you need todo a query, not a get,
get ist just a single instance by id, query is result of your query.
just do:
// Use class Query to assemble a query
Query q = new Query("Post");
// Use PreparedQuery interface to retrieve results
PreparedQuery pq = datastore.prepare(q);
for (Entity result : pq.asIterable()) {
String postName = (String) result.getProperty("PostName");
}