@Test public void toMutableMap() { int max = 5; List<List<Integer>> l = List.range(1, max + 1).map(n -> List.single(n)); TreeMap<List<Integer>, String> m2 = TreeMap.treeMap(Ord.listOrd(Ord.intOrd), l.zip(l.map(i -> i.toString()))); Map<List<Integer>, String> mm = m2.toMutableMap(); assertEquals(m2.keys(), List.fromIterable(mm.keySet())); }