java.time
Using java.time library built into Java 8 and later. Specifically the LocalDate and TemporalAdjusters classes.
import java.time.LocalDate
import static java.time.temporal.TemporalAdjusters.firstDayOfYear
import static java.time.temporal.TemporalAdjusters.lastDayOfYear
LocalDate now = LocalDate.now(); // 2015-11-23
LocalDate firstDay = now.with(firstDayOfYear()); // 2015-01-01
LocalDate lastDay = now.with(lastDayOfYear()); // 2015-12-31
If you need to add time information, you may use any available LocalDate to LocalDateTime conversion like
lastDay.atStartOfDay(); // 2015-12-31T00:00