalDateTime，然后通过LocalDateTime获取LocalTime。

6.6 LocalDateTime 转 Date

LocalDateTime localDateTime = LocalDateTime.now();

Instant toInstant = localDateTime.atZone(ZoneId.systemDefault()).toInstant();Date dateFromInstant = Date.from(toInstant);System.out.println(localDateTime);System.out.println(dateFromInstant);

输出结果：

2020-06-11T15:12:11.600

Thu Jun 11 15:12:11 CST 2020

6.7 LocalDate 转 Date

LocalDate today = LocalDate.now();

LocalDateTime localDateTime = localDate.atStartOfDay();Instant toInstant = localDateTime.atZone(ZoneId.systemDefault()).toInstant();Date dateFromLocalDate = Date.from(toInstant);System.out.println(dateFromLocalDate);

输出结果：

Thu Jun 11 00:00:00 CST 2020

6.8 LocalTime 转 Date

LocalDate localDate = LocalDate.now();LocalTime localTime = LocalTime.now();

LocalDateTime localDateTime = LocalDateTime.of(localDate, localTime);Instant instantFromLocalTime = localDateTime.atZone(ZoneId.systemDefault()).toInstant();Date dateFromLocalTime = Date.from(instantFromLocalTime);

System.out.println(dateFromLocalTime);

输出结果：

Thu Jun 11 15:24:18 CST 2020