driver-legacy/src/main/com/mongodb/DBCollection.java
@Nullable
    public DBObject findOne(final DBObject query, final DBObject projection, final DBObject sort) {
        return findOne(query, projection, sort, getReadPreference());
    }
@Nullable
    public DBObject findOne(final DBObject query, final DBObject projection, final DBObject sort) {
        return findOne(query, projection, sort, getReadPreference());
    }
Get a single document from collection.

@param query      the selection criteria using query operators.
@param projection specifies which fields MongoDB will return from the documents in the result set.
@param sort       A document whose fields specify the attributes on which to sort the result set.
@return A document that satisfies the query specified as the argument to this method.
@mongodb.driver.manual tutorial/query-documents/ Querying