Sorting a Map by key (date,string)

Question

Map<XMLGregorianCalendar, String> SortedByTimeForJourney = new HashMap<XMLGregorianCalendar, String>();

I have to sort it by key(XMLGregorianCalendar). I tried this

 SortedByTimeForJourney.entrySet().stream().sorted(Map.Entry.<XMLGregorianCalendar, String>comparingByKey()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e2, LinkedHashMap::new));

but it is showing Type Parameter 'javax.xml.datatype.XMLGregorianCalendar' is not withing its bound; should implement 'java.lang.Comparable'

I also tried to do by saving it in LinkedHashMap and also using Collections.util method but unable to do.

Someone Please Help me.

Answer 1

To sort a Map by key, use a TreeMap.

As the javadoc says:

The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.

Since XMLGregorianCalendar doesn't implement Comparable, i.e. doesn't have a natural ordering, you have to specify a Comparator in the constructor.

In Java 8+, use:

new TreeMap<>(XMLGregorianCalendar::compare)

In older versions of Java, use:

new TreeMap<>(new Comparator<XMLGregorianCalendar>() {
    @Override
    public int compare(XMLGregorianCalendar cal1, XMLGregorianCalendar cal2) {
        return cal1.compare(cal2);
    }
})