driver-legacy/src/main/com/mongodb/DBCollection.java
@Nullable
    public DBObject findOne(final Object id, final DBObject projection) {
        return findOne(new BasicDBObject("_id", id), new DBCollectionFindOptions().projection(projection));
    }
@Nullable
    public DBObject findOne(final Object id, final DBObject projection) {
        return findOne(new BasicDBObject("_id", id), new DBCollectionFindOptions().projection(projection));
    }
Get a single document from collection by '_id'.

@param id         value of '_id' field of a document we are looking for
@param projection specifies which projection MongoDB will return from the documents in the result set.
@return A document that satisfies the query specified as the argument to this method.
@mongodb.driver.manual tutorial/query-documents/ Querying