TemporalAdjusters.java

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 * Copyright (c) 2012-2013, Stephen Colebourne & Michael Nascimento Santos
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package java.time.temporal;

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.util.Objects;
import java.util.function.UnaryOperator;

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

/**
 * Common and useful TemporalAdjusters.
 * <p>
 * Adjusters are a key tool for modifying temporal objects.
 * They exist to externalize the process of adjustment, permitting different
 * approaches, as per the strategy design pattern.
 * Examples might be an adjuster that sets the date avoiding weekends, or one that
 * sets the date to the last day of the month.
 * <p>
 * There are two equivalent ways of using a {@code TemporalAdjuster}.
 * The first is to invoke the method on the interface directly.
 * The second is to use {@link Temporal#with(TemporalAdjuster)}:
 * <pre>
 *   // these two lines are equivalent, but the second approach is recommended
 *   temporal = thisAdjuster.adjustInto(temporal);
 *   temporal = temporal.with(thisAdjuster);
 * </pre>
 * It is recommended to use the second approach, {@code with(TemporalAdjuster)},
 * as it is a lot clearer to read in code.
 * <p>
 * This class contains a standard set of adjusters, available as static methods.
 * These include:
 * <ul>
 * <li>finding the first or last day of the month
 * <li>finding the first day of next month
 * <li>finding the first or last day of the year
 * <li>finding the first day of next year
 * <li>finding the first or last day-of-week within a month, such as "first Wednesday in June"
 * <li>finding the next or previous day-of-week, such as "next Thursday"
 * </ul>
 *
 * @implSpec All the implementations supplied by the static methods are immutable.
 * @see TemporalAdjuster
 * @since 1.8
 */
// 时间量整合器工厂，内部预设了多种实用的整合器
public final class TemporalAdjusters {
    
    /**
     * Private constructor since this is a utility class.
     */
    private TemporalAdjusters() {
    }
    
    /**
     * Obtains a {@code TemporalAdjuster} that wraps a date adjuster.
     * <p>
     * The {@code TemporalAdjuster} is based on the low level {@code Temporal} interface.
     * This method allows an adjustment from {@code LocalDate} to {@code LocalDate}
     * to be wrapped to match the temporal-based interface.
     * This is provided for convenience to make user-written adjusters simpler.
     * <p>
     * In general, user-written adjusters should be static constants:
     * <pre>{@code
     *  static TemporalAdjuster TWO_DAYS_LATER =
     *       TemporalAdjusters.ofDateAdjuster(date -> date.plusDays(2));
     * }</pre>
     *
     * @param dateBasedAdjuster the date-based adjuster, not null
     *
     * @return the temporal adjuster wrapping on the date adjuster, not null
     */
    // 返回一个时间量整合器，该整合期支持使用dateBasedAdjuster处理目标时间量中的LocalDate信息
    public static TemporalAdjuster ofDateAdjuster(UnaryOperator<LocalDate> dateBasedAdjuster) {
        Objects.requireNonNull(dateBasedAdjuster, "dateBasedAdjuster");
    
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                /*
                 * 从temporal中查询LocalDate部件。
                 *
                 * 如果没有现成的部件，通常需要从temporal中解析出纪元天，
                 * 然后使用纪元天构造LocalDate后返回。
                 */
                LocalDate input = LocalDate.from(temporal);
            
                // 使用dateBasedAdjuster处理input后返回处理结果
                LocalDate output = dateBasedAdjuster.apply(input);
            
                // 将output与temporal中其他类型的字段整合后返回
                return temporal.with(output);
            }
        };
    }
    
    /**
     * Returns the "first day of month" adjuster, which returns a new date set to
     * the first day of the current month.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2011-01-01.<br>
     * The input 2011-02-15 will return 2011-02-01.
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  temporal.with(DAY_OF_MONTH, 1);
     * </pre>
     *
     * @return the first day-of-month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在月的第一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster firstDayOfMonth() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                return temporal.with(ChronoField.DAY_OF_MONTH, 1);
            }
        };
    }
    
