### Question:
Matrix implements Serializable { public double get(int i, int j) { checkExpression(i >= 0 && i < mRow && j >= 0 && j < mCol, "Row or col is out of bounds"); return mArray[i * mCol + j]; } private  Matrix(double[] array, int row, int  col); int[] shape(); double[] getArray(); double[][] getArrays(); Matrix copy(); int getRow(); int getCol(); double get(int i, int j); void set(int i, int j, double value); Matrix times(@NonNull Matrix matrix); Matrix times(double num); Matrix timesTo(double num); Matrix timesTo(@NonNull Matrix matrix); Matrix plus(@NonNull Matrix matrix); Matrix plusTo(@NonNull Matrix matrix); Matrix minus(@NonNull Matrix matrix); Matrix minusTo(@NonNull Matrix matrix); Matrix dot(@NonNull Matrix matrix); Matrix transpose(); static Matrix array(@NonNull double[][] arrays); static Matrix array(@NonNull double[] array, int row); static Matrix randn(int row, int col); static Matrix plus(@NonNull Matrix a, @NonNull Matrix b); static Matrix plusTo(@NonNull Matrix a,@NonNull Matrix b); static Matrix minus(@NonNull Matrix a,@NonNull Matrix b); static Matrix minusTo(@NonNull Matrix a, @NonNull Matrix b); static Matrix times(@NonNull Matrix a, @NonNull Matrix b); static Matrix times(@NonNull Matrix matrix, double num); static Matrix timesTo(@NonNull Matrix matrix, double num); static Matrix timesTo(@NonNull Matrix a, @NonNull Matrix b); static Matrix dot(@NonNull Matrix a, @NonNull Matrix b); static Matrix transpose(@NonNull Matrix a); static Matrix zeroLike(Matrix matrix); static Matrix zeros(@NonNull int... shape);  }### Answer:
@Test public void testGet() { final Matrix a = Matrix.array(new double[]{ 1, 2, 3, 4, 5, 6 }, 3); assertEquals(a.get(2,1), 6, 0); }