Write & test subtract. Program output...
$ java -ea FractionTest
FractionTest tests verbose output:
test "That 1/0 should throw an IllegalArgumentException exception" passes.
test "That multiplying 1/2 by 3/5 gives 3/10" passes.
test "That 1/2 .equals 1/2" passes.
test "That 1/2 .equals 3/6" passes.
test "That -1/2 .equals 1/-2" passes.
test "That -1/-2 .equals 1/2" passes.
test "That 4/-8 does not .equals 1/2" passes.
test "Add: that 1/2 + 1/4 gives 3/4" passes.
test "Add: that 2/9 + 3/5 gives 37/45" passes.
FractionTest: All tests pass