    /**
     * Returns the "last day of month" adjuster, which returns a new date set to
     * the last day of the current month.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2011-01-31.<br>
     * The input 2011-02-15 will return 2011-02-28.<br>
     * The input 2012-02-15 will return 2012-02-29 (leap year).<br>
     * The input 2011-04-15 will return 2011-04-30.
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  long lastDay = temporal.range(DAY_OF_MONTH).getMaximum();
     *  temporal.with(DAY_OF_MONTH, lastDay);
     * </pre>
     *
     * @return the last day-of-month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在月的最后一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster lastDayOfMonth() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 获取ChronoField.DAY_OF_MONTH字段的上区间最大值
                long max = temporal.range(ChronoField.DAY_OF_MONTH).getMaximum();
                return temporal.with(ChronoField.DAY_OF_MONTH, max);
            }
        };
    }
    
    /**
     * Returns the "first day of next month" adjuster, which returns a new date set to
     * the first day of the next month.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2011-02-01.<br>
     * The input 2011-02-15 will return 2011-03-01.
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  temporal.with(DAY_OF_MONTH, 1).plus(1, MONTHS);
     * </pre>
     *
     * @return the first day of next month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在月的下个月的第一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster firstDayOfNextMonth() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                return temporal.with(ChronoField.DAY_OF_MONTH, 1).plus(1, MONTHS);
            }
        };
    }
    
    /**
     * Returns the "first day of year" adjuster, which returns a new date set to
     * the first day of the current year.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2011-01-01.<br>
     * The input 2011-02-15 will return 2011-01-01.<br>
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  temporal.with(DAY_OF_YEAR, 1);
     * </pre>
     *
     * @return the first day-of-year adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在年的第一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster firstDayOfYear() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                return temporal.with(ChronoField.DAY_OF_YEAR, 1);
            }
        };
    }
    
    /**
     * Returns the "last day of year" adjuster, which returns a new date set to
     * the last day of the current year.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2011-12-31.<br>
     * The input 2011-02-15 will return 2011-12-31.<br>
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  long lastDay = temporal.range(DAY_OF_YEAR).getMaximum();
     *  temporal.with(DAY_OF_YEAR, lastDay);
     * </pre>
     *
     * @return the last day-of-year adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在年的最后一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster lastDayOfYear() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 获取ChronoField.DAY_OF_YEAR字段的上区间最大值
                long max = temporal.range(ChronoField.DAY_OF_YEAR).getMaximum();
                return temporal.with(ChronoField.DAY_OF_YEAR, max);
            }
        };
    }
    
    /**
     * Returns the "first day of next year" adjuster, which returns a new date set to
     * the first day of the next year.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 will return 2012-01-01.
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It is equivalent to:
     * <pre>
     *  temporal.with(DAY_OF_YEAR, 1).plus(1, YEARS);
     * </pre>
     *
     * @return the first day of next month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器通常是用temporal所在年的下一年的第一天这个时间点来更新temporal的"天"部件
    public static TemporalAdjuster firstDayOfNextYear() {
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                return temporal.with(ChronoField.DAY_OF_YEAR, 1).plus(1, YEARS);
            }
        };
    }
    
    /**
     * Returns the first in month adjuster, which returns a new date
     * in the same month with the first matching day-of-week.
     * This is used for expressions like 'first Tuesday in March'.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-12-15 for (MONDAY) will return 2011-12-05.<br>
     * The input 2011-12-15 for (FRIDAY) will return 2011-12-02.<br>
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} and {@code DAY_OF_MONTH} fields
     * and the {@code DAYS} unit, and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week, not null
     *
     * @return the first in month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器将temporal调节到当月第一天之后的第1个dayOfWeek时间点
    public static TemporalAdjuster firstInMonth(DayOfWeek dayOfWeek) {
        return TemporalAdjusters.dayOfWeekInMonth(1, dayOfWeek);
    }
    
