How to convert java.util.Date to Java8 java.time.YearMonth

Question

How can I best convert a java.util.Date to a Java 8 java.time.YearMonth?

Unfortunately the following throws a DateTimeException:

Answer 1

Short answer:

// From Date to YearMonth
YearMonth yearMonth =
        YearMonth.from(date.toInstant()
                           .atZone(ZoneId.systemDefault())
                           .toLocalDate());

// From YearMonth to Date
// The same as the OP:s answer
final Date convertedFromYearMonth = 
        Date.from(yearMonth.atDay(1).atStartOfDay(ZoneId.systemDefault()).toInstant());

Explanation:

The JavaDoc of the YearMonth.from(TemporalAccessor)-method says:

The conversion extracts the YEAR and MONTH_OF_YEAR fields. The extraction is only permitted if the temporal object has an ISO chronology, or can be converted to a LocalDate.

So, you need to either be able to:

1. extract the YEAR and MONTH_OF_YEAR fields, or
2. you should use something that can be converted to a LocalDate.

Lets try it!

final Date date = new Date();
final Instant instant = date.toInstant();
instant.get(ChronoField.YEAR); // causes an error

This is not possible, an exception is thrown:

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: Year at java.time.Instant.get(Instant.java:571) ...

This means that alternative 1 goes out the window. The reason for is explained in this excellent answer about how to convert Date to LocalDate.

Despite its name, java.util.Date represents an instant on the time-line, not a "date". The actual data stored within the object is a long count of milliseconds since 1970-01-01T00:00Z (midnight at the start of 1970 GMT/UTC).

The equivalent class to java.util.Date in JSR-310 is Instant, thus there is a convenient method toInstant() to provide the conversion.

So, a Date can be converted to an Instant but that did not help us, did it?

Alternative 2 however proves to be successful. Convert the Instant to a LocalDate and then use the YearMonth.from(TemporalAccessor)-method.

    Date date = new Date();

    LocalDate localDate = date.toInstant()
                              .atZone(ZoneId.systemDefault())
                              .toLocalDate();

    YearMonth yearMonth = YearMonth.from(localDate);
    System.out.println("YearMonth: " + yearMonth);

The output is (since the code was executed in January 2015 ;):

YearMonth: 2015-01