ChronoLocalDate.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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) 2012, Stephen Colebourne & Michael Nascimento Santos
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  * Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
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package java.time.chrono;

import java.time.DateTimeException;
import java.time.LocalDate;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;
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.util.Comparator;
import java.util.Objects;

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

/**
 * A date without time-of-day or time-zone in an arbitrary chronology, intended
 * for advanced globalization use cases.
 * <p>
 * <b>Most applications should declare method signatures, fields and variables
 * as {@link LocalDate}, not this interface.</b>
 * <p>
 * A {@code ChronoLocalDate} is the abstract representation of a date where the
 * {@code Chronology chronology}, or calendar system, is pluggable.
 * The date is defined in terms of fields expressed by {@link TemporalField},
 * where most common implementations are defined in {@link ChronoField}.
 * The chronology defines how the calendar system operates and the meaning of
 * the standard fields.
 *
 * <h3>When to use this interface</h3>
 * The design of the API encourages the use of {@code LocalDate} rather than this
 * interface, even in the case where the application needs to deal with multiple
 * calendar systems.
 * <p>
 * This concept can seem surprising at first, as the natural way to globalize an
 * application might initially appear to be to abstract the calendar system.
 * However, as explored below, abstracting the calendar system is usually the wrong
 * approach, resulting in logic errors and hard to find bugs.
 * As such, it should be considered an application-wide architectural decision to choose
 * to use this interface as opposed to {@code LocalDate}.
 *
 * <h3>Architectural issues to consider</h3>
 * These are some of the points that must be considered before using this interface
 * throughout an application.
 * <p>
 * 1) Applications using this interface, as opposed to using just {@code LocalDate},
 * face a significantly higher probability of bugs. This is because the calendar system
 * in use is not known at development time. A key cause of bugs is where the developer
 * applies assumptions from their day-to-day knowledge of the ISO calendar system
 * to code that is intended to deal with any arbitrary calendar system.
 * The section below outlines how those assumptions can cause problems
 * The primary mechanism for reducing this increased risk of bugs is a strong code review process.
 * This should also be considered a extra cost in maintenance for the lifetime of the code.
 * <p>
 * 2) This interface does not enforce immutability of implementations.
 * While the implementation notes indicate that all implementations must be immutable
 * there is nothing in the code or type system to enforce this. Any method declared
 * to accept a {@code ChronoLocalDate} could therefore be passed a poorly or
 * maliciously written mutable implementation.
 * <p>
 * 3) Applications using this interface  must consider the impact of eras.
 * {@code LocalDate} shields users from the concept of eras, by ensuring that {@code getYear()}
 * returns the proleptic year. That decision ensures that developers can think of
 * {@code LocalDate} instances as consisting of three fields - year, month-of-year and day-of-month.
 * By contrast, users of this interface must think of dates as consisting of four fields -
 * era, year-of-era, month-of-year and day-of-month. The extra era field is frequently
 * forgotten, yet it is of vital importance to dates in an arbitrary calendar system.
 * For example, in the Japanese calendar system, the era represents the reign of an Emperor.
 * Whenever one reign ends and another starts, the year-of-era is reset to one.
 * <p>
 * 4) The only agreed international standard for passing a date between two systems
 * is the ISO-8601 standard which requires the ISO calendar system. Using this interface
 * throughout the application will inevitably lead to the requirement to pass the date
 * across a network or component boundary, requiring an application specific protocol or format.
 * <p>
 * 5) Long term persistence, such as a database, will almost always only accept dates in the
 * ISO-8601 calendar system (or the related Julian-Gregorian). Passing around dates in other
 * calendar systems increases the complications of interacting with persistence.
 * <p>
 * 6) Most of the time, passing a {@code ChronoLocalDate} throughout an application
 * is unnecessary, as discussed in the last section below.
 *
 * <h3>False assumptions causing bugs in multi-calendar system code</h3>
 * As indicated above, there are many issues to consider when try to use and manipulate a
 * date in an arbitrary calendar system. These are some of the key issues.