    /**
     * Returns the last in month adjuster, which returns a new date
     * in the same month with the last matching day-of-week.
     * This is used for expressions like 'last Tuesday in March'.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-12-15 for (MONDAY) will return 2011-12-26.<br>
     * The input 2011-12-15 for (FRIDAY) will return 2011-12-30.<br>
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} and {@code DAY_OF_MONTH} fields
     * and the {@code DAYS} unit, and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week, not null
     *
     * @return the first in month adjuster, not null
     */
    // 返回一个时间量整合器，该整合器将temporal调节到当月最后一天之前的第1个dayOfWeek时间点
    public static TemporalAdjuster lastInMonth(DayOfWeek dayOfWeek) {
        return TemporalAdjusters.dayOfWeekInMonth(-1, dayOfWeek);
    }
    
    /**
     * Returns the day-of-week in month adjuster, which returns a new date
     * with the ordinal day-of-week based on the month.
     * This is used for expressions like the 'second Tuesday in March'.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-12-15 for (1,TUESDAY) will return 2011-12-06.<br>
     * The input 2011-12-15 for (2,TUESDAY) will return 2011-12-13.<br>
     * The input 2011-12-15 for (3,TUESDAY) will return 2011-12-20.<br>
     * The input 2011-12-15 for (4,TUESDAY) will return 2011-12-27.<br>
     * The input 2011-12-15 for (5,TUESDAY) will return 2012-01-03.<br>
     * The input 2011-12-15 for (-1,TUESDAY) will return 2011-12-27 (last in month).<br>
     * The input 2011-12-15 for (-4,TUESDAY) will return 2011-12-06 (3 weeks before last in month).<br>
     * The input 2011-12-15 for (-5,TUESDAY) will return 2011-11-29 (4 weeks before last in month).<br>
     * The input 2011-12-15 for (0,TUESDAY) will return 2011-11-29 (last in previous month).<br>
     * <p>
     * For a positive or zero ordinal, the algorithm is equivalent to finding the first
     * day-of-week that matches within the month and then adding a number of weeks to it.
     * For a negative ordinal, the algorithm is equivalent to finding the last
     * day-of-week that matches within the month and then subtracting a number of weeks to it.
     * The ordinal number of weeks is not validated and is interpreted leniently
     * according to this algorithm. This definition means that an ordinal of zero finds
     * the last matching day-of-week in the previous month.
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} and {@code DAY_OF_MONTH} fields
     * and the {@code DAYS} unit, and assumes a seven day week.
     *
     * @param ordinal   the week within the month, unbounded but typically from -5 to 5
     * @param dayOfWeek the day-of-week, not null
     *
     * @return the day-of-week in month adjuster, not null
     */
    /*
     * 返回一个时间量整合器来调节temporal。
     *
     * 如果ordinal>=0：
     * > 先把temporal调节到当月第一天，
     * > 再将temporal前进到后面第|ordinal|个dayOfWeek时间点。
     *   如果ordinal为0，则是将temporal回退到前面第1个dayOfWeek时间点。
     *
     * 如果ordinal<0：
     * > 先把temporal调节到当月最后一天，
     * > 再将temporal回退到前面第|ordinal|个dayOfWeek时间点。
     */
    public static TemporalAdjuster dayOfWeekInMonth(int ordinal, DayOfWeek dayOfWeek) {
        Objects.requireNonNull(dayOfWeek, "dayOfWeek");
        
        // 返回dayOfWeek在一周中的天数，即周几
        int dowValue = dayOfWeek.getValue();
        
        if(ordinal >= 0) {
            return new TemporalAdjuster() {
                @Override
                public Temporal adjustInto(Temporal temporal) {
                    // 用temporal所在月的第一天这个时间点来更新temporal的"天"部件
                    Temporal temp = temporal.with(ChronoField.DAY_OF_MONTH, 1);
                    
