Quelle YearMonth.java Sprache: JAVA

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* Copyright (c) 2007-2012, Stephen Colebourne & Michael Nascimento Santos
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package java.time;

import static java.time.temporal.ChronoField.ERA;
import static java.time.temporal.ChronoField.MONTH_OF_YEAR;
import static java.time.temporal.ChronoField.PROLEPTIC_MONTH;
import static java.time.temporal.ChronoField.YEAR;
import static java.time.temporal.ChronoField.YEAR_OF_ERA;
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;

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;

/**
* A year-month in the ISO-8601 calendar system, such as {@code 2007-12}.
* 
* {@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.
* 
* 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}.
* 
* 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.
* 
* This is a <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>
* class; programmers should treat instances that are
* {@linkplain #equals(Object) equal} as interchangeable and should not
* use instances for synchronization, or unpredictable behavior may
* occur. For example, in a future release, synchronization may fail.
* The {@code equals} method should be used for comparisons.
*
* @implSpec
* This class is immutable and thread-safe.
*
* @since 1.8
*/
@jdk.internal.ValueBased
public final class YearMonth
 implements Temporal, TemporalAdjuster, Comparable<YearMonth>, Serializable {

 /**
 * Serialization version.
 */
 @java.io.Serial
 private static final long serialVersionUID = 4183400860270640070L;
 /**
 * Parser.
 */
 private static final DateTimeFormatter PARSER = new DateTimeFormatterBuilder()
 .appendValue(YEAR, 4, 10, SignStyle.EXCEEDS_PAD)
 .appendLiteral('-')
 .appendValue(MONTH_OF_YEAR, 2)
 .toFormatter();

 /**
 * The year.
 */
 private final int year;
 /**
 * The month-of-year, not null.
 */
 private final int month;

 //-----------------------------------------------------------------------
 /**
 * Obtains the current year-month from the system clock in the default time-zone.
 * 
 * This will query the {@link Clock#systemDefaultZone() system clock} in the default
 * time-zone to obtain the current year-month.
 * 
 * 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
 */
 public static YearMonth now() {
 return now(Clock.systemDefaultZone());
 }

 /**
 * Obtains the current year-month from the system clock in the specified time-zone.
 * 
 * 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.
 * 
 * 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
 */
 public static YearMonth now(ZoneId zone) {
 return now(Clock.system(zone));
 }

 /**
 * Obtains the current year-month from the specified clock.
 * 
 * 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
 */
 public static YearMonth now(Clock clock) {
 final LocalDate now = LocalDate.now(clock); // called once
 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
 */
 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
 */
 public static YearMonth of(int year, int month) {
 YEAR.checkValidValue(year);
 MONTH_OF_YEAR.checkValidValue(month);
 return new YearMonth(year, month);
 }

 //-----------------------------------------------------------------------
 /**
 * Obtains an instance of {@code YearMonth} from a temporal object.
 * 
 * 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}.
 * 
 * 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}.
 * 
 * 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}
 */
 public static YearMonth from(TemporalAccessor temporal) {
 if (temporal instanceof YearMonth) {
 return (YearMonth) temporal;
 }
 Objects.requireNonNull(temporal, "temporal");
 try {
 if (IsoChronology.INSTANCE.equals(Chronology.from(temporal)) == false) {
 temporal = LocalDate.from(temporal);
 }
 return of(temporal.get(YEAR), temporal.get(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}.
 * 
 * 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
 */
 public static YearMonth parse(CharSequence text) {
 return parse(text, PARSER);
 }

 /**
 * Obtains an instance of {@code YearMonth} from a text string using a specific formatter.
 * 
 * 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
 */
 public static YearMonth parse(CharSequence text, DateTimeFormatter formatter) {
 Objects.requireNonNull(formatter, "formatter");
 return formatter.parse(text, YearMonth::from);
 }

 //-----------------------------------------------------------------------
 /**
 * 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;
 }

 /**
 * 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
 */
 private YearMonth with(int newYear, int newMonth) {
 if (year == newYear && month == newMonth) {
 return this;
 }
 return new YearMonth(newYear, newMonth);
 }

