+ * {@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 will query the {@link java.time.Clock#systemDefaultZone() system clock} in the default + * time-zone to obtain the current year-month. + * The zone and offset will be set based on the time-zone in the clock. + *
+ * 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(java.time.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. + *
+ * A {@code TemporalAccessor} represents some form of date and time information. + * This factory converts the arbitrary temporal object 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 in queries 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; + } + try { + if (ISOChrono.INSTANCE.equals(Chrono.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.getClass(), 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 yyyy-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} and + * {@link #get(TemporalField) get} methods will throw an exception. + *
+ * 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 date-time. + * The supported fields are: + *
+ * If the field is not a {@code ChronoField}, then the result of this method + * is obtained by invoking {@code TemporalField.doIsSupported(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 == EPOCH_MONTH || field == YEAR_OF_ERA || field == ERA; + } + return field != null && field.doIsSupported(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 a {@code DateTimeException}. + *
+ * If the field is not a {@code ChronoField}, then the result of this method + * is obtained by invoking {@code TemporalField.doRange(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 + */ + @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 for 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 EPOCH_MONTH} which is too + * large to fit in an {@code int} and throw a {@code DateTimeException}. + * All other {@code ChronoField} instances will throw a {@code DateTimeException}. + *
+ * If the field is not a {@code ChronoField}, then the result of this method + * is obtained by invoking {@code TemporalField.doGet(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 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 for 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 a {@code DateTimeException}. + *
+ * If the field is not a {@code ChronoField}, then the result of this method + * is obtained by invoking {@code TemporalField.doGet(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 ArithmeticException if numeric overflow occurs + */ + @Override + public long getLong(TemporalField field) { + if (field instanceof ChronoField) { + switch ((ChronoField) field) { + case MONTH_OF_YEAR: return month; + case EPOCH_MONTH: return getEpochMonth(); + case YEAR_OF_ERA: return (year < 1 ? 1 - year : year); + case YEAR: return year; + case ERA: return (year < 1 ? 0 : 1); + } + throw new DateTimeException("Unsupported field: " + field.getName()); + } + return field.doGet(this); + } + + private long getEpochMonth() { + return ((year - 1970) * 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 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 + */ + 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 ISOChrono.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 new {@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 new {@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: + *
+ * 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 a {@code DateTimeException}. + *
+ * If the field is not a {@code ChronoField}, then the result of this method + * is obtained by invoking {@code TemporalField.doWith(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 ArithmeticException if numeric overflow occurs + */ + @Override + public YearMonth with(TemporalField field, long newValue) { + if (field instanceof ChronoField) { + ChronoField f = (ChronoField) field; + f.checkValidValue(newValue); + switch (f) { + case MONTH_OF_YEAR: return withMonth((int) newValue); + case EPOCH_MONTH: return plusMonths(newValue - getLong(EPOCH_MONTH)); + case YEAR_OF_ERA: return withYear((int) (year < 1 ? 1 - newValue : newValue)); + case YEAR: return withYear((int) newValue); + case ERA: return (getLong(ERA) == newValue ? this : withYear(1 - year)); + } + throw new DateTimeException("Unsupported field: " + field.getName()); + } + return field.doWith(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 period added. + *
+ * This method returns a new year-month based on this year-month with the specified period added. + * The adder is typically {@link java.time.Period} but may be any other type implementing + * the {@link TemporalAdder} interface. + * The calculation is delegated to the specified adjuster, which typically calls + * back to {@link #plus(long, TemporalUnit)}. + *
+ * This instance is immutable and unaffected by this method call. + * + * @param adder the adder to use, 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(TemporalAdder adder) { + return (YearMonth) adder.addTo(this); + } + + /** + * {@inheritDoc} + * @throws DateTimeException {@inheritDoc} + * @throws ArithmeticException {@inheritDoc} + */ + @Override + public YearMonth plus(long amountToAdd, TemporalUnit unit) { + if (unit instanceof ChronoUnit) { + switch ((ChronoUnit) unit) { + case MONTHS: return plusMonths(amountToAdd); + case YEARS: return plusYears(amountToAdd); + case DECADES: return plusYears(Math.multiplyExact(amountToAdd, 10)); + case CENTURIES: return plusYears(Math.multiplyExact(amountToAdd, 100)); + case MILLENNIA: return plusYears(Math.multiplyExact(amountToAdd, 1000)); + case ERAS: return with(ERA, Math.addExact(getLong(ERA), amountToAdd)); + } + throw new DateTimeException("Unsupported unit: " + unit.getName()); + } + return unit.doPlus(this, amountToAdd); + } + + /** + * Returns a copy of this year-month with the specified period in 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 year-month with the specified period in 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 = (int)Math.floorMod(calcMonths, 12) + 1; + return with(newYear, newMonth); + } + + //----------------------------------------------------------------------- + /** + * Returns a copy of this year-month with the specified period subtracted. + *
+ * This method returns a new year-month based on this year-month with the specified period subtracted. + * The subtractor is typically {@link java.time.Period} but may be any other type implementing + * the {@link TemporalSubtractor} interface. + * The calculation is delegated to the specified adjuster, which typically calls + * back to {@link #minus(long, TemporalUnit)}. + *
+ * This instance is immutable and unaffected by this method call. + * + * @param subtractor the subtractor to use, 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(TemporalSubtractor subtractor) { + return (YearMonth) subtractor.subtractFrom(this); + } + + /** + * {@inheritDoc} + * @throws DateTimeException {@inheritDoc} + * @throws ArithmeticException {@inheritDoc} + */ + @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 year-month with the specified period in 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 year-month with the specified period in 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
+ * 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#EPOCH_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)}:
+ *
+ * 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 (Chrono.from(temporal).equals(ISOChrono.INSTANCE) == false) {
+ throw new DateTimeException("Adjustment only supported on ISO date-time");
+ }
+ return temporal.with(EPOCH_MONTH, getEpochMonth());
+ }
+
+ /**
+ * Calculates the period between this year-month and another year-month in
+ * terms of the specified unit.
