driver-legacy/src/main/com/mongodb/DBCollection.java
@Nullable
    public DBObject findOne(final DBObject query, final DBObject projection, final ReadPreference readPreference) {
        return findOne(query, projection, null, readPreference);
    }
@Nullable
    public DBObject findOne(final DBObject query, final DBObject projection, final ReadPreference readPreference) {
        return findOne(query, projection, null, readPreference);
    }
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 readPreference {@link ReadPreference} to be used for this operation
@return A document that satisfies the query specified as the argument to this method.
@mongodb.driver.manual tutorial/query-documents/ Querying