 //-----------------------------------------------------------------------
 /**
 * Checks if the specified field is supported.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public boolean isSupported(TemporalField field) {
 if (field instanceof ChronoField) {
 return field == YEAR || field == MONTH_OF_YEAR ||
 field == PROLEPTIC_MONTH || field == YEAR_OF_ERA || field == ERA;
 }
 return field != null && field.isSupportedBy(this);
 }

 /**
 * Checks if the specified unit is supported.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @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);
 }

 //-----------------------------------------------------------------------
 /**
 * Gets the range of valid values for the specified field.
 * 
 * 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.
 * 
 * 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}.
 * 
 * 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
 */
 @Override
 public ValueRange range(TemporalField field) {
 if (field == YEAR_OF_ERA) {
 return (getYear() <= 0 ? ValueRange.of(1, Year.MAX_VALUE + 1) : 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}.
 * 
 * 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.
 * 
 * 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}.
 * 
 * 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
 */
 @Override // override for Javadoc
 public int get(TemporalField field) {
 return range(field).checkValidIntValue(getLong(field), field);
 }

 /**
 * Gets the value of the specified field from this year-month as a {@code long}.
 * 
 * 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.
 * 
 * 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}.
 * 
 * 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
 */
 @Override
 public long getLong(TemporalField field) {
 if (field instanceof ChronoField chronoField) {
 return switch (chronoField) {
 case MONTH_OF_YEAR -> month;
 case PROLEPTIC_MONTH -> getProlepticMonth();
 case YEAR_OF_ERA -> (year < 1 ? 1 - year : year);
 case YEAR -> year;
 case ERA -> (year < 1 ? 0 : 1);
 default -> throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
 };
 }
 return field.getFrom(this);
 }

 private long getProlepticMonth() {
 return (year * 12L + month - 1);
 }

 //-----------------------------------------------------------------------
 /**
 * Gets the year field.
 * 
 * This method returns the primitive {@code int} value for the year.
 * 
 * The year returned by this method is proleptic as per {@code get(YEAR)}.
 *
 * @return the year, from MIN_YEAR to MAX_YEAR
 */
 public int getYear() {
 return year;
 }

 /**
 * Gets the month-of-year field from 1 to 12.
 * 
 * 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()
 */
 public int getMonthValue() {
 return month;
 }

 /**
 * Gets the month-of-year field using the {@code Month} enum.
 * 
 * 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()
 */
 public Month getMonth() {
 return Month.of(month);
 }

 //-----------------------------------------------------------------------
 /**
 * Checks if the year is a leap year, according to the ISO proleptic
 * calendar system rules.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 public boolean isLeapYear() {
 return IsoChronology.INSTANCE.isLeapYear(year);
 }

 /**
 * Checks if the day-of-month is valid for this year-month.
 * 
 * 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
 */
 public boolean isValidDay(int dayOfMonth) {
 return dayOfMonth >= 1 && dayOfMonth <= lengthOfMonth();
 }

 /**
 * Returns the length of the month, taking account of the year.
 * 
 * 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
 */
 public int lengthOfMonth() {
 return getMonth().length(isLeapYear());
 }

 /**
 * Returns the length of the year.
 * 
 * This returns the length of the year in days, either 365 or 366.
 *
 * @return 366 if the year is leap, 365 otherwise
 */
 public int lengthOfYear() {
 return (isLeapYear() ? 366 : 365);
 }

 //-----------------------------------------------------------------------
 /**
 * Returns an adjusted copy of this year-month.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @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.
 * 
 * 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.
 * 
 * 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>
 * 
 * In all cases, if the new value is outside the valid range of values for the field
 * then a {@code DateTimeException} will be thrown.
 * 
 * All other {@code ChronoField} instances will throw an {@code UnsupportedTemporalTypeException}.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public YearMonth with(TemporalField field, long newValue) {
 if (field instanceof ChronoField chronoField) {
 chronoField.checkValidValue(newValue);
 return switch (chronoField) {
 case MONTH_OF_YEAR -> withMonth((int) newValue);
 case PROLEPTIC_MONTH -> plusMonths(newValue - getProlepticMonth());
 case YEAR_OF_ERA -> withYear((int) (year < 1 ? 1 - newValue : newValue));
 case YEAR -> withYear((int) newValue);
 case ERA -> (getLong(ERA) == newValue ? this : withYear(1 - year));
 default -> throw new UnsupportedTemporalTypeException("Unsupported field: " + field);
 };
 }
 return field.adjustInto(this, newValue);
 }

