Suppose, you coordinates are
/|\ y
| col8
+---+ ... +---+---+
| | | | | <- row 8
+---+ ... +---+---+
| | | | |
+---+ ... +---+---+
...............
+---+ ... +---+---+
| | | | | <- row 1
+---+ ... +---+---+--->
x
Extending your solution it will looks like
// Up right
if (y > x) { row = 8; column = 8-(y-x) } else { row = 8-(x-y); column = 8; }
// Down left
if (x > y) { row = 1; column = 1+(x-y) } else { row = 1+(y-x); column = 1; }
// Up left
if (9-x < y) { row = 8; column = x+y-8 } else { row = x+y-1; column = 1; }
// Down right
if (9-x > y) { row = 1; column = x+y-1 } else { row = x+y-8; column = 8; }