Don't be confused between a StringField and TextField. Although both the fields contain textual data, there are major differences between these two fields. A StringField is not tokenized and it's a good tool for exact match and sorting. A TextField is tokenized and it's useful for storing any unstructured text for indexing. When you pass the text into an Analyzer for indexing, a TextField is what's used to store the text content.
Similar to the way in which a StringField is set, adding a TextField is also very straightforward. Let's review how it's done:
Document document = new Document();
String text = "Lucene is an Information Retrieval library written in Java.";
doc.add(new TextField("text", text, Field.Store.YES));
indexWriter.addDocument(document);
indexWriter.commit();