### Question:
Matrix { public final void incrementAt(final int row, final int col, final double val) { final double originalVal = this.getAt(row, col); this.setAtUnchecked(row, col, originalVal + val); } protected  Matrix(final int numRows, final int numCols); final int getElementCount(); final int getRowCount(); final int getColumnCount(); final double getAt(final int row, final int col); final void setAt(final int row, final int col, final double val); final void incrementAt(final int row, final int col, final double val); final ColumnVector getRowSumVector(); final ColumnVector getColumnSumVector(); Collection<Integer> normalizeColumns(); void removeNegatives(); void removeLessThan(final double minValue); final void scale(final double scale); Matrix multiplyElementWise(final Matrix matrix); Matrix addElementWise(final Matrix matrix); final double absSum(); final double sum(); ColumnVector multiply(final ColumnVector vector); final Matrix transpose(); Matrix roundTo(final int numPlaces); Matrix multiply(final double scalar); Matrix add(final double scalar); Matrix abs(); Matrix sqrt(); final boolean isSameSize(final Matrix matrix); final boolean isZeroMatrix(); @Override int hashCode(); @Override boolean equals(final Object obj); void forEach(final ReadOnlyElementVisitorFunction func); abstract MatrixNonZeroElementRowIterator getNonZeroElementRowIterator(final int row);  }### Answer:
@Test public void matrixValuesCanBeIncremented() { final Matrix matrix = this.createMatrix(2, 3, new double[] {1, 4, 5, 7, 2, 11}); matrix.incrementAt(0, 0, 4); matrix.incrementAt(0, 1, 2); matrix.incrementAt(0, 2, 3); matrix.incrementAt(1, 0, 4); matrix.incrementAt(1, 1, 5); matrix.incrementAt(1, 2, 6); matrix.incrementAt(0, 2, 7); MatcherAssert.assertThat( matrix, IsEqual.equalTo(this.createMatrix(2, 3, new double[] {5, 6, 15, 11, 7, 17}))); }