item.categories = new String[] { "aaa", "bbb", "ccc" };
SolrInputDocument out = new SolrInputDocument();
converter.write(item, out);
Assert.assertEquals(item.id, out.getFieldValue("id"));
SolrInputField catfield = out.getField("cat");
Assert.assertEquals(3, catfield.getValueCount());
List<String> catValues = (List<String>) catfield.getValue();
Assert.assertEquals("aaa", catValues.get(0));
Assert.assertEquals("bbb", catValues.get(1));
Assert.assertEquals("ccc", catValues.get(2));
}