                    // 获取temp当前是周几
                    int curDow = temp.get(ChronoField.DAY_OF_WEEK);
                    
                    // temp到dayOfWeek的天数
                    int dowDiff = (dowValue - curDow + 7) % 7;
                    dowDiff += (ordinal - 1L) * 7L;
                    
                    return temp.plus(dowDiff, DAYS);
                }
            };
        } else {
            return new TemporalAdjuster() {
                @Override
                public Temporal adjustInto(Temporal temporal) {
                    // 获取ChronoField.DAY_OF_MONTH字段的上区间最大值
                    long max = temporal.range(ChronoField.DAY_OF_MONTH).getMaximum();
                    
                    // 用temporal所在月的最后一天这个时间点来更新temporal的"天"部件
                    Temporal temp = temporal.with(ChronoField.DAY_OF_MONTH, max);
                    
                    // 获取temp当前是周几
                    int curDow = temp.get(ChronoField.DAY_OF_WEEK);
                    
                    // temp到dayOfWeek的天数
                    int daysDiff = dowValue - curDow;
                    daysDiff = (daysDiff == 0 ? 0 : (daysDiff>0 ? daysDiff - 7 : daysDiff));
                    daysDiff -= (-ordinal - 1L) * 7L;
                    
                    return temp.plus(daysDiff, DAYS);
                }
            };
        }
    }
    
    /**
     * Returns the next day-of-week adjuster, which adjusts the date to the
     * first occurrence of the specified day-of-week after the date being adjusted.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 (a Saturday) for parameter (MONDAY) will return 2011-01-17 (two days later).<br>
     * The input 2011-01-15 (a Saturday) for parameter (WEDNESDAY) will return 2011-01-19 (four days later).<br>
     * The input 2011-01-15 (a Saturday) for parameter (SATURDAY) will return 2011-01-22 (seven days later).
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} field and the {@code DAYS} unit,
     * and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week to move the date to, not null
     *
     * @return the next day-of-week adjuster, not null
     */
    /*
     * 返回一个时间量整合器，该整合器将temporal前进到下一个dayOfWeek时间点。
     * 即使temporal已经在dayOfWeek这个时间点，仍需要加上7天进入下周。
     */
    public static TemporalAdjuster next(DayOfWeek dayOfWeek) {
        // 返回dayOfWeek在一周中的天数，即周几
        int dowValue = dayOfWeek.getValue();
        
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 计算temporal是一周的第几天(周几)
                int calDow = temporal.get(ChronoField.DAY_OF_WEEK);
                
                // dayOfWeek到temporal的天数
                int daysDiff = calDow - dowValue;
                
                // 加上指定的天数，进入下一周
                return temporal.plus(daysDiff >= 0 ? 7 - daysDiff : -daysDiff, DAYS);
            }
        };
    }
    
    /**
     * Returns the next-or-same day-of-week adjuster, which adjusts the date to the
     * first occurrence of the specified day-of-week after the date being adjusted
     * unless it is already on that day in which case the same object is returned.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 (a Saturday) for parameter (MONDAY) will return 2011-01-17 (two days later).<br>
     * The input 2011-01-15 (a Saturday) for parameter (WEDNESDAY) will return 2011-01-19 (four days later).<br>
     * The input 2011-01-15 (a Saturday) for parameter (SATURDAY) will return 2011-01-15 (same as input).
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} field and the {@code DAYS} unit,
     * and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week to check for or move the date to, not null
     *
     * @return the next-or-same day-of-week adjuster, not null
     */
    /*
     * 返回一个时间量整合器，该整合器将temporal前进到下一个dayOfWeek时间点。
     * 如果temporal已经在dayOfWeek这个时间点，则不做任何操作。
     */
    public static TemporalAdjuster nextOrSame(DayOfWeek dayOfWeek) {
        // 返回dayOfWeek在一周中的天数，即周几
        int dowValue = dayOfWeek.getValue();
        
