allKeysFromListOfMaps.java

  List<Map<String, Object>> listOfMaps = Lists.newArrayList();
        listOfMaps.add(ImmutableMap.of("a",1));
        listOfMaps.add(ImmutableMap.of("b",1));
        listOfMaps.add(ImmutableMap.of("b",1));
        List<String> allKeys = listOfMaps.stream()
                .flatMap(map -> map.keySet().stream())
                .collect(Collectors.toList());
        assertThat(allKeys, Matchers.containsInAnyOrder("a","b","b"));