}

  public void testMapSplitter_emptySeparator() {
    assertThrows(IllegalArgumentException.class, () -> COMMA_SPLITTER.withKeyValueSeparator(""));
  }

  public void testMapSplitter_malformedEntry() {
    assertThrows(
        IllegalArgumentException.class,
        () -> COMMA_SPLITTER.withKeyValueSeparator("=").split("a=1,b,c=2"));
  }

  /**

    Joiner j = Joiner.on("x").useForNull("y");
    assertThrows(UnsupportedOperationException.class, () -> j.useForNull("y"));
  }

  public void testMap() {
    MapJoiner j = Joiner.on(';').withKeyValueSeparator(':');
    assertEquals("", j.join(ImmutableMap.of()));
    assertEquals(":", j.join(ImmutableMap.of("", "")));

    Map<@Nullable String, @Nullable String> mapWithNulls = new LinkedHashMap<>();