### Question:
MatrixUtils { public static void addTo(long row, long column, float value, LongObjectMap<LongFloatMap> RbyRow, LongObjectMap<LongFloatMap> RbyColumn) { addToByRow(row, column, value, RbyRow); addToByRow(column, row, value, RbyColumn); } private  MatrixUtils(); static void addTo(long row,
                           long column,
                           float value,
                           LongObjectMap<LongFloatMap> RbyRow,
                           LongObjectMap<LongFloatMap> RbyColumn); static void remove(long row,
                            long column,
                            LongObjectMap<LongFloatMap> RbyRow,
                            LongObjectMap<LongFloatMap> RbyColumn); static boolean isNonSingular(RealMatrix M); static Solver getSolver(RealMatrix M); static double[][] accessMatrixDataDirectly(RealMatrix matrix); static RealMatrix transposeTimesSelf(LongObjectMap<float[]> M);  }### Answer:
@Test public void testAddTo() { LongObjectMap<LongFloatMap> byRow = new LongObjectMap<LongFloatMap>(); assertNull(byRow.get(0L)); assertNull(byRow.get(1L)); assertNull(byRow.get(4L)); LongObjectMap<LongFloatMap> byCol = new LongObjectMap<LongFloatMap>(); MatrixUtils.addTo(0L, 0L, -1.0f, byRow, byCol); MatrixUtils.addTo(4L, 1L, 2.0f, byRow, byCol); assertEquals(-1.0f, byRow.get(0L).get(0L)); assertEquals(-1.0f, byCol.get(0L).get(0L)); assertNull(byRow.get(1L)); assertEquals(2.0f, byRow.get(4L).get(1L)); assertEquals(2.0f, byCol.get(1L).get(4L)); assertNaN(byRow.get(4L).get(0L)); }