YearMonth.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Copyright (c) 2007-2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
 *
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 * modification, are permitted provided that the following conditions are met:
 *
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package java.time;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.time.chrono.Chronology;
import java.time.chrono.IsoChronology;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.format.DateTimeParseException;
import java.time.format.SignStyle;
import java.time.temporal.ChronoField;
import java.time.temporal.ChronoUnit;
import java.time.temporal.Temporal;
import java.time.temporal.TemporalAccessor;
import java.time.temporal.TemporalAdjuster;
import java.time.temporal.TemporalAmount;
import java.time.temporal.TemporalField;
import java.time.temporal.TemporalQueries;
import java.time.temporal.TemporalQuery;
import java.time.temporal.TemporalUnit;
import java.time.temporal.UnsupportedTemporalTypeException;
import java.time.temporal.ValueRange;
import java.util.Objects;

import static java.time.temporal.ChronoUnit.CENTURIES;
import static java.time.temporal.ChronoUnit.DECADES;
import static java.time.temporal.ChronoUnit.ERAS;
import static java.time.temporal.ChronoUnit.MILLENNIA;
import static java.time.temporal.ChronoUnit.MONTHS;
import static java.time.temporal.ChronoUnit.YEARS;

/**
 * A year-month in the ISO-8601 calendar system, such as {@code 2007-12}.
 * <p>
 * {@code YearMonth} is an immutable date-time object that represents the combination
 * of a year and month. Any field that can be derived from a year and month, such as
 * quarter-of-year, can be obtained.
 * <p>
 * This class does not store or represent a day, time or time-zone.
 * For example, the value "October 2007" can be stored in a {@code YearMonth}.
 * <p>
 * The ISO-8601 calendar system is the modern civil calendar system used today
 * in most of the world. It is equivalent to the proleptic Gregorian calendar
 * system, in which today's rules for leap years are applied for all time.
 * For most applications written today, the ISO-8601 rules are entirely suitable.
 * However, any application that makes use of historical dates, and requires them
 * to be accurate will find the ISO-8601 approach unsuitable.
 *
 * <p>
 * This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code YearMonth} may have unpredictable results and should be avoided.
 * The {@code equals} method should be used for comparisons.
 *
 * @implSpec This class is immutable and thread-safe.
 * @since 1.8
 */
// x年x月
public final class YearMonth implements Temporal, TemporalAdjuster, Comparable<YearMonth>, Serializable {
    
    /**
     * Parser.
     */
    private static final DateTimeFormatter PARSER = new DateTimeFormatterBuilder().appendValue(ChronoField.YEAR, 4, 10, SignStyle.EXCEEDS_PAD).appendLiteral('-').appendValue(ChronoField.MONTH_OF_YEAR, 2).toFormatter();
    
    /**
     * The year.
     */
    private final int year;  // "Proleptic年"部件[-999_999_999, 999_999_999]
    
    /**
     * The month-of-year, not null.
     */
    private final int month; // "月份"部件[1, 12]
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructor.
     *
     * @param year  the year to represent, validated from MIN_YEAR to MAX_YEAR
     * @param month the month-of-year to represent, validated from 1 (January) to 12 (December)
     */
    private YearMonth(int year, int month) {
        this.year = year;
        this.month = month;
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Obtains the current year-month from the system clock in the default time-zone.
     * <p>
     * This will query the {@link Clock#systemDefaultZone() system clock} in the default
     * time-zone to obtain the current year-month.
     * <p>
     * Using this method will prevent the ability to use an alternate clock for testing
     * because the clock is hard-coded.
     *
     * @return the current year-month using the system clock and default time-zone, not null
     */
    // 基于此刻的UTC时间，构造属于系统默认时区的YearMonth对象
    public static YearMonth now() {
        // 获取一个系统时钟，其预设的时区ID为系统默认的时区ID
        Clock clock = Clock.systemDefaultZone();
        return now(clock);
    }
    
    /**
     * Obtains the current year-month from the system clock in the specified time-zone.
     * <p>
     * This will query the {@link Clock#system(ZoneId) system clock} to obtain the current year-month.
     * Specifying the time-zone avoids dependence on the default time-zone.
     * <p>
     * Using this method will prevent the ability to use an alternate clock for testing
     * because the clock is hard-coded.
     *
     * @param zone the zone ID to use, not null
     *
     * @return the current year-month using the system clock, not null
     */
    // 基于此刻的UTC时间，构造属于zone时区的YearMonth对象
    public static YearMonth now(ZoneId zone) {
        // 获取一个系统时钟，其预设的时区ID为zone
        Clock clock = Clock.system(zone);
        return now(clock);
    }
    
    /**
     * Obtains the current year-month from the specified clock.
     * <p>
     * This will query the specified clock to obtain the current year-month.
     * Using this method allows the use of an alternate clock for testing.
     * The alternate clock may be introduced using {@link Clock dependency injection}.
     *
     * @param clock the clock to use, not null
     *
     * @return the current year-month, not null
     */
    // 基于clock提供的时间戳和时区ID构造YearMonth对象
    public static YearMonth now(Clock clock) {
        final LocalDate now = LocalDate.now(clock);
        return YearMonth.of(now.getYear(), now.getMonth());
    }
    
    /**
     * Obtains an instance of {@code YearMonth} from a year and month.
     *
     * @param year  the year to represent, from MIN_YEAR to MAX_YEAR
     * @param month the month-of-year to represent, not null
     *
     * @return the year-month, not null
     *
     * @throws DateTimeException if the year value is invalid
     */
    // 根据Proleptic年份、月份构造YearMonth对象
    public static YearMonth of(int year, Month month) {
        Objects.requireNonNull(month, "month");
        return of(year, month.getValue());
    }
    