 //-----------------------------------------------------------------------
 /**
 * Returns a copy of this {@code YearMonth} with the year altered.
 * 
 * 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) {
 YEAR.checkValidValue(year);
 return with(year, month);
 }

 /**
 * Returns a copy of this {@code YearMonth} with the month-of-year altered.
 * 
 * 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) {
 MONTH_OF_YEAR.checkValidValue(month);
 return with(year, month);
 }

 //-----------------------------------------------------------------------
 /**
 * Returns a copy of this year-month with the specified amount added.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public YearMonth plus(TemporalAmount amountToAdd) {
 return (YearMonth) amountToAdd.addTo(this);
 }

 /**
 * Returns a copy of this year-month with the specified amount added.
 * 
 * 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.
 * 
 * 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>
 * 
 * All other {@code ChronoUnit} instances will throw an {@code UnsupportedTemporalTypeException}.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public YearMonth plus(long amountToAdd, TemporalUnit unit) {
 if (unit instanceof ChronoUnit chronoUnit) {
 return switch (chronoUnit) {
 case MONTHS -> plusMonths(amountToAdd);
 case YEARS -> plusYears(amountToAdd);
 case DECADES -> plusYears(Math.multiplyExact(amountToAdd, 10));
 case CENTURIES -> plusYears(Math.multiplyExact(amountToAdd, 100));
 case MILLENNIA -> plusYears(Math.multiplyExact(amountToAdd, 1000));
 case ERAS -> with(ERA, Math.addExact(getLong(ERA), amountToAdd));
 default -> 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.
 * 
 * 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
 */
 public YearMonth plusYears(long yearsToAdd) {
 if (yearsToAdd == 0) {
 return this;
 }
 int newYear = YEAR.checkValidIntValue(year + yearsToAdd); // safe overflow
 return with(newYear, month);
 }

 /**
 * Returns a copy of this {@code YearMonth} with the specified number of months added.
 * 
 * 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
 */
 public YearMonth plusMonths(long monthsToAdd) {
 if (monthsToAdd == 0) {
 return this;
 }
 long monthCount = year * 12L + (month - 1);
 long calcMonths = monthCount + monthsToAdd; // safe overflow
 int newYear = YEAR.checkValidIntValue(Math.floorDiv(calcMonths, 12));
 int newMonth = Math.floorMod(calcMonths, 12) + 1;
 return with(newYear, newMonth);
 }

 //-----------------------------------------------------------------------
 /**
 * Returns a copy of this year-month with the specified amount subtracted.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public YearMonth minus(TemporalAmount amountToSubtract) {
 return (YearMonth) amountToSubtract.subtractFrom(this);
 }

 /**
 * Returns a copy of this year-month with the specified amount subtracted.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public YearMonth minus(long amountToSubtract, TemporalUnit unit) {
 return (amountToSubtract == Long.MIN_VALUE ? plus(Long.MAX_VALUE, unit).plus(1, unit) : plus(-amountToSubtract, unit));
 }

 /**
 * Returns a copy of this {@code YearMonth} with the specified number of years subtracted.
 * 
 * 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
 */
 public YearMonth minusYears(long yearsToSubtract) {
 return (yearsToSubtract == Long.MIN_VALUE ? plusYears(Long.MAX_VALUE).plusYears(1) : plusYears(-yearsToSubtract));
 }

 /**
 * Returns a copy of this {@code YearMonth} with the specified number of months subtracted.
 * 
 * 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
 */
 public YearMonth minusMonths(long monthsToSubtract) {
 return (monthsToSubtract == Long.MIN_VALUE ? plusMonths(Long.MAX_VALUE).plusMonths(1) : plusMonths(-monthsToSubtract));
 }

 //-----------------------------------------------------------------------
 /**
 * Queries this year-month using the specified query.
 * 
 * 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.
 * 
 * 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)
 */
 @SuppressWarnings("unchecked")
 @Override
 public <R> R query(TemporalQuery<R> query) {
 if (query == TemporalQueries.chronology()) {
 return (R) IsoChronology.INSTANCE;
 } else if (query == TemporalQueries.precision()) {
 return (R) MONTHS;
 }
 return Temporal.super.query(query);
 }

