*/
    String getFileExtension();

    /**
     * Returns the default index file name for this format (e.g. "index.html", "index.json").
     *
     * @return the index file name
     */
    String getIndexFileName();

    /**
     * Formats a document source map into the target format string.
     *
     * @param source the document source map
     * @param excludeFields the set of field names to exclude from output

    public HtmlIndexExportFormatter() {
        // default constructor
    }

    @Override
    public String getFileExtension() {
        return ".html";
    }

    @Override
    public String getIndexFileName() {
        return "index.html";
    }

    @Override
    public String format(final Map<String, Object> source, final Set<String> excludeFields) {

    public JsonIndexExportFormatter() {
        // default constructor
    }

    @Override
    public String getFileExtension() {
        return ".json";
    }

    @Override
    public String getIndexFileName() {
        return "index.json";
    }

    @Override
    public String format(final Map<String, Object> source, final Set<String> excludeFields) {
        final StringBuilder json = new StringBuilder();

        final JsonIndexExportFormatter formatter = new JsonIndexExportFormatter();
        assertEquals(".json", formatter.getFileExtension());
        assertEquals("index.json", formatter.getIndexFileName());
    }

    @Test
    public void test_jsonFormatter_emptyCollection() {
        final Map<String, Object> source = new LinkedHashMap<>();
        source.put("title", "Test");