### Question:
IncrementalRelativeFileSets { @NonNull public static ImmutableMap<RelativeFile, FileStatus> union( @NonNull Iterable<ImmutableMap<RelativeFile, FileStatus>> sets) { Map<RelativeFile, FileStatus> map = Maps.newHashMap(); for (ImmutableMap<RelativeFile, FileStatus> set : sets) { map.putAll(set); } return ImmutableMap.copyOf(map); } private  IncrementalRelativeFileSets(); @NonNull static ImmutableMap<RelativeFile, FileStatus> fromDirectory(@NonNull File directory); @NonNull static ImmutableMap<RelativeFile, FileStatus> fromZip(@NonNull File zip); @NonNull static ImmutableMap<RelativeFile, FileStatus> fromZip(
            @NonNull File zip,
            FileStatus status); @NonNull static ImmutableMap<RelativeFile, FileStatus> fromZip(
            @NonNull File zip,
            @NonNull FileCacheByPath cache,
            @NonNull Set<Runnable> cacheUpdates); @NonNull static ImmutableMap<RelativeFile, FileStatus> union(
            @NonNull Iterable<ImmutableMap<RelativeFile, FileStatus>> sets); static int getBaseDirectoryCount(@NonNull ImmutableMap<RelativeFile, FileStatus> set); @NonNull static ImmutableMap<RelativeFile, FileStatus> fromZipsAndDirectories(
            @NonNull Iterable<File> files); @NonNull static ImmutableMap<RelativeFile, FileStatus> makeFromBaseFiles(
            @NonNull Collection<File> baseFiles,
            @NonNull Map<File, FileStatus> updates,
            @NonNull FileCacheByPath cache,
            @NonNull Set<Runnable> cacheUpdates,
            @NonNull FileDeletionPolicy fileDeletionPolicy);  }### Answer:
@Test public void unionOfEmptySet() throws Exception { ImmutableMap<RelativeFile, FileStatus> set = IncrementalRelativeFileSets.union(new HashSet<>()); assertEquals(0, set.size()); }