TemporalQuery.java

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 * Copyright (c) 2012, Stephen Colebourne & Michael Nascimento Santos
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package java.time.temporal;

import java.time.DateTimeException;

/**
 * Strategy for querying a temporal object.
 * <p>
 * Queries are a key tool for extracting information from temporal objects.
 * They exist to externalize the process of querying, permitting different
 * approaches, as per the strategy design pattern.
 * Examples might be a query that checks if the date is the day before February 29th
 * in a leap year, or calculates the number of days to your next birthday.
 * <p>
 * The {@link TemporalField} interface provides another mechanism for querying
 * temporal objects. That interface is limited to returning a {@code long}.
 * By contrast, queries can return any type.
 * <p>
 * There are two equivalent ways of using a {@code TemporalQuery}.
 * The first is to invoke the method on this interface directly.
 * The second is to use {@link TemporalAccessor#query(TemporalQuery)}:
 * <pre>
 *   // these two lines are equivalent, but the second approach is recommended
 *   temporal = thisQuery.queryFrom(temporal);
 *   temporal = temporal.query(thisQuery);
 * </pre>
 * It is recommended to use the second approach, {@code query(TemporalQuery)},
 * as it is a lot clearer to read in code.
 * <p>
 * The most common implementations are method references, such as
 * {@code LocalDate::from} and {@code ZoneId::from}.
 * Additional common queries are provided as static methods in {@link TemporalQueries}.
 *
 * @implSpec
 * This interface places no restrictions on the mutability of implementations,
 * however immutability is strongly recommended.
 *
 * @param <R> the type returned from the query
 *
 * @since 1.8
 */
/*
 * 时间量查询器的接口，用来从指定的时间量中查询目标信息。
 * 该接口同时也是一个函数表达式，具体查询行为由实现类指定。
 * 注：TemporalQueries类中预设了多种实用的查询器。
 */
@FunctionalInterface
public interface TemporalQuery<R> {

    /**
     * Queries the specified temporal object.
     * <p>
     * This queries the specified temporal object to return an object using the logic
     * encapsulated in the implementing class.
     * Examples might be a query that checks if the date is the day before February 29th
     * in a leap year, or calculates the number of days to your next birthday.
     * <p>
     * There are two equivalent ways of using this method.
     * The first is to invoke this method directly.
     * The second is to use {@link TemporalAccessor#query(TemporalQuery)}:
     * <pre>
     *   // these two lines are equivalent, but the second approach is recommended
     *   temporal = thisQuery.queryFrom(temporal);
     *   temporal = temporal.query(thisQuery);
     * </pre>
     * It is recommended to use the second approach, {@code query(TemporalQuery)},
     * as it is a lot clearer to read in code.
     *
     * @implSpec
     * The implementation must take the input object and query it.
     * The implementation defines the logic of the query and is responsible for
     * documenting that logic.
     * It may use any method on {@code TemporalAccessor} to determine the result.
     * The input object must not be altered.
     * <p>
     * The input temporal object may be in a calendar system other than ISO.
     * Implementations may choose to document compatibility with other calendar systems,
     * or reject non-ISO temporal objects by {@link TemporalQueries#chronology() querying the chronology}.
     * <p>
     * This method may be called from multiple threads in parallel.
     * It must be thread-safe when invoked.
     *
     * @param temporal  the temporal object to query, not null
     * @return the queried value, may return null to indicate not found
     * @throws DateTimeException if unable to query
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 从时间量temporal中查询出目标对象
    R queryFrom(TemporalAccessor temporal);

}