TemporalAccessor.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;
import java.util.Objects;

/**
 * Framework-level interface defining read-only access to a temporal object,
 * such as a date, time, offset or some combination of these.
 * <p>
 * This is the base interface type for date, time and offset objects.
 * It is implemented by those classes that can provide information
 * as {@linkplain TemporalField fields} or {@linkplain TemporalQuery queries}.
 * <p>
 * Most date and time information can be represented as a number.
 * These are modeled using {@code TemporalField} with the number held using
 * a {@code long} to handle large values. Year, month and day-of-month are
 * simple examples of fields, but they also include instant and offsets.
 * See {@link ChronoField} for the standard set of fields.
 * <p>
 * Two pieces of date/time information cannot be represented by numbers,
 * the {@linkplain java.time.chrono.Chronology chronology} and the
 * {@linkplain java.time.ZoneId time-zone}.
 * These can be accessed via {@linkplain #query(TemporalQuery) queries} using
 * the static methods defined on {@link TemporalQuery}.
 * <p>
 * A sub-interface, {@link Temporal}, extends this definition to one that also
 * supports adjustment and manipulation on more complete temporal objects.
 * <p>
 * This interface is a framework-level interface that should not be widely
 * used in application code. Instead, applications should create and pass
 * around instances of concrete types, such as {@code LocalDate}.
 * There are many reasons for this, part of which is that implementations
 * of this interface may be in calendar systems other than ISO.
 * See {@link java.time.chrono.ChronoLocalDate} for a fuller discussion of the issues.
 *
 * @implSpec
 * This interface places no restrictions on the mutability of implementations,
 * however immutability is strongly recommended.
 *
 * @since 1.8
 */
/*
 * 时间量访问器的接口，用来访问时间量中的字段信息。
 *
 * 注：所有时间量均实现了该接口。
 */
public interface TemporalAccessor {

    /**
     * Checks if the specified field is supported.
     * <p>
     * This checks if the date-time can be queried for the specified field.
     * If false, then calling the {@link #range(TemporalField) range} and {@link #get(TemporalField) get}
     * methods will throw an exception.
     *
     * @implSpec
     * Implementations must check and handle all fields defined in {@link ChronoField}.
     * If the field is supported, then true must be returned, otherwise false must be returned.
     * <p>
     * If the field is not a {@code ChronoField}, then the result of this method
     * is obtained by invoking {@code TemporalField.isSupportedBy(TemporalAccessor)}
     * passing {@code this} as the argument.
     * <p>
     * Implementations must ensure that no observable state is altered when this
     * read-only method is invoked.
     *
     * @param field  the field to check, null returns false
     * @return true if this date-time can be queried for the field, false if not
     */
    // 判断当前时间量是否支持指定的时间量字段
    boolean isSupported(TemporalField field);

    /**
     * Gets the range of valid values for the specified field.
     * <p>
     * All fields can be expressed as a {@code long} integer.
     * This method returns an object that describes the valid range for that value.
     * The value of this temporal object is used to enhance the accuracy of the returned range.
     * If the date-time cannot return the range, because the field is unsupported or for
     * some other reason, an exception will be thrown.
     * <p>
     * Note that the result only describes the minimum and maximum valid values
     * and it is important not to read too much into them. For example, there
     * could be values within the range that are invalid for the field.
     *
     * @implSpec
     * Implementations must check and handle all fields defined in {@link ChronoField}.
     * If the field is supported, then the range of the field must be returned.
     * If unsupported, then an {@code UnsupportedTemporalTypeException} must be thrown.
     * <p>
     * If the field is not a {@code ChronoField}, then the result of this method
     * is obtained by invoking {@code TemporalField.rangeRefinedBy(TemporalAccessorl)}
     * passing {@code this} as the argument.
     * <p>
     * Implementations must ensure that no observable state is altered when this
     * read-only method is invoked.
     * <p>
     * The default implementation must behave equivalent to this code:
     * <pre>
     *  if (field instanceof ChronoField) {
     *    if (isSupported(field)) {
     *      return field.range();
     *    }
     *    throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
     *  }
     *  return field.rangeRefinedBy(this);
     * </pre>
     *
     * @param field  the field to query the range for, not null
     * @return the range of valid values for the field, not null
     * @throws DateTimeException if the range for the field cannot be obtained
     * @throws UnsupportedTemporalTypeException if the field is not supported
     */
    // 返回时间量字段field的取值区间，通常要求当前时间量支持该时间量字段
    default ValueRange range(TemporalField field) {
        Objects.requireNonNull(field, "field");
    
        if(field instanceof ChronoField) {
            // 确保当前时间量支持指定的时间量字段
            if(isSupported(field)) {
                return field.range();
            }
        