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 计算temporal是一周的第几天(周几)
                int calDow = temporal.get(ChronoField.DAY_OF_WEEK);
                if(calDow == dowValue) {
                    return temporal;
                }
                
                // dayOfWeek到temporal的天数
                int daysDiff = calDow - dowValue;
                
                // 加上指定的天数，进入下一周
                return temporal.plus(daysDiff >= 0 ? 7 - daysDiff : -daysDiff, DAYS);
            }
        };
    }
    
    /**
     * Returns the previous day-of-week adjuster, which adjusts the date to the
     * first occurrence of the specified day-of-week before the date being adjusted.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 (a Saturday) for parameter (MONDAY) will return 2011-01-10 (five days earlier).<br>
     * The input 2011-01-15 (a Saturday) for parameter (WEDNESDAY) will return 2011-01-12 (three days earlier).<br>
     * The input 2011-01-15 (a Saturday) for parameter (SATURDAY) will return 2011-01-08 (seven days earlier).
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} field and the {@code DAYS} unit,
     * and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week to move the date to, not null
     *
     * @return the previous day-of-week adjuster, not null
     */
    /*
     * 返回一个时间量整合器，该整合器将temporal回退到上一个dayOfWeek时间点。
     * 即使temporal已经在dayOfWeek这个时间点，仍需要减去7天进入上周。
     */
    public static TemporalAdjuster previous(DayOfWeek dayOfWeek) {
        // 返回dayOfWeek在一周中的天数，即周几
        int dowValue = dayOfWeek.getValue();
        
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 计算temporal是一周的第几天(周几)
                int calDow = temporal.get(ChronoField.DAY_OF_WEEK);
                
                // temporal到dayOfWeek的天数
                int daysDiff = dowValue - calDow;
                
                // 减去指定的天数，进入上一周
                return temporal.minus(daysDiff >= 0 ? 7 - daysDiff : -daysDiff, DAYS);
            }
        };
    }
    
    /**
     * Returns the previous-or-same day-of-week adjuster, which adjusts the date to the
     * first occurrence of the specified day-of-week before the date being adjusted
     * unless it is already on that day in which case the same object is returned.
     * <p>
     * The ISO calendar system behaves as follows:<br>
     * The input 2011-01-15 (a Saturday) for parameter (MONDAY) will return 2011-01-10 (five days earlier).<br>
     * The input 2011-01-15 (a Saturday) for parameter (WEDNESDAY) will return 2011-01-12 (three days earlier).<br>
     * The input 2011-01-15 (a Saturday) for parameter (SATURDAY) will return 2011-01-15 (same as input).
     * <p>
     * The behavior is suitable for use with most calendar systems.
     * It uses the {@code DAY_OF_WEEK} field and the {@code DAYS} unit,
     * and assumes a seven day week.
     *
     * @param dayOfWeek the day-of-week to check for or move the date to, not null
     *
     * @return the previous-or-same day-of-week adjuster, not null
     */
    /*
     * 返回一个时间量整合器，该整合器将temporal回退到上一个dayOfWeek时间点。
     * 如果temporal已经在dayOfWeek这个时间点，则不做任何操作。
     */
    public static TemporalAdjuster previousOrSame(DayOfWeek dayOfWeek) {
        // 返回dayOfWeek在一周中的天数，即周几
        int dowValue = dayOfWeek.getValue();
        
        return new TemporalAdjuster() {
            @Override
            public Temporal adjustInto(Temporal temporal) {
                // 计算temporal是一周的第几天(周几)
                int calDow = temporal.get(ChronoField.DAY_OF_WEEK);
                if(calDow == dowValue) {
                    return temporal;
                }
                
                // temporal到dayOfWeek的天数
                int daysDiff = dowValue - calDow;
                
                // 减去指定的天数，进入上一周
                return temporal.minus(daysDiff >= 0 ? 7 - daysDiff : -daysDiff, DAYS);
            }
        };
    }
    
}