    /**
     * Obtains an instance of {@code YearMonth} from a year and month.
     *
     * @param year  the year to represent, from MIN_YEAR to MAX_YEAR
     * @param month the month-of-year to represent, from 1 (January) to 12 (December)
     *
     * @return the year-month, not null
     *
     * @throws DateTimeException if either field value is invalid
     */
    // 根据Proleptic年份、月份构造YearMonth对象
    public static YearMonth of(int year, int month) {
        ChronoField.YEAR.checkValidValue(year);
        ChronoField.MONTH_OF_YEAR.checkValidValue(month);
    
        return new YearMonth(year, month);
    }
    
    /**
     * Obtains an instance of {@code YearMonth} from a temporal object.
     * <p>
     * This obtains a year-month based on the specified temporal.
     * A {@code TemporalAccessor} represents an arbitrary set of date and time information,
     * which this factory converts to an instance of {@code YearMonth}.
     * <p>
     * The conversion extracts the {@link ChronoField#YEAR YEAR} and
     * {@link ChronoField#MONTH_OF_YEAR MONTH_OF_YEAR} fields.
     * The extraction is only permitted if the temporal object has an ISO
     * chronology, or can be converted to a {@code LocalDate}.
     * <p>
     * This method matches the signature of the functional interface {@link TemporalQuery}
     * allowing it to be used as a query via method reference, {@code YearMonth::from}.
     *
     * @param temporal the temporal object to convert, not null
     *
     * @return the year-month, not null
     *
     * @throws DateTimeException if unable to convert to a {@code YearMonth}
     */
    // 从temporal中查询YearMonth对象
    public static YearMonth from(TemporalAccessor temporal) {
        Objects.requireNonNull(temporal, "temporal");
    
        if(temporal instanceof YearMonth) {
            return (YearMonth) temporal;
        }
    
        try {
            if(!IsoChronology.INSTANCE.equals(Chronology.from(temporal))) {
                temporal = LocalDate.from(temporal);
            }
        
            return of(temporal.get(ChronoField.YEAR), temporal.get(ChronoField.MONTH_OF_YEAR));
        } catch(DateTimeException ex) {
            throw new DateTimeException("Unable to obtain YearMonth from TemporalAccessor: " + temporal + " of type " + temporal.getClass().getName(), ex);
        }
    }
    
    /**
     * Obtains an instance of {@code YearMonth} from a text string such as {@code 2007-12}.
     * <p>
     * The string must represent a valid year-month.
     * The format must be {@code uuuu-MM}.
     * Years outside the range 0000 to 9999 must be prefixed by the plus or minus symbol.
     *
     * @param text the text to parse such as "2007-12", not null
     *
     * @return the parsed year-month, not null
     *
     * @throws DateTimeParseException if the text cannot be parsed
     */
    // 从指定的文本中解析出YearMonth信息，要求该文本符合ISO规范，即类似：2020-05
    public static YearMonth parse(CharSequence text) {
        return parse(text, PARSER);
    }
    
    /**
     * Obtains an instance of {@code YearMonth} from a text string using a specific formatter.
     * <p>
     * The text is parsed using the formatter, returning a year-month.
     *
     * @param text      the text to parse, not null
     * @param formatter the formatter to use, not null
     *
     * @return the parsed year-month, not null
     *
     * @throws DateTimeParseException if the text cannot be parsed
     */
    // 从指定的文本中解析出YearMonth信息，要求该文本符合指定的格式规范
    public static YearMonth parse(CharSequence text, DateTimeFormatter formatter) {
        Objects.requireNonNull(formatter, "formatter");
        return formatter.parse(text, YearMonth::from);
    }
    
    /*▲ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 转换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Combines this year-month with a day-of-month to create a {@code LocalDate}.
     * <p>
     * This returns a {@code LocalDate} formed from this year-month and the specified day-of-month.
     * <p>
     * The day-of-month value must be valid for the year-month.
     * <p>
     * This method can be used as part of a chain to produce a date:
     * <pre>
     *  LocalDate date = year.atMonth(month).atDay(day);
     * </pre>
     *
     * @param dayOfMonth the day-of-month to use, from 1 to 31
     *
     * @return the date formed from this year-month and the specified day, not null
     *
     * @throws DateTimeException if the day is invalid for the year-month
     * @see #isValidDay(int)
     */
    // 将当前YearMonth和月份中的天数dayOfMonth整合成一个"本地日期"对象后返回
    public LocalDate atDay(int dayOfMonth) {
        return LocalDate.of(year, month, dayOfMonth);
    }
    
    /**
     * Returns a {@code LocalDate} at the end of the month.
     * <p>
     * This returns a {@code LocalDate} based on this year-month.
     * The day-of-month is set to the last valid day of the month, taking
     * into account leap years.
     * <p>
     * This method can be used as part of a chain to produce a date:
     * <pre>
     *  LocalDate date = year.atMonth(month).atEndOfMonth();
     * </pre>
     *
     * @return the last valid date of this year-month, not null
     */
    // 将当前YearMonth和月份中的最后一天整合成一个"本地日期"对象后返回
    public LocalDate atEndOfMonth() {
        return LocalDate.of(year, month, lengthOfMonth());
    }
    