 /**
 * Adjusts the specified temporal object to have this year-month.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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>
 * 
 * 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
 */
 @Override
 public Temporal adjustInto(Temporal temporal) {
 if (Chronology.from(temporal).equals(IsoChronology.INSTANCE) == false) {
 throw new DateTimeException("Adjustment only supported on ISO date-time");
 }
 return temporal.with(PROLEPTIC_MONTH, getProlepticMonth());
 }

 /**
 * Calculates the amount of time until another year-month in terms of the specified unit.
 * 
 * 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)}.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @Override
 public long until(Temporal endExclusive, TemporalUnit unit) {
 YearMonth end = YearMonth.from(endExclusive);
 if (unit instanceof ChronoUnit chronoUnit) {
 long monthsUntil = end.getProlepticMonth() - getProlepticMonth(); // no overflow
 return switch (chronoUnit) {
 case MONTHS -> monthsUntil;
 case YEARS -> monthsUntil / 12;
 case DECADES -> monthsUntil / 120;
 case CENTURIES -> monthsUntil / 1200;
 case MILLENNIA -> monthsUntil / 12000;
 case ERAS -> end.getLong(ERA) - getLong(ERA);
 default -> throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
 };
 }
 return unit.between(this, end);
 }

 /**
 * Formats this year-month using the specified formatter.
 * 
 * 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);
 }

 //-----------------------------------------------------------------------
 /**
 * Combines this year-month with a day-of-month to create a {@code LocalDate}.
 * 
 * This returns a {@code LocalDate} formed from this year-month and the specified day-of-month.
 * 
 * The day-of-month value must be valid for the year-month.
 * 
 * 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)
 */
 public LocalDate atDay(int dayOfMonth) {
 return LocalDate.of(year, month, dayOfMonth);
 }

 /**
 * Returns a {@code LocalDate} at the end of the month.
 * 
 * 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.
 * 
 * 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
 */
 public LocalDate atEndOfMonth() {
 return LocalDate.of(year, month, lengthOfMonth());
 }

 //-----------------------------------------------------------------------
 /**
 * Compares this year-month to another year-month.
 * 
 * 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;
 }

 /**
 * 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
 */
 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
 */
 public boolean isBefore(YearMonth other) {
 return compareTo(other) < 0;
 }

 //-----------------------------------------------------------------------
 /**
 * Checks if this year-month is equal to another year-month.
 * 
 * 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;
 }
 return (obj instanceof YearMonth other)
 && year == other.year
 && month == other.month;
 }

 /**
 * A hash code for this year-month.
 *
 * @return a suitable hash code
 */
 @Override
 public int hashCode() {
 return year ^ (month << 27);
 }

 //-----------------------------------------------------------------------
 /**
 * Outputs this year-month as a {@code String}, such as {@code 2007-12}.
 * 
 * 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();
 }

 //-----------------------------------------------------------------------
 /**
 * Writes the object using a
 * <a href="{@docRoot}/serialized-form.html#java.time.Ser">dedicated serialized form</a>.
 * @serialData
 * <pre>
 * out.writeByte(12); // identifies a YearMonth
 * out.writeInt(year);
 * out.writeByte(month);
 * </pre>
 *
 * @return the instance of {@code Ser}, not null
 */
 @java.io.Serial
 private Object writeReplace() {
 return new Ser(Ser.YEAR_MONTH_TYPE, this);
 }

 /**
 * Defend against malicious streams.
 *
 * @param s the stream to read
 * @throws InvalidObjectException always
 */
 @java.io.Serial
 private void readObject(ObjectInputStream s) throws InvalidObjectException {
 throw new InvalidObjectException("Deserialization via serialization delegate");
 }

 void writeExternal(DataOutput out) throws IOException {
 out.writeInt(year);
 out.writeByte(month);
 }

 static YearMonth readExternal(DataInput in) throws IOException {
 int year = in.readInt();
 byte month = in.readByte();
 return YearMonth.of(year, month);
 }

}

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