Era.java

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

import java.time.DateTimeException;
import java.time.format.DateTimeFormatterBuilder;
import java.time.format.TextStyle;
import java.time.temporal.ChronoField;
import java.time.temporal.Temporal;
import java.time.temporal.TemporalAccessor;
import java.time.temporal.TemporalAdjuster;
import java.time.temporal.TemporalField;
import java.time.temporal.TemporalQueries;
import java.time.temporal.TemporalQuery;
import java.time.temporal.UnsupportedTemporalTypeException;
import java.time.temporal.ValueRange;
import java.util.Locale;

import static java.time.temporal.ChronoUnit.ERAS;

/**
 * An era of the time-line.
 * <p>
 * Most calendar systems have a single epoch dividing the time-line into two eras.
 * However, some calendar systems, have multiple eras, such as one for the reign
 * of each leader.
 * In all cases, the era is conceptually the largest division of the time-line.
 * Each chronology defines the Era's that are known Eras and a
 * {@link Chronology#eras Chronology.eras} to get the valid eras.
 * <p>
 * For example, the Thai Buddhist calendar system divides time into two eras,
 * before and after a single date. By contrast, the Japanese calendar system
 * has one era for the reign of each Emperor.
 * <p>
 * Instances of {@code Era} may be compared using the {@code ==} operator.
 *
 * @implSpec
 * This interface must be implemented with care to ensure other classes operate correctly.
 * All implementations must be singletons - final, immutable and thread-safe.
 * It is recommended to use an enum whenever possible.
 *
 * @since 1.8
 */
/*
 * 纪元
 *
 * Era支持的纪元包括：
 * IsoEra          - ISO纪元，基于ISO-8601历法系统
 * HijrahEra       - 伊斯兰历纪元
 * ThaiBuddhistEra - 泰国佛教历纪元
 * JapaneseEra     - 日本历纪元
 * MinguoEra       - 中华民国历纪元
 */
public interface Era extends TemporalAccessor, TemporalAdjuster {
    
    /**
     * Gets the numeric value associated with the era as defined by the chronology.
     * Each chronology defines the predefined Eras and methods to list the Eras
     * of the chronology.
     * <p>
     * All fields, including eras, have an associated numeric value.
     * The meaning of the numeric value for era is determined by the chronology
     * according to these principles:
     * <ul>
     * <li>The era in use at the epoch 1970-01-01 (ISO) has the value 1.
     * <li>Later eras have sequentially higher values.
     * <li>Earlier eras have sequentially lower values, which may be negative.
     * </ul>
     *
     * @return the numeric era value
     */
    // 返回纪元标识；在ISO历法系统中，0是公元前，1是公元(后)
    int getValue();
    
    /**
     * Checks if the specified field is supported.
     * <p>
     * This checks if this era 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.
     * <p>
     * If the field is a {@link ChronoField} then the query is implemented here.
     * The {@code ERA} field returns true.
     * All other {@code ChronoField} instances will return false.
     * <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.
     * Whether the field is supported is determined by the field.
     *
     * @param field  the field to check, null returns false
     * @return true if the field is supported on this era, false if not
     */
    // 判断当前时间量是否支持指定的时间量字段
    @Override
    default boolean isSupported(TemporalField field) {
        if (field instanceof ChronoField) {
            return field == ChronoField.ERA;
        }
        return field != null && field.isSupportedBy(this);
    }
    
    /**
     * Gets the range of valid values for the specified field.
     * <p>
     * The range object expresses the minimum and maximum valid values for a field.
     * This era is used to enhance the accuracy of the returned range.
     * If it is not possible to return the range, because the field is not supported
     * or for some other reason, an exception is thrown.
     * <p>
     * If the field is a {@link ChronoField} then the query is implemented here.
     * The {@code ERA} field returns the range.
     * All other {@code ChronoField} instances will throw an {@code UnsupportedTemporalTypeException}.
     * <p>
     * If the field is not a {@code ChronoField}, then the result of this method
     * is obtained by invoking {@code TemporalField.rangeRefinedBy(TemporalAccessor)}
     * passing {@code this} as the argument.
     * Whether the range can be obtained is determined by the field.
     * <p>
     * The default implementation must return a range for {@code ERA} from
     * zero to one, suitable for two era calendar systems such as ISO.
     *
     * @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 unit 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
    @Override
    default ValueRange range(TemporalField field) {
        return TemporalAccessor.super.range(field);
    }
    
