Question

I want to copy my elements from a List<Set<String>> into a SortedMap<Set<String>,Integer>,
but I always get:

java.lang.ClassCastException: java.util.HashSet cannot be cast to java.lang.Comparable. (or HashMap could be a TreeSet too, vice versa)

Some places I’ve been reading say it isn’t possible, but is this correct?

I can’t believe I can’t copy a original List or Set into a Map.

This is what I have tried:

List<Set<String> > tempnewOut= new ArrayList<>(); 
SortedMap<Set<String>,Integer> freqSetsWithCount= new TreeMap<>(); 
for (Set<String> set : tempnewOut) 
{ 
    freqSetsWithCount.put(set, 0); 
}

Answer 1

The class that you use as a key in a TreeMap (one of the implementations of interface SortedMap) must either implement interface Comparable, or you must create the TreeMap by providing a Comparator to the constructor.

You’re trying to use a HashSet<String> for the keys. HashSet doesn’t implement Comparable, and you’re not supplying a Comparator, so you get a ClassCastException.

One solution is to create the TreeMap by passing it a Comparator to the constructor. You’ll have to implement the compare method of the Comparator to specify how the sets should be sorted in the map.

List<Set<String>> list = new ArrayList<>();

Comparator<Set<String>> comparator = new Comparator<Set<String>>() {
    @Override
    public int compare(Set<String> o1, Set<String> o2) {
        // TODO: Implement your logic to compare the sets
    }
};

SortedMap<Set<String>, Integer> set = new TreeMap<>(comparator);

// TODO: Fill the set