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Converting List to TreeMap java lambda

I’m trying to convert a List<String> to a TreeMap<Long, CustomClass> The key is the same as the list items but just parsed to Long, the value is just a call to new CustomClass(). How can I achieve this using lambda functions?

List<String> list = List.of("1","2","3");
TreeMap<Long, CustomClass> resultMap =
                                           .collect(Collectors.toMap(i -> Long.parseLong(i), v -> new CustomClass()));

the above code errors out, with the following error,

Required type:    TreeMap<Long,CustomClass>
Provided:         Map<Object,Object>
no instance(s) of type variable(s) K, U exist so that Map<K, U> conforms to TreeMap<Long, CustomClass>



For example:

final Map<Long, CustomClass> result =, ignore -> new CustomClass(), (x, y) -> y, TreeMap::new));