driver-core/src/main/com/mongodb/operation/MapReduceToCollectionOperation.java
@Override
    public MapReduceStatistics execute(final WriteBinding binding) {
        return withConnection(binding, new OperationHelper.CallableWithConnection<MapReduceStatistics>() {
            @Override
            public MapReduceStatistics call(final Connection connection) {
                validateCollation(connection, collation);
                return executeCommand(binding, namespace.getDatabaseName(), getCommand(connection.getDescription()),
                        connection, transformer());
            }
        });
    }
@Override
    public MapReduceStatistics execute(final WriteBinding binding) {
        return withConnection(binding, new OperationHelper.CallableWithConnection<MapReduceStatistics>() {
            @Override
            public MapReduceStatistics call(final Connection connection) {
                validateCollation(connection, collation);
                return executeCommand(binding, namespace.getDatabaseName(), getCommand(connection.getDescription()),
                        connection, transformer());
            }
        });
    }
Executing this will return a cursor with your results in.

@param binding the binding
@return a MongoCursor that can be iterated over to find all the results of the Map Reduce operation.