 * <p>
 * Code that queries the day-of-month and assumes that the value will never be more than
 * 31 is invalid. Some calendar systems have more than 31 days in some months.
 * <p>
 * Code that adds 12 months to a date and assumes that a year has been added is invalid.
 * Some calendar systems have a different number of months, such as 13 in the Coptic or Ethiopic.
 * <p>
 * Code that adds one month to a date and assumes that the month-of-year value will increase
 * by one or wrap to the next year is invalid. Some calendar systems have a variable number
 * of months in a year, such as the Hebrew.
 * <p>
 * Code that adds one month, then adds a second one month and assumes that the day-of-month
 * will remain close to its original value is invalid. Some calendar systems have a large difference
 * between the length of the longest month and the length of the shortest month.
 * For example, the Coptic or Ethiopic have 12 months of 30 days and 1 month of 5 days.
 * <p>
 * Code that adds seven days and assumes that a week has been added is invalid.
 * Some calendar systems have weeks of other than seven days, such as the French Revolutionary.
 * <p>
 * Code that assumes that because the year of {@code date1} is greater than the year of {@code date2}
 * then {@code date1} is after {@code date2} is invalid. This is invalid for all calendar systems
 * when referring to the year-of-era, and especially untrue of the Japanese calendar system
 * where the year-of-era restarts with the reign of every new Emperor.
 * <p>
 * Code that treats month-of-year one and day-of-month one as the start of the year is invalid.
 * Not all calendar systems start the year when the month value is one.
 * <p>
 * In general, manipulating a date, and even querying a date, is wide open to bugs when the
 * calendar system is unknown at development time. This is why it is essential that code using
 * this interface is subjected to additional code reviews. It is also why an architectural
 * decision to avoid this interface type is usually the correct one.
 *
 * <h3>Using LocalDate instead</h3>
 * The primary alternative to using this interface throughout your application is as follows.
 * <ul>
 * <li>Declare all method signatures referring to dates in terms of {@code LocalDate}.
 * <li>Either store the chronology (calendar system) in the user profile or lookup
 *  the chronology from the user locale
 * <li>Convert the ISO {@code LocalDate} to and from the user's preferred calendar system during
 *  printing and parsing
 * </ul>
 * This approach treats the problem of globalized calendar systems as a localization issue
 * and confines it to the UI layer. This approach is in keeping with other localization
 * issues in the java platform.
 * <p>
 * As discussed above, performing calculations on a date where the rules of the calendar system
 * are pluggable requires skill and is not recommended.
 * Fortunately, the need to perform calculations on a date in an arbitrary calendar system
 * is extremely rare. For example, it is highly unlikely that the business rules of a library
 * book rental scheme will allow rentals to be for one month, where meaning of the month
 * is dependent on the user's preferred calendar system.
 * <p>
 * A key use case for calculations on a date in an arbitrary calendar system is producing
 * a month-by-month calendar for display and user interaction. Again, this is a UI issue,
 * and use of this interface solely within a few methods of the UI layer may be justified.
 * <p>
 * In any other part of the system, where a date must be manipulated in a calendar system
 * other than ISO, the use case will generally specify the calendar system to use.
 * For example, an application may need to calculate the next Islamic or Hebrew holiday
 * which may require manipulating the date.
 * This kind of use case can be handled as follows:
 * <ul>
 * <li>start from the ISO {@code LocalDate} being passed to the method
 * <li>convert the date to the alternate calendar system, which for this use case is known
 *  rather than arbitrary
 * <li>perform the calculation
 * <li>convert back to {@code LocalDate}
 * </ul>
 * Developers writing low-level frameworks or libraries should also avoid this interface.
 * Instead, one of the two general purpose access interfaces should be used.
 * Use {@link TemporalAccessor} if read-only access is required, or use {@link Temporal}
 * if read-write access is required.
 *
 * @implSpec This interface must be implemented with care to ensure other classes operate correctly.
 * All implementations that can be instantiated must be final, immutable and thread-safe.
 * Subclasses should be Serializable wherever possible.
 * <p>
 * Additional calendar systems may be added to the system.
 * See {@link Chronology} for more details.
 * @since 1.8
 */
// "本地日期"接口，允许在子类中将"日期"部件绑定到某种历法系统
public interface ChronoLocalDate extends Temporal, TemporalAdjuster, Comparable<ChronoLocalDate> {
    