    /*▲ 转换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 部件 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Gets the year field.
     * <p>
     * This method returns the primitive {@code int} value for the year.
     * <p>
     * The year returned by this method is proleptic as per {@code get(YEAR)}.
     *
     * @return the year, from MIN_YEAR to MAX_YEAR
     */
    // 返回"年份"部件
    public int getYear() {
        return year;
    }
    
    /**
     * Gets the month-of-year field from 1 to 12.
     * <p>
     * This method returns the month as an {@code int} from 1 to 12.
     * Application code is frequently clearer if the enum {@link Month}
     * is used by calling {@link #getMonth()}.
     *
     * @return the month-of-year, from 1 to 12
     *
     * @see #getMonth()
     */
    // 返回"月份"部件
    public int getMonthValue() {
        return month;
    }
    
    /**
     * Gets the month-of-year field using the {@code Month} enum.
     * <p>
     * This method returns the enum {@link Month} for the month.
     * This avoids confusion as to what {@code int} values mean.
     * If you need access to the primitive {@code int} value then the enum
     * provides the {@link Month#getValue() int value}.
     *
     * @return the month-of-year, not null
     *
     * @see #getMonthValue()
     */
    // 返回"月份"部件
    public Month getMonth() {
        return Month.of(month);
    }
    
    /*▲ 部件 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 增加 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this year-month with the specified amount added.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the specified amount added.
     * The amount is typically {@link Period} but may be any other type implementing
     * the {@link TemporalAmount} interface.
     * <p>
     * The calculation is delegated to the amount object by calling
     * {@link TemporalAmount#addTo(Temporal)}. The amount implementation is free
     * to implement the addition in any way it wishes, however it typically
     * calls back to {@link #plus(long, TemporalUnit)}. Consult the documentation
     * of the amount implementation to determine if it can be successfully added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToAdd the amount to add, not null
     *
     * @return a {@code YearMonth} based on this year-month with the addition made, not null
     *
     * @throws DateTimeException   if the addition cannot be made
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 对当前时间量的值与参数中的"时间段"求和
     *
     * 如果求和后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"求和"后的新对象再返回。
     */
    @Override
    public YearMonth plus(TemporalAmount amountToAdd) {
        return (YearMonth) amountToAdd.addTo(this);
    }
    
    /**
     * Returns a copy of this year-month with the specified amount added.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the amount
     * in terms of the unit added. If it is not possible to add the amount, because the
     * unit is not supported or for some other reason, an exception is thrown.
     * <p>
     * If the field is a {@link ChronoUnit} then the addition is implemented here.
     * The supported fields behave as follows:
     * <ul>
     * <li>{@code MONTHS} -
     *  Returns a {@code YearMonth} with the specified number of months added.
     *  This is equivalent to {@link #plusMonths(long)}.
     * <li>{@code YEARS} -
     *  Returns a {@code YearMonth} with the specified number of years added.
     *  This is equivalent to {@link #plusYears(long)}.
     * <li>{@code DECADES} -
     *  Returns a {@code YearMonth} with the specified number of decades added.
     *  This is equivalent to calling {@link #plusYears(long)} with the amount
     *  multiplied by 10.
     * <li>{@code CENTURIES} -
     *  Returns a {@code YearMonth} with the specified number of centuries added.
     *  This is equivalent to calling {@link #plusYears(long)} with the amount
     *  multiplied by 100.
     * <li>{@code MILLENNIA} -
     *  Returns a {@code YearMonth} with the specified number of millennia added.
     *  This is equivalent to calling {@link #plusYears(long)} with the amount
     *  multiplied by 1,000.
     * <li>{@code ERAS} -
     *  Returns a {@code YearMonth} with the specified number of eras added.
     *  Only two eras are supported so the amount must be one, zero or minus one.
     *  If the amount is non-zero then the year is changed such that the year-of-era
     *  is unchanged.
     * </ul>
     * <p>
     * All other {@code ChronoUnit} instances will throw an {@code UnsupportedTemporalTypeException}.
     * <p>
     * If the field is not a {@code ChronoUnit}, then the result of this method
     * is obtained by invoking {@code TemporalUnit.addTo(Temporal, long)}
     * passing {@code this} as the argument. In this case, the unit determines
     * whether and how to perform the addition.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToAdd the amount of the unit to add to the result, may be negative
     * @param unit        the unit of the amount to add, not null
     *
     * @return a {@code YearMonth} based on this year-month with the specified amount added, not null
     *
     * @throws DateTimeException                if the addition cannot be made
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     * @throws ArithmeticException              if numeric overflow occurs
     */
    /*
     * 对当前时间量的值累加amountToAdd个unit单位的时间量
     *
     * 如果累加后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    @Override
    public YearMonth plus(long amountToAdd, TemporalUnit unit) {
        if(unit instanceof ChronoUnit) {
            switch((ChronoUnit) unit) {
                case MONTHS:
                    return plusMonths(amountToAdd);
                case YEARS:
                    return plusYears(amountToAdd);
                case DECADES:
                    return plusYears(Math.multiplyExact(amountToAdd, 10));
                case CENTURIES:
                    return plusYears(Math.multiplyExact(amountToAdd, 100));
                case MILLENNIA:
                    return plusYears(Math.multiplyExact(amountToAdd, 1000));
                case ERAS:
                    return with(ChronoField.ERA, Math.addExact(getLong(ChronoField.ERA), amountToAdd));
            }
            