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
    
        return field.rangeRefinedBy(this);
    }

    /**
     * Gets the value of the specified field as an {@code int}.
     * <p>
     * This queries the date-time for the value of the specified field.
     * The returned value will always be within the valid range of values for the field.
     * If the date-time cannot return the value, because the field is unsupported or for
     * some other reason, an exception will be thrown.
     *
     * @implSpec
     * Implementations must check and handle all fields defined in {@link ChronoField}.
     * If the field is supported and has an {@code int} range, then the value of
     * the field must be returned.
     * If unsupported, then an {@code UnsupportedTemporalTypeException} must be thrown.
     * <p>
     * If the field is not a {@code ChronoField}, then the result of this method
     * is obtained by invoking {@code TemporalField.getFrom(TemporalAccessor)}
     * passing {@code this} as the argument.
     * <p>
     * Implementations must ensure that no observable state is altered when this
     * read-only method is invoked.
     * <p>
     * The default implementation must behave equivalent to this code:
     * <pre>
     *  if (range(field).isIntValue()) {
     *    return range(field).checkValidIntValue(getLong(field), field);
     *  }
     *  throw new UnsupportedTemporalTypeException("Invalid field " + field + " + for get() method, use getLong() instead");
     * </pre>
     *
     * @param field  the field to get, not null
     * @return the value for the field, within the valid range of values
     * @throws DateTimeException if a value for the field cannot be obtained or
     *         the value is outside the range of valid values for the field
     * @throws UnsupportedTemporalTypeException if the field is not supported or
     *         the range of values exceeds an {@code int}
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 以int形式返回时间量字段field的值
    default int get(TemporalField field) {
        // 返回时间量字段field的取值区间，通常要求当前时间量支持该时间量字段
        ValueRange range = range(field);
    
        // 确保field的取值区间是否在int的范围内
        if(!range.isIntValue()) {
            throw new UnsupportedTemporalTypeException("Invalid field " + field + " for get() method, use getLong() instead");
        }
    
        // 以long形式返回时间量字段field的值
        long value = getLong(field);
        // 确保给定的值落在字段的取值区间中
        if(!range.isValidValue(value)) {
            throw new DateTimeException("Invalid value for " + field + " (valid values " + range + "): " + value);
        }
    
        // 将long截断为int
        return (int) value;
    }

    /**
     * Gets the value of the specified field as a {@code long}.
     * <p>
     * This queries the date-time for the value of the specified field.
     * The returned value may be outside the valid range of values for the field.
     * If the date-time cannot return the value, because the field is unsupported or for
     * some other reason, an exception will be thrown.
     *
     * @implSpec
     * Implementations must check and handle all fields defined in {@link ChronoField}.
     * If the field is supported, then the value of the field must be returned.
     * If unsupported, then an {@code UnsupportedTemporalTypeException} must be thrown.
     * <p>
     * If the field is not a {@code ChronoField}, then the result of this method
     * is obtained by invoking {@code TemporalField.getFrom(TemporalAccessor)}
     * passing {@code this} as the argument.
     * <p>
     * Implementations must ensure that no observable state is altered when this
     * read-only method is invoked.
     *
     * @param field  the field to get, not null
     * @return the value for the field
     * @throws DateTimeException if a value for the field cannot be obtained
     * @throws UnsupportedTemporalTypeException if the field is not supported
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 以long形式返回时间量字段field的值
    long getLong(TemporalField field);

    /**
     * Queries this date-time.
     * <p>
     * This queries this date-time using the specified query strategy object.
     * <p>
     * Queries are a key tool for extracting information from date-times.
     * They exists 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 most common query implementations are method references, such as
     * {@code LocalDate::from} and {@code ZoneId::from}.
     * Additional implementations are provided as static methods on {@link TemporalQuery}.
     *
     * @implSpec
     * The default implementation must behave equivalent to this code:
     * <pre>
     *  if (query == TemporalQueries.zoneId() ||
     *        query == TemporalQueries.chronology() || query == TemporalQueries.precision()) {
     *    return null;
     *  }
     *  return query.queryFrom(this);
     * </pre>
     * Future versions are permitted to add further queries to the if statement.
     * <p>
     * All classes implementing this interface and overriding this method must call
     * {@code TemporalAccessor.super.query(query)}. JDK classes may avoid calling
     * super if they provide behavior equivalent to the default behaviour, however
     * non-JDK classes may not utilize this optimization and must call {@code super}.
     * <p>
     * If the implementation can supply a value for one of the queries listed in the
     * if statement of the default implementation, then it must do so.
     * For example, an application-defined {@code HourMin} class storing the hour
     * and minute must override this method as follows:
     * <pre>
     *  if (query == TemporalQueries.precision()) {
     *    return MINUTES;
     *  }
     *  return TemporalAccessor.super.query(query);
     * </pre>
     * <p>
     * Implementations must ensure that no observable state is altered when this
     * read-only method is invoked.
     *
     * @param <R> the type of the result
     * @param query  the query to invoke, not null
     * @return the query result, null may be returned (defined by the query)
     * @throws DateTimeException if unable to query
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 使用指定的时间量查询器，从当前时间量中查询目标信息
     *
     * 注：这里查询的目标信息，往往就是某个时间量字段的值
     */
    default <R> R query(TemporalQuery<R> query) {
        if(query == TemporalQueries.zoneId() || query == TemporalQueries.chronology() || query == TemporalQueries.precision()) {
            return null;
        }
    
        return query.queryFrom(this);
    }

}