MoonPhaseCalc

MoonPhaseCalc.java

/**
 * Moon Phase Calculator
 * 
 * @author Zilvinas Vilutis
 */
public class MoonPhaseCalc {

    public final static String format = "yyyy-MM-dd";

    private static final int moonPhaseHours = 709;

    private final Date start;

    public MoonPhaseCalc() throws ParseException {
        this( null );
    }

    public MoonPhaseCalc( Date start ) throws ParseException {
        if ( start != null ) {
            this.start = start;
        }
        else {
            this.start = new SimpleDateFormat( format ).parse( "2014-06-13" );
        }
    }

    /**
     * Finds the next full moon phase date matching the given day of month and
     * day of week. Used to find the next Friday, 13th with Full Moon :)
     * 
     * @param dayOfMonth Day Of Month to match ( e.g. 13 ) - see {@link Calendar#DAY_OF_MONTH}
     * @param dayOfWeek Day of Week to match ( 6 for Friday ) - see {@link Calendar#DAY_OF_WEEK}
     * @return
     */
    public Date getNextFullMoon( int dayOfMonth, int dayOfWeek ) {

        final Calendar cal = Calendar.getInstance();

        // validate input
        if ( cal.getMinimum( Calendar.DAY_OF_MONTH ) > dayOfMonth || cal.getMaximum( Calendar.DAY_OF_MONTH ) < dayOfMonth ) {
            throw new IllegalArgumentException( "Invalid day of month provided!" );
        }
        if ( cal.getMinimum( Calendar.DAY_OF_WEEK ) > dayOfWeek || cal.getMaximum( Calendar.DAY_OF_WEEK ) < dayOfWeek ) {
            throw new IllegalArgumentException( "Invalid day of week provided!" );
        }

        cal.setTime( start );

        while ( true ) {
            cal.add( Calendar.HOUR, moonPhaseHours );
            if ( cal.get( Calendar.DAY_OF_MONTH ) == dayOfMonth && cal.get( Calendar.DAY_OF_WEEK ) == dayOfWeek ) {
                return cal.getTime();
            }
        }
    }

    public static void main( String[] args ) throws ParseException {
        final MoonPhaseCalc moonPhaseCalc = new MoonPhaseCalc();
        final Date nextFullMoon = moonPhaseCalc.getNextFullMoon( 13, Calendar.FRIDAY );
        final String nexFullMoonOnFriday = new SimpleDateFormat( format ).format( nextFullMoon );
        System.out.println( nexFullMoonOnFriday );
    }

}