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
        
        return unit.addTo(this, amountToAdd);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the specified number of years added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param yearsToAdd the years to add, may be negative
     *
     * @return a {@code YearMonth} based on this year-month with the years added, not null
     *
     * @throws DateTimeException if the result exceeds the supported range
     */
    /*
     * 在当前时间量的值上累加yearsToAdd年
     *
     * 如果累加后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public YearMonth plusYears(long yearsToAdd) {
        if(yearsToAdd == 0) {
            return this;
        }
        
        int newYear = ChronoField.YEAR.checkValidIntValue(year + yearsToAdd);  // safe overflow
        
        return with(newYear, month);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the specified number of months added.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param monthsToAdd the months to add, may be negative
     *
     * @return a {@code YearMonth} based on this year-month with the months added, not null
     *
     * @throws DateTimeException if the result exceeds the supported range
     */
    /*
     * 在当前时间量的值上累加monthsToAdd月
     *
     * 如果累加后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"累加"操作后的新对象再返回。
     */
    public YearMonth plusMonths(long monthsToAdd) {
        if(monthsToAdd == 0) {
            return this;
        }
        long monthCount = year * 12L + (month - 1);
        long calcMonths = monthCount + monthsToAdd;  // safe overflow
        int newYear = ChronoField.YEAR.checkValidIntValue(Math.floorDiv(calcMonths, 12));
        int newMonth = Math.floorMod(calcMonths, 12) + 1;
        return with(newYear, newMonth);
    }
    
    /*▲ 增加 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 减少 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a copy of this year-month with the specified amount subtracted.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the specified amount subtracted.
     * The amount is typically {@link Period} but may be any other type implementing
     * the {@link TemporalAmount} interface.
     * <p>
     * The calculation is delegated to the amount object by calling
     * {@link TemporalAmount#subtractFrom(Temporal)}. The amount implementation is free
     * to implement the subtraction in any way it wishes, however it typically
     * calls back to {@link #minus(long, TemporalUnit)}. Consult the documentation
     * of the amount implementation to determine if it can be successfully subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToSubtract the amount to subtract, not null
     *
     * @return a {@code YearMonth} based on this year-month with the subtraction made, not null
     *
     * @throws DateTimeException   if the subtraction cannot be made
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 对当前时间量的值与参数中的"时间段"求差
     *
     * 如果求差后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"求差"后的新对象再返回。
     */
    @Override
    public YearMonth minus(TemporalAmount amountToSubtract) {
        return (YearMonth) amountToSubtract.subtractFrom(this);
    }
    
    /**
     * Returns a copy of this year-month with the specified amount subtracted.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the amount
     * in terms of the unit subtracted. If it is not possible to subtract the amount,
     * because the unit is not supported or for some other reason, an exception is thrown.
     * <p>
     * This method is equivalent to {@link #plus(long, TemporalUnit)} with the amount negated.
     * See that method for a full description of how addition, and thus subtraction, works.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param amountToSubtract the amount of the unit to subtract from the result, may be negative
     * @param unit             the unit of the amount to subtract, not null
     *
     * @return a {@code YearMonth} based on this year-month with the specified amount subtracted, not null
     *
     * @throws DateTimeException                if the subtraction cannot be made
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     * @throws ArithmeticException              if numeric overflow occurs
     */
    /*
     * 对当前时间量的值减去amountToSubtract个unit单位的时间量
     *
     * 如果减去后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    @Override
    public YearMonth minus(long amountToSubtract, TemporalUnit unit) {
        if(amountToSubtract == Long.MIN_VALUE) {
            return plus(Long.MAX_VALUE, unit).plus(1, unit);
        }
        
        return plus(-amountToSubtract, unit);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the specified number of years subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param yearsToSubtract the years to subtract, may be negative
     *
     * @return a {@code YearMonth} based on this year-month with the years subtracted, not null
     *
     * @throws DateTimeException if the result exceeds the supported range
     */
    /*
     * 在当前时间量的值上减去yearsToSubtract年
     *
     * 如果减去后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public YearMonth minusYears(long yearsToSubtract) {
        if(yearsToSubtract == Long.MIN_VALUE) {
            return plusYears(Long.MAX_VALUE).plusYears(1);
        }
        
        return plusYears(-yearsToSubtract);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the specified number of months subtracted.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param monthsToSubtract the months to subtract, may be negative
     *
     * @return a {@code YearMonth} based on this year-month with the months subtracted, not null
     *
     * @throws DateTimeException if the result exceeds the supported range
     */
    /*
     * 在当前时间量的值上减去monthsToSubtract月
     *
     * 如果减去后的值与当前时间量的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"减去"操作后的新对象再返回。
     */
    public YearMonth minusMonths(long monthsToSubtract) {
        if(monthsToSubtract == Long.MIN_VALUE) {
            return plusMonths(Long.MAX_VALUE).plusMonths(1);
        }
        
        return plusMonths(-monthsToSubtract);
    }
    
    /*▲ 减少 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 时间量单位 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Checks if the specified unit is supported.
     * <p>
     * This checks if the specified unit can be added to, or subtracted from, this year-month.
     * If false, then calling the {@link #plus(long, TemporalUnit)} and
     * {@link #minus(long, TemporalUnit) minus} methods will throw an exception.
     * <p>
     * If the unit is a {@link ChronoUnit} then the query is implemented here.
     * The supported units are:
     * <ul>
     * <li>{@code MONTHS}
     * <li>{@code YEARS}
     * <li>{@code DECADES}
     * <li>{@code CENTURIES}
     * <li>{@code MILLENNIA}
     * <li>{@code ERAS}
     * </ul>
     * All other {@code ChronoUnit} instances will return false.
     * <p>
     * If the unit is not a {@code ChronoUnit}, then the result of this method
     * is obtained by invoking {@code TemporalUnit.isSupportedBy(Temporal)}
     * passing {@code this} as the argument.
     * Whether the unit is supported is determined by the unit.
     *
     * @param unit the unit to check, null returns false
     *
     * @return true if the unit can be added/subtracted, false if not
     */
    // 判断当前时间量是否支持指定的时间量单位
    @Override
    public boolean isSupported(TemporalUnit unit) {
        if(unit instanceof ChronoUnit) {
            return unit == MONTHS || unit == YEARS || unit == DECADES || unit == CENTURIES || unit == MILLENNIA || unit == ERAS;
        }
        return unit != null && unit.isSupportedBy(this);
    }
    