    /*▼ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Obtains an instance of {@code ChronoLocalDate} from a temporal object.
     * <p>
     * This obtains a local date 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 ChronoLocalDate}.
     * <p>
     * The conversion extracts and combines the chronology and the date
     * from the temporal object. The behavior is equivalent to using
     * {@link Chronology#date(TemporalAccessor)} with the extracted chronology.
     * Implementations are permitted to perform optimizations such as accessing
     * those fields that are equivalent to the relevant objects.
     * <p>
     * This method matches the signature of the functional interface {@link TemporalQuery}
     * allowing it to be used as a query via method reference, {@code ChronoLocalDate::from}.
     *
     * @param temporal the temporal object to convert, not null
     *
     * @return the date, not null
     *
     * @throws DateTimeException if unable to convert to a {@code ChronoLocalDate}
     * @see Chronology#date(TemporalAccessor)
     */
    // 从temporal中获取/构造ChronoLocalDate对象
    static ChronoLocalDate from(TemporalAccessor temporal) {
        Objects.requireNonNull(temporal, "temporal");
    
        if(temporal instanceof ChronoLocalDate) {
            return (ChronoLocalDate) temporal;
        }
    
        // 查询时间量的历法系统
        Chronology chrono = temporal.query(TemporalQueries.chronology());
        if(chrono == null) {
            throw new DateTimeException("Unable to obtain ChronoLocalDate from TemporalAccessor: " + temporal.getClass());
        }
    
        // 使用指定的时间量构造ChronoLocalDate
        return chrono.date(temporal);
    }
    
    /*▲ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 转换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Combines this date with a time to create a {@code ChronoLocalDateTime}.
     * <p>
     * This returns a {@code ChronoLocalDateTime} formed from this date at the specified time.
     * All possible combinations of date and time are valid.
     *
     * @param localTime the local time to use, not null
     *
     * @return the local date-time formed from this date and the specified time, not null
     */
    // 将当前"本地日期"和指定的"本地时间"整合成一个"本地日期-时间"对象后返回
    @SuppressWarnings("unchecked")
    default ChronoLocalDateTime<?> atTime(LocalTime localTime) {
        return ChronoLocalDateTimeImpl.of(this, localTime);
    }
    
    /**
     * Converts this date to the Epoch Day.
     * <p>
     * The {@link ChronoField#EPOCH_DAY Epoch Day count} is a simple
     * incrementing count of days where day 0 is 1970-01-01 (ISO).
     * This definition is the same for all chronologies, enabling conversion.
     * <p>
     * This default implementation queries the {@code EPOCH_DAY} field.
     *
     * @return the Epoch Day equivalent to this date
     */
    // 返回当前时间量的纪元天
    default long toEpochDay() {
        return getLong(ChronoField.EPOCH_DAY);
    }
    
    /*▲ 转换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 增加 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException   {@inheritDoc}
     * @throws ArithmeticException {@inheritDoc}
     */
    /*
     * 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
    default ChronoLocalDate plus(TemporalAmount amount) {
        return ChronoLocalDateImpl.ensureValid(getChronology(), Temporal.super.plus(amount));
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException   {@inheritDoc}
     * @throws ArithmeticException {@inheritDoc}
     */
    /*
     * 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
    default ChronoLocalDate plus(long amountToAdd, TemporalUnit unit) {
        if(unit instanceof ChronoUnit) {
            throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
        }
        
