This Java library can convert JTS geometries to GeoJSON and back. Its API is similar to other io.* classes in JTS.
GeoJSONWriter writer = new GeoJSONWriter();
GeoJSON json = writer.write(geometry);
String jsonstring = json.toString();
GeoJSONReader reader = new GeoJSONReader();
Geometry geometry = reader.read(json);
JTS does not have anything like GeoJSON Feature or FeatureCollection but they can be parsed by this library. Example:
// parse Feature or FeatureCollection
Feature feature = (Feature) GeoJSONFactory.create(json);
FeatureCollection featureCollection = (FeatureCollection) GeoJSONFactory.create(json);
// parse Geometry from Feature
GeoJSONReader reader = new GeoJSONReader();
Geometry geometry = reader.read(feature.getGeometry());
geometry = reader.read(featureCollection.getFeatures()[0].getGeometry());
// create and serialize a FeatureCollection
List<Features> features = new ArrayList<Features>();
Map<String, Object> properties = new HashMap<String, Object>();
features.add(new Feature(geometry, properties);
GeoJSONWriter writer = new GeoJSONWriter();
GeoJSON json = writer.write(features);