import org.apache.commons.math3.linear.BlockRealMatrix;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class StreamAdder {

    public static void main(String[] args) {

        List<BlockRealMatrix> matrixHolder = new ArrayList<>();

        matrixHolder.add(new BlockRealMatrix(2, 3));
        matrixHolder.add(new BlockRealMatrix(2, 3));

        matrixHolder.get(0).setRow(0, new double[]{1, 2, 3});
        matrixHolder.get(0).setRow(1, new double[]{4, 5, 6});
        matrixHolder.get(1).setRow(0, new double[]{7, 8, 9});
        matrixHolder.get(1).setRow(1, new double[]{10, 11, 12});

        matrixHolder.stream().forEach(i -> System.out.println(i));

        BlockRealMatrix averages = new BlockRealMatrix(matrixHolder.get(0).getRowDimension(), 1);

        for (int i = 0; i < averages.getRowDimension(); i++) {
            double average = 0;
            for (int j = 0; j < matrixHolder.size(); j++) {
                average += Arrays.stream(matrixHolder.get(j).getRow(i)).sum();
            }
            average /= (matrixHolder.size() * matrixHolder.get(0).getColumnDimension());
            averages.setEntry(i, 0, average);
            average = 0;
        }

        System.out.println("Averages n" + averages);

    }
}

BlockRealMatrix{{1.0,2.0,3.0},{4.0,5.0,6.0}}
BlockRealMatrix{{7.0,8.0,9.0},{10.0,11.0,12.0}}
Averages
BlockRealMatrix{{5.0},{8.0}}