        ChronoLocalDate date = unit.addTo(this, amountToAdd);
        
        return ChronoLocalDateImpl.ensureValid(getChronology(), date);
    }
    
    /*▲ 增加 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 减少 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException   {@inheritDoc}
     * @throws ArithmeticException {@inheritDoc}
     */
    /*
     * 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
    default ChronoLocalDate minus(TemporalAmount amount) {
        return ChronoLocalDateImpl.ensureValid(getChronology(), Temporal.super.minus(amount));
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException                {@inheritDoc}
     * @throws UnsupportedTemporalTypeException {@inheritDoc}
     * @throws ArithmeticException              {@inheritDoc}
     */
    /*
     * 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
    default ChronoLocalDate minus(long amountToSubtract, TemporalUnit unit) {
        Temporal temporal = Temporal.super.minus(amountToSubtract, unit);
        return ChronoLocalDateImpl.ensureValid(getChronology(), temporal);
    }
    
    /*▲ 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 date.
     * If false, then calling the {@link #plus(long, TemporalUnit)} and
     * {@link #minus(long, TemporalUnit) minus} methods will throw an exception.
     * <p>
     * The set of supported units is defined by the chronology and normally includes
     * all {@code ChronoUnit} date units except {@code FOREVER}.
     * <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
    default boolean isSupported(TemporalUnit unit) {
        if(unit instanceof ChronoUnit) {
            // "日期"单位都被支持
            return unit.isDateBased();
        }
        
        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 the specified field can be queried on this date.
     * If false, then calling the {@link #range(TemporalField) range},
     * {@link #get(TemporalField) get} and {@link #with(TemporalField, long)}
     * methods will throw an exception.
     * <p>
     * The set of supported fields is defined by the chronology and normally includes
     * all {@code ChronoField} date fields.
     * <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 can be queried, 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.isDateBased();
        }
        return field != null && field.isSupportedBy(this);
    }
    
    /**
     * Queries this date using the specified query.
     * <p>
     * This queries this date 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) {
        // 查询时间量支持的最小时间量单位
        if(query == TemporalQueries.precision()) {
            return (R) DAYS;
        }
    
        // 查询时间量的历法系统
        if(query == TemporalQueries.chronology()) {
            return (R) getChronology();
        }
    
        // 一些优化操作，即当前时间量不支持下面这些查询操作
        if(query == TemporalQueries.zoneId() || query == TemporalQueries.zone() || query == TemporalQueries.offset() || query == TemporalQueries.localTime()) {
            return null;
        }
    
        /*
         * inline TemporalAccessor.super.query(query) as an optimization
         * non-JDK classes are not permitted to make this optimization
         */
        return query.queryFrom(this);
    }
    
    /*▲ 时间量字段操作(TemporalAccessor) ███████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 整合 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException   {@inheritDoc}
     * @throws ArithmeticException {@inheritDoc}
     */
    /*
     * 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
    default ChronoLocalDate with(TemporalAdjuster adjuster) {
        Temporal temporal = Temporal.super.with(adjuster);
        return ChronoLocalDateImpl.ensureValid(getChronology(), temporal);
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws DateTimeException                {@inheritDoc}
     * @throws UnsupportedTemporalTypeException {@inheritDoc}
     * @throws ArithmeticException              {@inheritDoc}
     */
    /*
     * 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
     *
     * 默认是不支持ChronoField类型的字段的，但是子类的实现中可以支持它
     */
    @Override
    default ChronoLocalDate with(TemporalField field, long newValue) {
        if(field instanceof ChronoField) {
            throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
        }
        
