Subset.java

public class Subset {

    @SafeVarargs
    public static <T> List<List<T>> subsets(T... items) {
        List<T> elements = new ArrayList<>(new HashSet<>(Arrays.asList(items)));
        List<List<T>> results = new ArrayList<>();
        for (T e : elements) {
            // Record size as the list will change
            int size = results.size();
            for (int i = 0; i < size; i++) {
                List<T> subset = new ArrayList<>(results.get(i));
                subset.add(e);
                results.add(subset);
            }
            results.add(Collections.singletonList(e));
        }
        return results;
    }

    public static void main(String[] args) {
        List<List<String>> results = subsets("a", "b", "c");
        for (List<String> result : results) {
            System.out.println(result);
        }
    }
}