Trail: Date Time
Lesson: Standard Calendar
Section: The Temporal Package

Temporal Query

可以使用 TemporalQuery 来从基于时间的对象中获取信息。

预定义查询

TemporalQueries 类(注意复数)提供了几个预定义查询，包括在应用程序无法识别基于时间的对象类型时有用的方法。与调节器一样，预定义查询被定义为静态方法，旨在与 static import 语句一起使用。

例如，precision 查询返回可由特定基于时间的对象返回的最小 ChronoUnit。以下示例对几种类型的基于时间的对象使用 precision 查询：

TemporalQueries query = TemporalQueries.precision();
System.out.printf("LocalDate precision is %s%n",
                  LocalDate.now().query(query));
System.out.printf("LocalDateTime precision is %s%n",
                  LocalDateTime.now().query(query));
System.out.printf("Year precision is %s%n",
                  Year.now().query(query));
System.out.printf("YearMonth precision is %s%n",
                  YearMonth.now().query(query));
System.out.printf("Instant precision is %s%n",
                  Instant.now().query(query));

输出如下所示：

LocalDate precision is Days
LocalDateTime precision is Nanos
Year precision is Years
YearMonth precision is Months
Instant precision is Nanos

自定义查询

你还可以创建自己的自定义查询。一种方法是使用 queryFrom(TemporalAccessor) 方法创建一个实现 TemporalQuery 接口的类。CheckDate 示例实现了两个自定义查询。第一个自定义查询可以在 FamilyVacations 类中找到，该类实现 TemporalQuery 接口。queryFrom 方法将传入的日期与预定的休假日期进行比较，如果它落在这些日期范围内，则返回 TRUE。

// Returns true if the passed-in date occurs during one of the
// family vacations. Because the query compares the month and day only,
// the check succeeds even if the Temporal types are not the same.
public Boolean queryFrom(TemporalAccessor date) {
    int month = date.get(ChronoField.MONTH_OF_YEAR);
    int day   = date.get(ChronoField.DAY_OF_MONTH);

    // Disneyland over Spring Break
    if ((month == Month.APRIL.getValue()) && ((day >= 3) && (day <= 8)))
        return Boolean.TRUE;

    // Smith family reunion on Lake Saugatuck
    if ((month == Month.AUGUST.getValue()) && ((day >= 8) && (day <= 14)))
        return Boolean.TRUE;

    return Boolean.FALSE;
}

第二个自定义查询在 FamilyBirthdays 类中实现。此类提供 isFamilyBirthday 方法，该方法将传入的日期与几个生日进行比较，如果匹配则返回 TRUE。

// Returns true if the passed-in date is the same as one of the
// family birthdays. Because the query compares the month and day only,
// the check succeeds even if the Temporal types are not the same.
public static Boolean isFamilyBirthday(TemporalAccessor date) {
    int month = date.get(ChronoField.MONTH_OF_YEAR);
    int day   = date.get(ChronoField.DAY_OF_MONTH);

    // Angie's birthday is on April 3.
    if ((month == Month.APRIL.getValue()) && (day == 3))
        return Boolean.TRUE;

    // Sue's birthday is on June 18.
    if ((month == Month.JUNE.getValue()) && (day == 18))
        return Boolean.TRUE;

    // Joe's birthday is on May 29.
    if ((month == Month.MAY.getValue()) && (day == 29))
        return Boolean.TRUE;

    return Boolean.FALSE;
}

FamilyBirthday 类没有实现 TemporalQuery 接口，可以用作 lambda expression 的一部分。以下代码来自 CheckDate 示例，显示了如何调用这两个自定义查询。

// Invoking the query without using a lambda expression.
Boolean isFamilyVacation = date.query(new FamilyVacations());

// Invoking the query using a lambda expression.
Boolean isFamilyBirthday = date.query(FamilyBirthdays::isFamilyBirthday);

if (isFamilyVacation.booleanValue() || isFamilyBirthday.booleanValue())
    System.out.printf("%s is an important date!%n", date);
else
    System.out.printf("%s is not an important date.%n", date);