        ChronoLocalDate chronoLocalDate = field.adjustInto(this, newValue);
        
        return ChronoLocalDateImpl.ensureValid(getChronology(), chronoLocalDate);
    }
    
    /**
     * Adjusts the specified temporal object to have the same date as this object.
     * <p>
     * This returns a temporal object of the same observable type as the input
     * with the date changed to be the same as this.
     * <p>
     * The adjustment is equivalent to using {@link Temporal#with(TemporalField, long)}
     * passing {@link ChronoField#EPOCH_DAY} 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 = thisLocalDate.adjustInto(temporal);
     *   temporal = temporal.with(thisLocalDate);
     * </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.EPOCH_DAY - 当前时间量中包含的纪元天
     *
     * 目标时间量temporal的默认取值可以是：
     * LocalDateTime
     * OffsetDateTime
     * ZonedDateTime
     * ChronoLocalDateTimeImpl
     * ChronoZonedDateTimeImpl
     */
    @Override
    default Temporal adjustInto(Temporal temporal) {
        // 获取当前时间量中包含的纪元天
        long epochDay = toEpochDay();
        
        return temporal.with(ChronoField.EPOCH_DAY, epochDay);
    }
    
    /*▲ 整合 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Calculates the amount of time until another date in terms of the specified unit.
     * <p>
     * This calculates the amount of time between two {@code ChronoLocalDate}
     * objects in terms of a single {@code TemporalUnit}.
     * The start and end points are {@code this} and the specified date.
     * The result will be negative if the end is before the start.
     * The {@code Temporal} passed to this method is converted to a
     * {@code ChronoLocalDate} using {@link Chronology#date(TemporalAccessor)}.
     * The calculation returns a whole number, representing the number of
     * complete units between the two dates.
     * For example, the amount in days between two dates can be calculated
     * using {@code startDate.until(endDate, DAYS)}.
     * <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 DAYS}, {@code WEEKS}, {@code MONTHS}, {@code YEARS},
     * {@code DECADES}, {@code CENTURIES}, {@code MILLENNIA} and {@code ERAS}
     * should be supported by all implementations.
     * 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 ChronoLocalDate} in the same chronology, not null
     * @param unit         the unit to measure the amount in, not null
     *
     * @return the amount of time between this date and the end date
     *
     * @throws DateTimeException                if the amount cannot be calculated, or the end
     *                                          temporal cannot be converted to a {@code ChronoLocalDate}
     * @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
    long until(Temporal endExclusive, TemporalUnit unit);
    
    /**
     * Calculates the period between this date and another date as a {@code ChronoPeriod}.
     * <p>
     * This calculates the period between two dates. All supplied chronologies
     * calculate the period using years, months and days, however the
     * {@code ChronoPeriod} API allows the period to be represented using other units.
     * <p>
     * The start and end points are {@code this} and the specified date.
     * The result will be negative if the end is before the start.
     * The negative sign will be the same in each of year, month and day.
     * <p>
     * The calculation is performed using the chronology of this date.
     * If necessary, the input date will be converted to match.
     * <p>
     * This instance is immutable and unaffected by this method call.
     *
     * @param endDateExclusive the end date, exclusive, which may be in any chronology, not null
     *
     * @return the period between this date and the end date, not null
     *
     * @throws DateTimeException   if the period cannot be calculated
     * @throws ArithmeticException if numeric overflow occurs
     */
    // 计算当前时间量到目标时间量之间相差的"时间段"
    ChronoPeriod until(ChronoLocalDate endDateExclusive);
    
    /**
     * Returns the length of the year represented by this date, as defined by the calendar system.
     * <p>
     * This returns the length of the year in days.
     * <p>
     * The default implementation uses {@link #isLeapYear()} and returns 365 or 366.
     *
     * @return the length of the year in days
     */
    // 返回当前日期的年份包含的天数
    default int lengthOfYear() {
        return (isLeapYear() ? 366 : 365);
    }
    
    /**
     * Returns the length of the month represented by this date, as defined by the calendar system.
     * <p>
     * This returns the length of the month in days.
     *
     * @return the length of the month in days
     */
    // 返回当前日期的月份包含的天数
    int lengthOfMonth();
    
