How to sort collection with null's and invert the list afterwards?

Question

So I'm working with a list of Dates and some of the values are "" ie null. I used the answer from How to handle nulls when using Java collection sort

public int compare(MyBean o1, MyBean o2) {
    if (o1.getDate() == null) {
        return (o2.getDate() == null) ? 0 : -1;
    }
    if (o2.getDate() == null) {
        return 1;
    }
    return o2.getDate().compareTo(o1.getDate());
} 

to sort the list in both ascending which puts the nulls first.

What I want is in ascending order to have nulls first and then the values afterwards in ascending order like the above code does. Then when descending is selected to literally flip the list. IE first values in list are in descending order then all the nulls.

I've attempted the following after I sorted the list in ascending order Collections.reverseOrder(); This kept the nulls first and then sorted the dates in descending order.

I also tried Collections.reverse(List). This put the nulls at the end of the list but kept the Dates in ascending order.

Answer 1

In Java 8 this whole thing can be

Collections.sort(list, 
     Comparator.comparing(MyBean::getDate, 
        Comparator.nullsFirst(Comparator.naturalOrder()))
     .reversed());

Answer 2

You can achieve this by a simple comparator. Modify this as per your custom bean object. Something like this -

public class DateComparator implements Comparator<Date> {

    private boolean reverse;

    public DateComparator(boolean reverse) {
        this.reverse = reverse;
    }

    public int compare(Date o1, Date o2) {
        if (o1 == null || o2 == null) {
            return o2 != null ? (reverse ? 1 : -1) : (o1 != null ? (reverse ? -1 : 1) : 0);
        }
        int result = o1.compareTo(o2);
        return reverse ? result * -1 : result;
    }

    public static void main(String[] args) throws ParseException {
        SimpleDateFormat dateFormat = new SimpleDateFormat("dd-MM-yyyy");
        Date[] dates = new Date[]{null, dateFormat.parse("10-10-2013"), null, dateFormat.parse("10-10-2012"), dateFormat.parse("10-10-2015"), dateFormat.parse("10-10-2011"), null};
        List<Date> list = Arrays.asList(dates);
        Collections.sort(list, new DateComparator(false));
        System.out.println(list);
        Collections.sort(list, new DateComparator(true));
        System.out.println(list);
    }
}