    /*▲ 时间量单位 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 时间量字段操作(TemporalAccessor) ███████████████████████████████████████████████████████┓ */
    
    /**
     * Checks if the specified field is supported.
     * <p>
     * This checks if this year-month can be queried for the specified field.
     * If false, then calling the {@link #range(TemporalField) range},
     * {@link #get(TemporalField) get} and {@link #with(TemporalField, long)}
     * methods will throw an exception.
     * <p>
     * If the field is a {@link ChronoField} then the query is implemented here.
     * The supported fields are:
     * <ul>
     * <li>{@code MONTH_OF_YEAR}
     * <li>{@code PROLEPTIC_MONTH}
     * <li>{@code YEAR_OF_ERA}
     * <li>{@code YEAR}
     * <li>{@code ERA}
     * </ul>
     * 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 year-month, false if not
     */
    // 判断当前时间量是否支持指定的时间量字段
    @Override
    public boolean isSupported(TemporalField field) {
        if(field instanceof ChronoField) {
            return field == ChronoField.YEAR || field == ChronoField.MONTH_OF_YEAR || field == ChronoField.PROLEPTIC_MONTH || field == ChronoField.YEAR_OF_ERA || 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 year-month 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 {@link #isSupported(TemporalField) supported fields} will return
     * appropriate range instances.
     * 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.
     *
     * @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
    @Override
    public ValueRange range(TemporalField field) {
        if(field == ChronoField.YEAR_OF_ERA) {
            if(getYear()<=0) {
                return ValueRange.of(1, Year.MAX_VALUE + 1);
            }
            
            return ValueRange.of(1, Year.MAX_VALUE);
        }
        
        return Temporal.super.range(field);
    }
    
    /**
     * Gets the value of the specified field from this year-month as an {@code int}.
     * <p>
     * This queries this year-month 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 {@link #isSupported(TemporalField) supported fields} will return valid
     * values based on this year-month, except {@code PROLEPTIC_MONTH} which is too
     * large to fit in an {@code int} and throw a {@code DateTimeException}.
     * 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.YEAR            - 返回"Proleptic年"部件
     * ChronoField.MONTH_OF_YEAR   - 返回"月份"部件
     * .......................................................................
     * ChronoField.YEAR_OF_ERA     - 将"Proleptic年"转换为位于纪元中的[年]后返回
     * ChronoField.PROLEPTIC_MONTH ×
     * ChronoField.ERA             - 计算"Proleptic年"年所在的纪元；在公历系统中，0是公元前，1是公元(后)
     */
    @Override
    public int get(TemporalField field) {
        long longValue = getLong(field);
        return range(field).checkValidIntValue(longValue, field);
    }
    
    /**
     * Gets the value of the specified field from this year-month as a {@code long}.
     * <p>
     * This queries this year-month 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 {@link #isSupported(TemporalField) supported fields} will return valid
     * values based on this year-month.
     * 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.YEAR            - 返回"Proleptic年"部件
     * ChronoField.MONTH_OF_YEAR   - 返回"月份"部件
     * .......................................................................
     * ChronoField.YEAR_OF_ERA     - 将"Proleptic年"转换为位于纪元中的[年]后返回
     * ChronoField.PROLEPTIC_MONTH = 计算当前时间量包含的"Proleptic月"
     * ChronoField.ERA             - 计算"Proleptic年"年所在的纪元；在公历系统中，0是公元前，1是公元(后)
     */
    @Override
    public long getLong(TemporalField field) {
        if(field instanceof ChronoField) {
            switch((ChronoField) field) {
                case MONTH_OF_YEAR:
                    return month;
                case PROLEPTIC_MONTH:
                    return getProlepticMonth();
                case YEAR_OF_ERA:
                    return (year >= 1 ? year : 1 - year);
                case YEAR:
                    return year;
                case ERA:
                    return (year<1 ? 0 : 1);
            }
            
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        
        return field.getFrom(this);
    }
    
    /**
     * Queries this year-month using the specified query.
     * <p>
     * This queries this year-month 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
    public <R> R query(TemporalQuery<R> query) {
    
        if(query == TemporalQueries.precision()) {
            return (R) MONTHS;
        }
    
        if(query == TemporalQueries.chronology()) {
            return (R) IsoChronology.INSTANCE;
        }
    
        return Temporal.super.query(query);
    }
    
    /*▲ 时间量字段操作(TemporalAccessor) ███████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 整合 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an adjusted copy of this year-month.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the year-month adjusted.
     * The adjustment takes place using the specified adjuster strategy object.
     * Read the documentation of the adjuster to understand what adjustment will be made.
     * <p>
     * A simple adjuster might simply set the one of the fields, such as the year field.
     * A more complex adjuster might set the year-month to the next month that
     * Halley's comet will pass the Earth.
     * <p>
     * The result of this method is obtained by invoking the
     * {@link TemporalAdjuster#adjustInto(Temporal)} method on the
     * specified adjuster passing {@code this} as the argument.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param adjuster the adjuster to use, not null
     *
     * @return a {@code YearMonth} based on {@code this} with the adjustment made, not null
     *
     * @throws DateTimeException   if the adjustment cannot be made
     * @throws ArithmeticException if numeric overflow occurs
     */
    /*
     * 使用指定的时间量整合器adjuster来构造时间量对象。
     *
     * 如果整合后的值与当前时间量中的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"整合"后的新对象再返回。
     */
    @Override
    public YearMonth with(TemporalAdjuster adjuster) {
        return (YearMonth) adjuster.adjustInto(this);
    }
    
