Please use the following code:

  long r[][] = {{-1, -1}};
  int type = TIFFTag.TIFF_RATIONAL;
  TIFFField f = new TIFFField(new TIFFTag("tag", 666, 1 << type), type, r.length, r);
  System.out.println(f.getValueAsString(0));


Output:
  r[][] = {{-1, -1}}; -> "1/1"
  r[][] = {{-3, -3}}; -> "1/1"
  r[][] = {{0, -3}}; -> "0/1"
  r[][] = {{-4, 2}}; -> "-2/1"
  r[][] = {{-4, -2}}; -> "2/1"

but, at the same time:
  long r[][] = {{-2, -3}}; -> "-2/-3" (expected: "2/3")
and
  long r[][] = {{2, -4}}; -> "2/-4" (expected: at least, "-2/4", but much better: "1/2")

So some rudiments of reducing fractions are implemented in getValueAsString(), but it would be nice to complete them (at least in part of the signs) or remove them completely to avoid the mess.

The same for TIFFTag.TIFF_SRATIONAL