+ *
+ * This calculates the period between two year-months in terms of a single unit.
+ * 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 must be a {@code YearMonth}.
+ * For example, the period in years between two year-months can be calculated
+ * using {@code startYearMonth.periodUntil(endYearMonth, YEARS)}.
+ *
+ * The calculation returns a whole number, representing the number of
+ * complete units between the two year-months.
+ * For example, the period in decades between 2012-06 and 2032-05
+ * will only be one decade as it is one month short of two decades.
+ *
+ * This method operates in association with {@link TemporalUnit#between}.
+ * The result of this method is a {@code long} representing the amount of
+ * the specified unit. By contrast, the result of {@code between} is an
+ * object that can be used directly in addition/subtraction:
+ *
+ * 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 input temporal as
+ * the second argument.
+ *
+ * This instance is immutable and unaffected by this method call.
+ *
+ * @param endYearMonth the end year-month, which must be a {@code YearMonth}, not null
+ * @param unit the unit to measure the period in, not null
+ * @return the amount of the period between this year-month and the end year-month
+ * @throws DateTimeException if the period cannot be calculated
+ * @throws ArithmeticException if numeric overflow occurs
+ */
+ @Override
+ public long periodUntil(Temporal endYearMonth, TemporalUnit unit) {
+ if (endYearMonth instanceof YearMonth == false) {
+ Objects.requireNonNull(endYearMonth, "endYearMonth");
+ throw new DateTimeException("Unable to calculate period between objects of two different types");
+ }
+ YearMonth end = (YearMonth) endYearMonth;
+ if (unit instanceof ChronoUnit) {
+ long monthsUntil = end.getEpochMonth() - getEpochMonth(); // no overflow
+ switch ((ChronoUnit) unit) {
+ case MONTHS: return monthsUntil;
+ case YEARS: return monthsUntil / 12;
+ case DECADES: return monthsUntil / 120;
+ case CENTURIES: return monthsUntil / 1200;
+ case MILLENNIA: return monthsUntil / 12000;
+ case ERAS: return end.getLong(ERA) - getLong(ERA);
+ }
+ throw new DateTimeException("Unsupported unit: " + unit.getName());
+ }
+ return unit.between(this, endYearMonth).getAmount();
+ }
+
+ //-----------------------------------------------------------------------
+ /**
+ * Returns a date formed from this year-month at the specified day-of-month.
+ *
+ * This combines this year-month and the specified day-of-month to form a {@code LocalDate}.
+ * 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:
+ *
+ * This instance is immutable and unaffected by this method call.
+ *
+ * @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 when the day is invalid for the year-month
+ * @see #isValidDay(int)
+ */
+ public LocalDate atDay(int dayOfMonth) {
+ return LocalDate.of(year, month, dayOfMonth);
+ }
+
+ //-----------------------------------------------------------------------
+ /**
+ * 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;
+ }
+
+ /**
+ * Is this year-month 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;
+ }
+
+ /**
+ * Is this year-month 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;
+ }
+ if (obj instanceof YearMonth) {
+ YearMonth other = (YearMonth) obj;
+ return year == other.year && month == other.month;
+ }
+ return false;
+ }
+
+ /**
+ * A hash code for this year-month.
+ *
+ * @return a suitable hash code
+ */
+ @Override
+ public int hashCode() {
+ return year ^ (month << 27);
+ }
+
+ //-----------------------------------------------------------------------
+ /**
+ * Outputs this year-month as a {@code String}, such as {@code 2007-12}.
+ *
+ * The output will be in the format {@code yyyy-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();
+ }
+
+ /**
+ * Outputs this year-month as a {@code String} using the formatter.
+ *
+ * This year-month will be passed to the formatter
+ * {@link DateTimeFormatter#print(TemporalAccessor) print method}.
+ *
+ * @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 toString(DateTimeFormatter formatter) {
+ Objects.requireNonNull(formatter, "formatter");
+ return formatter.print(this);
+ }
+
+ //-----------------------------------------------------------------------
+ /**
+ * Writes the object using a
+ * dedicated serialized form.
+ *
+ * // these two lines are equivalent, but the second approach is recommended
+ * temporal = thisYearMonth.adjustInto(temporal);
+ * temporal = temporal.with(thisYearMonth);
+ *
+ *
+ * long period = start.periodUntil(end, YEARS); // this method
+ * dateTime.plus(YEARS.between(start, end)); // use in plus/minus
+ *
+ *
+ * LocalDate date = year.atMonth(month).atDay(day);
+ *
+ *
+ * out.writeByte(5); // identifies this as a Year
+ * out.writeInt(year);
+ * out.writeByte(month);
+ *
+ *
+ * @return the instance of {@code Ser}, not null
+ */
+ private Object writeReplace() {
+ return new Ser(Ser.YEAR_MONTH_TYPE, this);
+ }
+
+ /**
+ * Defend against malicious streams.
+ * @return never
+ * @throws InvalidObjectException always
+ */
+ private Object readResolve() throws ObjectStreamException {
+ 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);
+ }
+
+}