    /**
     * Returns a copy of this year-month with the specified field set to a new value.
     * <p>
     * This returns a {@code YearMonth}, based on this one, with the value
     * for the specified field changed.
     * This can be used to change any supported field, such as the year or month.
     * If it is not possible to set 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 adjustment is implemented here.
     * The supported fields behave as follows:
     * <ul>
     * <li>{@code MONTH_OF_YEAR} -
     *  Returns a {@code YearMonth} with the specified month-of-year.
     *  The year will be unchanged.
     * <li>{@code PROLEPTIC_MONTH} -
     *  Returns a {@code YearMonth} with the specified proleptic-month.
     *  This completely replaces the year and month of this object.
     * <li>{@code YEAR_OF_ERA} -
     *  Returns a {@code YearMonth} with the specified year-of-era
     *  The month and era will be unchanged.
     * <li>{@code YEAR} -
     *  Returns a {@code YearMonth} with the specified year.
     *  The month will be unchanged.
     * <li>{@code ERA} -
     *  Returns a {@code YearMonth} with the specified era.
     *  The month and year-of-era will be unchanged.
     * </ul>
     * <p>
     * In all cases, if the new value is outside the valid range of values for the field
     * then a {@code DateTimeException} will be thrown.
     * <p>
     * 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.adjustInto(Temporal, long)}
     * passing {@code this} as the argument. In this case, the field determines
     * whether and how to adjust the instant.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param field    the field to set in the result, not null
     * @param newValue the new value of the field in the result
     *
     * @return a {@code YearMonth} based on {@code this} with the specified field set, not null
     *
     * @throws DateTimeException                if the field cannot be set
     * @throws UnsupportedTemporalTypeException if the field is not supported
     * @throws ArithmeticException              if numeric overflow occurs
     */
    /*
     * 通过整合指定类型的字段和当前时间量中的其他类型的字段来构造时间量对象。
     *
     * 如果整合后的值与当前时间量中的值相等，则直接返回当前时间量对象。
     * 否则，需要构造"整合"后的新对象再返回。
     *
     * field   : 待整合的字段(类型)
     * newValue: field的原始值，需要根据filed的类型进行放缩
     *
     * 目前支持传入的字段值包括：
     *
     * ChronoField.YEAR            - 与[Proleptic-年]整合，只会覆盖当前时间量的"Proleptic年"部件
     * ChronoField.MONTH_OF_YEAR   - 与年中的[月](1, 12)整合，只会覆盖当前时间量的"月份"部件
     * ....................................................................................
     * ChronoField.YEAR_OF_ERA     - 与位于纪元中的[年]整合，这会将该年份进行转换后覆盖"Proleptic年"部件
     * ChronoField.PROLEPTIC_MONTH - 与[Proleptic-月]整合，这会在当前时间量的基础上增/减一定的月数，以达到给定的字段值表示的目标月
     * ChronoField.ERA             - 与[纪元]整合，即切换公元前与公元(后)
     */
    @Override
    public YearMonth with(TemporalField field, long newValue) {
        if(field instanceof ChronoField) {
            ChronoField f = (ChronoField) field;
            f.checkValidValue(newValue);
            
            switch(f) {
                case MONTH_OF_YEAR:
                    return withMonth((int) newValue);
                case PROLEPTIC_MONTH:
                    return plusMonths(newValue - getProlepticMonth());
                case YEAR_OF_ERA:
                    return withYear((int) (year >= 1 ? newValue : 1 - newValue));
                case YEAR:
                    return withYear((int) newValue);
                case ERA:
                    return (getLong(ChronoField.ERA) == newValue ? this : withYear(1 - year));
            }
            
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        
        return field.adjustInto(this, newValue);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the year altered.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param year the year to set in the returned year-month, from MIN_YEAR to MAX_YEAR
     *
     * @return a {@code YearMonth} based on this year-month with the requested year, not null
     *
     * @throws DateTimeException if the year value is invalid
     */
    public YearMonth withYear(int year) {
        ChronoField.YEAR.checkValidValue(year);
        return with(year, month);
    }
    
    /**
     * Returns a copy of this {@code YearMonth} with the month-of-year altered.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param month the month-of-year to set in the returned year-month, from 1 (January) to 12 (December)
     *
     * @return a {@code YearMonth} based on this year-month with the requested month, not null
     *
     * @throws DateTimeException if the month-of-year value is invalid
     */
    public YearMonth withMonth(int month) {
        ChronoField.MONTH_OF_YEAR.checkValidValue(month);
        return with(year, month);
    }
    
