使用新的日期时间API格式化日期 [英] Format a date using the new date time API

问题描述

我正在玩新的日期时间API，但运行时：

I was playing with the new date time API but when running this:

public class Test {         
    public static void main(String[] args){
        String dateFormatted = LocalDate.now()
                                        .format(DateTimeFormatter
                                              .ofPattern("yyyy-MM-dd HH:mm:ss"));
        System.out.println(dateFormatted);
    }
}

它抛出：

Exception in thread "main" java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: HourOfDay
    at java.time.LocalDate.get0(LocalDate.java:680)
    at java.time.LocalDate.getLong(LocalDate.java:659)
    at java.time.format.DateTimePrintContext.getValue(DateTimePrintContext.java:298)
    at java.time.format.DateTimeFormatterBuilder$NumberPrinterParser.format(DateTimeFormatterBuilder.java:2543)
    at java.time.format.DateTimeFormatterBuilder$CompositePrinterParser.format(DateTimeFormatterBuilder.java:2182)
    at java.time.format.DateTimeFormatter.formatTo(DateTimeFormatter.java:1745)
    at java.time.format.DateTimeFormatter.format(DateTimeFormatter.java:1719)
    at java.time.LocalDate.format(LocalDate.java:1685)
    at Test.main(Test.java:23)

查看源代码的LocalDate类，我是ee：

When looking at the source code of the LocalDate class, I see:

  private int get0(TemporalField field) {
        switch ((ChronoField) field) {
            case DAY_OF_WEEK: return getDayOfWeek().getValue();
            case ALIGNED_DAY_OF_WEEK_IN_MONTH: return ((day - 1) % 7) + 1;
            case ALIGNED_DAY_OF_WEEK_IN_YEAR: return ((getDayOfYear() - 1) % 7) + 1;
            case DAY_OF_MONTH: return day;
            case DAY_OF_YEAR: return getDayOfYear();
            case EPOCH_DAY: throw new UnsupportedTemporalTypeException("Invalid field 'EpochDay' for get() method, use getLong() instead");
            case ALIGNED_WEEK_OF_MONTH: return ((day - 1) / 7) + 1;
            case ALIGNED_WEEK_OF_YEAR: return ((getDayOfYear() - 1) / 7) + 1;
            case MONTH_OF_YEAR: return month;
            case PROLEPTIC_MONTH: throw new UnsupportedTemporalTypeException("Invalid field 'ProlepticMonth' for get() method, use getLong() instead");
            case YEAR_OF_ERA: return (year >= 1 ? year : 1 - year);
            case YEAR: return year;
            case ERA: return (year >= 1 ? 1 : 0);
        }
        throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
    }

正如文档中所述：

此方法将基于类文档中描述的
字母和符号的简单模式创建一个格式化程序。

This method will create a formatter based on a simple pattern of letters and symbols as described in the class documentation.

所有这些字母都是定义。

那么为什么 DateTimeFormatter.ofPattern 不允许我们使用一些图案字母？

So why DateTimeFormatter.ofPattern doesn't allow us to use some pattern letters?

推荐答案

LocalDate 仅代表日期，而不是DateTime。因此，格式化 LocalDate 时，HH：mm：ss无效。假设您要代表日期和时间，请使用 LocalDateTime 。

LocalDate represents just a date, not a DateTime. So "HH:mm:ss" make no sense when formatting a LocalDate. Use a LocalDateTime instead, assuming you want to represent both a date and time.

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