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

@param query the selection criteria using query operators.
@return A document that satisfies the query specified as the argument to this method.
@mongodb.driver.manual tutorial/query-documents/ Querying