import java.util.*; class MyTest { { Set<Integer> set = new TreeSet<>(Comparator.comparing(b -> "")); Set<Integer> set1 = new TreeSet<>(Comparator.comparing(b -> b != null ? "" : "")); } void f(List<String> l) { setCategories(l.stream().toArray(size -> new String[size])); setCategories(l.stream().toArray(String[]::new)); } private void setCategories(String... strings) {} }