    /**
     * Checks if the year is a leap year, as defined by the calendar system.
     * <p>
     * A leap-year is a year of a longer length than normal.
     * The exact meaning is determined by the chronology with the constraint that
     * a leap-year must imply a year-length longer than a non leap-year.
     * <p>
     * This default implementation uses {@link Chronology#isLeapYear(long)}.
     *
     * @return true if this date is in a leap year, false otherwise
     */
    // 判断当前日期的年份是否为"闰年"
    default boolean isLeapYear() {
        return getChronology().isLeapYear(getLong(ChronoField.YEAR));
    }
    
    /**
     * Gets the chronology of this date.
     * <p>
     * The {@code Chronology} represents the calendar system in use.
     * The era and other fields in {@link ChronoField} are defined by the chronology.
     *
     * @return the chronology, not null
     */
    // 返回当前日期所用的历法系统
    Chronology getChronology();
    
    /**
     * Gets the era, as defined by the chronology.
     * <p>
     * The era is, conceptually, the largest division of the time-line.
     * Most calendar systems have a single epoch dividing the time-line into two eras.
     * However, some have multiple eras, such as one for the reign of each leader.
     * The exact meaning is determined by the {@code Chronology}.
     * <p>
     * All correctly implemented {@code Era} classes are singletons, thus it
     * is valid code to write {@code date.getEra() == SomeChrono.ERA_NAME)}.
     * <p>
     * This default implementation uses {@link Chronology#eraOf(int)}.
     *
     * @return the chronology specific era constant applicable at this date, not null
     */
    // 返回当前日期所属的纪元
    default Era getEra() {
        return getChronology().eraOf(get(ChronoField.ERA));
    }
    
    /**
     * Checks if this date is after the specified date ignoring the chronology.
     * <p>
     * This method differs from the comparison in {@link #compareTo} in that it
     * only compares the underlying date and not the chronology.
     * This allows dates in different calendar systems to be compared based
     * on the time-line position.
     * This is equivalent to using {@code date1.toEpochDay() > date2.toEpochDay()}.
     * <p>
     * This default implementation performs the comparison based on the epoch-day.
     *
     * @param other the other date to compare to, not null
     *
     * @return true if this is after the specified date
     */
    // 判断当前日期是否晚于参数中指定的日期
    default boolean isAfter(ChronoLocalDate other) {
        return this.toEpochDay()>other.toEpochDay();
    }
    
    /**
     * Checks if this date is before the specified date ignoring the chronology.
     * <p>
     * This method differs from the comparison in {@link #compareTo} in that it
     * only compares the underlying date and not the chronology.
     * This allows dates in different calendar systems to be compared based
     * on the time-line position.
     * This is equivalent to using {@code date1.toEpochDay() < date2.toEpochDay()}.
     * <p>
     * This default implementation performs the comparison based on the epoch-day.
     *
     * @param other the other date to compare to, not null
     *
     * @return true if this is before the specified date
     */
    // 判断当前日期是否早于参数中指定的日期
    default boolean isBefore(ChronoLocalDate other) {
        return this.toEpochDay()<other.toEpochDay();
    }
    
    /**
     * Checks if this date is equal to the specified date ignoring the chronology.
     * <p>
     * This method differs from the comparison in {@link #compareTo} in that it
     * only compares the underlying date and not the chronology.
     * This allows dates in different calendar systems to be compared based
     * on the time-line position.
     * This is equivalent to using {@code date1.toEpochDay() == date2.toEpochDay()}.
     * <p>
     * This default implementation performs the comparison based on the epoch-day.
     *
     * @param other the other date to compare to, not null
     *
     * @return true if the underlying date is equal to the specified date
     */
    // 判断当前日期与参数中指定的日期是否相等
    default boolean isEqual(ChronoLocalDate other) {
        return this.toEpochDay() == other.toEpochDay();
    }
    