    /**
     * Adjusts the specified temporal object to have this year-month.
     * <p>
     * This returns a temporal object of the same observable type as the input
     * with the year and month changed to be the same as this.
     * <p>
     * The adjustment is equivalent to using {@link Temporal#with(TemporalField, long)}
     * passing {@link ChronoField#PROLEPTIC_MONTH} as the field.
     * If the specified temporal object does not use the ISO calendar system then
     * a {@code DateTimeException} is thrown.
     * <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 = thisYearMonth.adjustInto(temporal);
     *   temporal = temporal.with(thisYearMonth);
     * </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.PROLEPTIC_MONTH - 当前时间量包含的"Proleptic月"
     *
     * 目标时间量temporal的取值可以是：
     * YearMonth
     * LocalDate
     * LocalDateTime
     * OffsetDateTime
     * ZonedDateTime
     * ChronoLocalDateTimeImpl
     * ChronoZonedDateTimeImpl
     */
    @Override
    public Temporal adjustInto(Temporal temporal) {
        // 不是ISO历法系统则抛异常
        if(!Chronology.from(temporal).equals(IsoChronology.INSTANCE)) {
            throw new DateTimeException("Adjustment only supported on ISO date-time");
        }
        
        // 获取当前时间量包含的"Proleptic-月"
        long prolepticMonth = getProlepticMonth();
        
        return temporal.with(ChronoField.PROLEPTIC_MONTH, prolepticMonth);
    }
    
    /*▲ 整合 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Calculates the amount of time until another year-month in terms of the specified unit.
     * <p>
     * This calculates the amount of time between two {@code YearMonth}
     * objects in terms of a single {@code TemporalUnit}.
     * The start and end points are {@code this} and the specified year-month.
     * The result will be negative if the end is before the start.
     * The {@code Temporal} passed to this method is converted to a
     * {@code YearMonth} using {@link #from(TemporalAccessor)}.
     * For example, the amount in years between two year-months can be calculated
     * using {@code startYearMonth.until(endYearMonth, YEARS)}.
     * <p>
     * The calculation returns a whole number, representing the number of
     * complete units between the two year-months.
     * For example, the amount in decades between 2012-06 and 2032-05
     * will only be one decade as it is one month short of two decades.
     * <p>
     * There are two equivalent ways of using this method.
     * The first is to invoke this method.
     * The second is to use {@link TemporalUnit#between(Temporal, Temporal)}:
     * <pre>
     *   // these two lines are equivalent
     *   amount = start.until(end, MONTHS);
     *   amount = MONTHS.between(start, end);
     * </pre>
     * The choice should be made based on which makes the code more readable.
     * <p>
     * The calculation is implemented in this method for {@link ChronoUnit}.
     * The units {@code MONTHS}, {@code YEARS}, {@code DECADES},
     * {@code CENTURIES}, {@code MILLENNIA} and {@code ERAS} are supported.
     * Other {@code ChronoUnit} values will throw an exception.
     * <p>
     * If the unit is not a {@code ChronoUnit}, then the result of this method
     * is obtained by invoking {@code TemporalUnit.between(Temporal, Temporal)}
     * passing {@code this} as the first argument and the converted input temporal
     * as the second argument.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param endExclusive the end date, exclusive, which is converted to a {@code YearMonth}, not null
     * @param unit         the unit to measure the amount in, not null
     *
     * @return the amount of time between this year-month and the end year-month
     *
     * @throws DateTimeException                if the amount cannot be calculated, or the end
     *                                          temporal cannot be converted to a {@code YearMonth}
     * @throws UnsupportedTemporalTypeException if the unit is not supported
     * @throws ArithmeticException              if numeric overflow occurs
     */
    // 计算当前时间量到目标时间量endExclusive之间相差多少个unit单位的时间值
    @Override
    public long until(Temporal endExclusive, TemporalUnit unit) {
        YearMonth end = YearMonth.from(endExclusive);
    
        if(unit instanceof ChronoUnit) {
            long monthsUntil = end.getProlepticMonth() - getProlepticMonth();
        
            switch((ChronoUnit) unit) {
                case MONTHS:
                    return monthsUntil;
                case YEARS:
                    return monthsUntil / 12;
                case DECADES:
                    return monthsUntil / 120;
                case CENTURIES:
                    return monthsUntil / 1200;
                case MILLENNIA:
                    return monthsUntil / 12000;
                case ERAS:
                    return end.getLong(ChronoField.ERA) - getLong(ChronoField.ERA);
            }
        
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
    
        return unit.between(this, end);
    }
    
    /**
     * Checks if this year-month is after the specified year-month.
     *
     * @param other the other year-month to compare to, not null
     *
     * @return true if this is after the specified year-month
     */
    // 判断当前日期是否晚于参数中指定的日期
    public boolean isAfter(YearMonth other) {
        return compareTo(other)>0;
    }
    
    /**
     * Checks if this year-month is before the specified year-month.
     *
     * @param other the other year-month to compare to, not null
     *
     * @return true if this point is before the specified year-month
     */
    // 判断当前日期是否早于参数中指定的日期
    public boolean isBefore(YearMonth other) {
        return compareTo(other)<0;
    }
    
    /**
     * Checks if the year is a leap year, according to the ISO proleptic
     * calendar system rules.
     * <p>
     * This method applies the current rules for leap years across the whole time-line.
     * In general, a year is a leap year if it is divisible by four without
     * remainder. However, years divisible by 100, are not leap years, with
     * the exception of years divisible by 400 which are.
     * <p>
     * For example, 1904 is a leap year it is divisible by 4.
     * 1900 was not a leap year as it is divisible by 100, however 2000 was a
     * leap year as it is divisible by 400.
     * <p>
     * The calculation is proleptic - applying the same rules into the far future and far past.
     * This is historically inaccurate, but is correct for the ISO-8601 standard.
     *
     * @return true if the year is leap, false otherwise
     */
    // 判断当前"年份"是否为闰年
    public boolean isLeapYear() {
        return IsoChronology.INSTANCE.isLeapYear(year);
    }
    