    /**
     * Gets the value of the specified field from this era as an {@code int}.
     * <p>
     * This queries this era for the value of the specified field.
     * The returned value will always be within the valid range of values for the field.
     * If it is not possible to return the value, because the field is not supported
     * or for some other reason, an exception is thrown.
     * <p>
     * If the field is a {@link ChronoField} then the query is implemented here.
     * The {@code ERA} field returns the value of the era.
     * All other {@code ChronoField} instances will throw an {@code UnsupportedTemporalTypeException}.
     * <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. Whether the value can be obtained,
     * and what the value represents, is determined by the field.
     *
     * @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 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
     *
     * 目前支持的字段包括：
     * ChronoField.ERA - [纪元]；在ISO历法系统中，0是公元前，1是公元(后)
     */
    @Override
    default int get(TemporalField field) {
        if(field == ChronoField.ERA) {
            return getValue();
        }
        
        return TemporalAccessor.super.get(field);
    }
    
    /**
     * Gets the value of the specified field from this era as a {@code long}.
     * <p>
     * This queries this era for the value of the specified field.
     * If it is not possible to return the value, because the field is not supported
     * or for some other reason, an exception is thrown.
     * <p>
     * If the field is a {@link ChronoField} then the query is implemented here.
     * The {@code ERA} field returns the value of the era.
     * All other {@code ChronoField} instances will throw an {@code UnsupportedTemporalTypeException}.
     * <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. Whether the value can be obtained,
     * and what the value represents, is determined by the field.
     *
     * @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
     *
     * 目前支持的字段包括：
     * ChronoField.ERA - [纪元]；在ISO历法系统中，0是公元前，1是公元(后)
     */
    @Override
    default long getLong(TemporalField field) {
        if(field instanceof ChronoField) {
            if(field == ChronoField.ERA) {
                return getValue();
            }
            
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        
        return field.getFrom(this);
    }
    
    /**
     * Queries this era using the specified query.
     * <p>
     * This queries this era using the specified query strategy object.
     * The {@code TemporalQuery} object defines the logic to be used to
     * obtain the result. Read the documentation of the query to understand
     * what the result of this method will be.
     * <p>
     * The result of this method is obtained by invoking the
     * {@link TemporalQuery#queryFrom(TemporalAccessor)} method on the
     * specified query passing {@code this} as the argument.
     *
     * @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 (defined by the query)
     * @throws ArithmeticException if numeric overflow occurs (defined by the query)
     */
    // 使用指定的时间量查询器，从当前时间量中查询目标信息
    @SuppressWarnings("unchecked")
    @Override
    default <R> R query(TemporalQuery<R> query) {
        if (query == TemporalQueries.precision()) {
            return (R) ERAS;
        }
    
        return TemporalAccessor.super.query(query);
    }

    /**
     * Adjusts the specified temporal object to have the same era as this object.
     * <p>
     * This returns a temporal object of the same observable type as the input
     * with the era changed to be the same as this.
     * <p>
     * The adjustment is equivalent to using {@link Temporal#with(TemporalField, long)}
     * passing {@link ChronoField#ERA} as the field.
     * <p>
     * In most cases, it is clearer to reverse the calling pattern by using
     * {@link Temporal#with(TemporalAdjuster)}:
     * <pre>
     *   // these two lines are equivalent, but the second approach is recommended
     *   temporal = thisEra.adjustInto(temporal);
     *   temporal = temporal.with(thisEra);
     * </pre>
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param temporal  the target object to be adjusted, not null
     * @return the adjusted object, not null
     * @throws DateTimeException if unable to make the adjustment
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 拿当前时间量中的特定字段与时间量temporal中的其他字段进行整合。
     *
     * 如果整合后的值与temporal中原有的值相等，则可以直接使用temporal本身；否则，会返回新构造的时间量对象。
     *
     * 注：通常，这会用到当前时间量的所有部件信息
     *
     *
     * 当前时间量参与整合字段包括：
     * ChronoField.ERA - 当前时间量的纪元标识；在ISO历法系统中，0是公元前，1是公元(后)
     *
     * 目标时间量temporal的取值可以是：
     * Year
     * YearMonth
     * LocalDate
     * LocalDateTime
     * OffsetDateTime
     * ZonedDateTime
     * ChronoLocalDateTimeImpl的子类
     * ChronoZonedDateTimeImpl的子类
     */
    @Override
    default Temporal adjustInto(Temporal temporal) {
        // 获取当前时间量的纪元标识；在ISO历法系统中，0是公元前，1是公元(后)
        int value = getValue();
    
        return temporal.with(ChronoField.ERA, value);
    }
    
    /**
     * Gets the textual representation of this era.
     * <p>
     * This returns the textual name used to identify the era,
     * suitable for presentation to the user.
     * The parameters control the style of the returned text and the locale.
     * <p>
     * If no textual mapping is found then the {@link #getValue() numeric value} is returned.
     *
     * @apiNote This default implementation is suitable for most implementations.
     *
     * @param style  the style of the text required, not null
     * @param locale  the locale to use, not null
     * @return the text value of the era, not null
     */
    // 返回当前纪元的文本显示
    default String getDisplayName(TextStyle style, Locale locale) {
        return new DateTimeFormatterBuilder().appendText(ChronoField.ERA, style).toFormatter(locale).format(this);
    }
    
    /* NOTE: methods to convert year-of-era/proleptic-year cannot be here as they may depend on month/day (Japanese) */
    
}