    /**
     * Gets a comparator that compares {@code ChronoLocalDate} in
     * time-line order ignoring the chronology.
     * <p>
     * This comparator differs from the comparison in {@link #compareTo} in that it
     * only compares the underlying date and not the chronology.
     * This allows dates in different calendar systems to be compared based
     * on the position of the date on the local time-line.
     * The underlying comparison is equivalent to comparing the epoch-day.
     *
     * @return a comparator that compares in time-line order ignoring the chronology
     *
     * @see #isAfter
     * @see #isBefore
     * @see #isEqual
     */
    // 返回一个外部比较器，以比较两个"本地日期"的早晚
    static Comparator<ChronoLocalDate> timeLineOrder() {
        return new Comparator<ChronoLocalDate>() {
            @Override
            public int compare(ChronoLocalDate date1, ChronoLocalDate date2) {
                return Long.compare(date1.toEpochDay(), date2.toEpochDay());
            }
        };
    }
    
    /**
     * Formats this date using the specified formatter.
     * <p>
     * This date will be passed to the formatter to produce a string.
     * <p>
     * The default implementation must behave as follows:
     * <pre>
     *  return formatter.format(this);
     * </pre>
     *
     * @param formatter the formatter to use, not null
     *
     * @return the formatted date string, not null
     *
     * @throws DateTimeException if an error occurs during printing
     */
    // 将当前日期转换为一个指定格式的字符串后返回
    default String format(DateTimeFormatter formatter) {
        Objects.requireNonNull(formatter, "formatter");
        return formatter.format(this);
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /**
     * Compares this date to another date, including the chronology.
     * <p>
     * The comparison is based first on the underlying time-line date, then
     * on the chronology.
     * It is "consistent with equals", as defined by {@link Comparable}.
     * <p>
     * For example, the following is the comparator order:
     * <ol>
     * <li>{@code 2012-12-03 (ISO)}</li>
     * <li>{@code 2012-12-04 (ISO)}</li>
     * <li>{@code 2555-12-04 (ThaiBuddhist)}</li>
     * <li>{@code 2012-12-05 (ISO)}</li>
     * </ol>
     * Values #2 and #3 represent the same date on the time-line.
     * When two values represent the same date, the chronology ID is compared to distinguish them.
     * This step is needed to make the ordering "consistent with equals".
     * <p>
     * If all the date objects being compared are in the same chronology, then the
     * additional chronology stage is not required and only the local date is used.
     * To compare the dates of two {@code TemporalAccessor} instances, including dates
     * in two different chronologies, use {@link ChronoField#EPOCH_DAY} as a comparator.
     * <p>
     * This default implementation performs the comparison defined above.
     *
     * @param other the other date to compare to, not null
     *
     * @return the comparator value, negative if less, positive if greater
     */
    @Override
    default int compareTo(ChronoLocalDate other) {
        int cmp = Long.compare(toEpochDay(), other.toEpochDay());
        if(cmp == 0) {
            cmp = getChronology().compareTo(other.getChronology());
        }
        return cmp;
    }
    
    /**
     * Outputs this date as a {@code String}.
     * <p>
     * The output will include the full local date.
     *
     * @return the formatted date, not null
     */
    @Override
    String toString();
    
    /**
     * Checks if this date is equal to another date, including the chronology.
     * <p>
     * Compares this date with another ensuring that the date and chronology are the same.
     * <p>
     * To compare the dates of two {@code TemporalAccessor} instances, including dates
     * in two different chronologies, use {@link ChronoField#EPOCH_DAY} as a comparator.
     *
     * @param obj the object to check, null returns false
     *
     * @return true if this is equal to the other date
     */
    @Override
    boolean equals(Object obj);
    
    /**
     * A hash code for this date.
     *
     * @return a suitable hash code
     */
    @Override
    int hashCode();
    
}