    /**
     * Checks if the day-of-month is valid for this year-month.
     * <p>
     * This method checks whether this year and month and the input day form
     * a valid date.
     *
     * @param dayOfMonth the day-of-month to validate, from 1 to 31, invalid value returns false
     *
     * @return true if the day is valid for this year-month
     */
    // 判断指定的天数是否位于当前"月份"的范围内
    public boolean isValidDay(int dayOfMonth) {
        return dayOfMonth >= 1 && dayOfMonth<=lengthOfMonth();
    }
    
    /**
     * Returns the length of the month, taking account of the year.
     * <p>
     * This returns the length of the month in days.
     * For example, a date in January would return 31.
     *
     * @return the length of the month in days, from 28 to 31
     */
    // 返回"月份"部件包含的天数
    public int lengthOfMonth() {
        return getMonth().length(isLeapYear());
    }
    
    /**
     * Returns the length of the year.
     * <p>
     * This returns the length of the year in days, either 365 or 366.
     *
     * @return 366 if the year is leap, 365 otherwise
     */
    // 返回"年份"部件包含的天数
    public int lengthOfYear() {
        return (isLeapYear() ? 366 : 365);
    }
    
    /**
     * Formats this year-month using the specified formatter.
     * <p>
     * This year-month will be passed to the formatter to produce a string.
     *
     * @param formatter the formatter to use, not null
     *
     * @return the formatted year-month string, not null
     *
     * @throws DateTimeException if an error occurs during printing
     */
    public String format(DateTimeFormatter formatter) {
        Objects.requireNonNull(formatter, "formatter");
        return formatter.format(this);
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    // 返回当前时间量包含的"Proleptic-月"
    private long getProlepticMonth() {
        return (year * 12L + month - 1);
    }
    
    /**
     * Returns a copy of this year-month with the new year and month, checking
     * to see if a new object is in fact required.
     *
     * @param newYear  the year to represent, validated from MIN_YEAR to MAX_YEAR
     * @param newMonth the month-of-year to represent, validated not null
     *
     * @return the year-month, not null
     */
    // 使用指定的年和月构造YearMonth对象
    private YearMonth with(int newYear, int newMonth) {
        if(year == newYear && month == newMonth) {
            return this;
        }
        
        return new YearMonth(newYear, newMonth);
    }
    
    
    /**
     * Compares this year-month to another year-month.
     * <p>
     * The comparison is based first on the value of the year, then on the value of the month.
     * It is "consistent with equals", as defined by {@link Comparable}.
     *
     * @param other the other year-month to compare to, not null
     *
     * @return the comparator value, negative if less, positive if greater
     */
    @Override
    public int compareTo(YearMonth other) {
        int cmp = (year - other.year);
        if(cmp == 0) {
            cmp = (month - other.month);
        }
        return cmp;
    }
    
    /**
     * Outputs this year-month as a {@code String}, such as {@code 2007-12}.
     * <p>
     * The output will be in the format {@code uuuu-MM}:
     *
     * @return a string representation of this year-month, not null
     */
    @Override
    public String toString() {
        int absYear = Math.abs(year);
        StringBuilder buf = new StringBuilder(9);
        if(absYear<1000) {
            if(year<0) {
                buf.append(year - 10000).deleteCharAt(1);
            } else {
                buf.append(year + 10000).deleteCharAt(0);
            }
        } else {
            buf.append(year);
        }
        return buf.append(month<10 ? "-0" : "-").append(month).toString();
    }
    
    /**
     * Checks if this year-month is equal to another year-month.
     * <p>
     * The comparison is based on the time-line position of the year-months.
     *
     * @param obj the object to check, null returns false
     *
     * @return true if this is equal to the other year-month
     */
    @Override
    public boolean equals(Object obj) {
        if(this == obj) {
            return true;
        }
        if(obj instanceof YearMonth) {
            YearMonth other = (YearMonth) obj;
            return year == other.year && month == other.month;
        }
        return false;
    }
    
    /**
     * A hash code for this year-month.
     *
     * @return a suitable hash code
     */
    @Override
    public int hashCode() {
        return year ^ (month << 27);
    }
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Serialization version.
     */
    private static final long serialVersionUID = 4183400860270640070L;
    
    /**
     * Defend against malicious streams.
     *
     * @param s the stream to read
     *
     * @throws InvalidObjectException always
     */
    private void readObject(ObjectInputStream s) throws InvalidObjectException {
        throw new InvalidObjectException("Deserialization via serialization delegate");
    }
    
    /**
     * Writes the object using a
     * <a href="../../serialized-form.html#java.time.Ser">dedicated serialized form</a>.
     *
     * @return the instance of {@code Ser}, not null
     *
     * @serialData <pre>
     *  out.writeByte(12);  // identifies a YearMonth
     *  out.writeInt(year);
     *  out.writeByte(month);
     * </pre>
     */
    private Object writeReplace() {
        return new Ser(Ser.YEAR_MONTH_TYPE, this);
    }
    
    static YearMonth readExternal(DataInput in) throws IOException {
        int year = in.readInt();
        byte month = in.readByte();
        return YearMonth.of(year, month);
    }
    
    void writeExternal(DataOutput out) throws IOException {
        out.writeInt(year);
        out.writeByte(month);
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
}