[BAEL-2996] Apache Commons Math3 example

libraries-apache-commons/src/main/java/com/baeldung/commons/math3/MatrixMultiplication.java

@@ -0,0 +1,6 @@
+package com.baeldung.commons.math3;
+
+public class MatrixMultiplication {
+
+
+}

libraries-apache-commons/src/test/java/com/baeldung/commons/math/RealMatrixUnitTest.java

@@ -0,0 +1,41 @@
+package com.baeldung.commons.math;
+
+import org.apache.commons.math3.linear.Array2DRowRealMatrix;
+import org.apache.commons.math3.linear.RealMatrix;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class RealMatrixUnitTest {
+
+    @Test
+    void givenTwoMatrices_whenMultiply_thenMultiplicatedMatrix() {
+        RealMatrix firstMatrix = new Array2DRowRealMatrix(
+          new double[][] {
+            new double[] {1d, 5d},
+            new double[] {2d, 3d},
+            new double[] {1d ,7d}
+          }
+        );
+
+        RealMatrix secondMatrix = new Array2DRowRealMatrix(
+          new double[][] {
+            new double[] {1d, 2d, 3d, 7d},
+            new double[] {5d, 2d, 8d, 1d}
+          }
+        );
+
+        RealMatrix expected = new Array2DRowRealMatrix(
+          new double[][] {
+            new double[] {26d, 12d, 43d, 12d},
+            new double[] {17d, 10d, 30d, 17d},
+            new double[] {36d, 16d, 59d, 14d}
+          }
+        );
+
+        RealMatrix actual = firstMatrix.multiply(secondMatrix);
+
+        assertThat(actual).isEqualTo(expected);
+    }
+
+}