Is not tail rec (can go on stackoverflow) and moreover Java does not support tail rec optimization.
If you really want to go with rec mode you should use an accumulator.

Yep. The code will indeed go into an infinite recursion if n is less than 1 (negative number or 0), resulting in a stack overflow error.

Either the tests are lacking or that was added after the translation was made and it wasn't added to the tests.

surprising that this works, i thougt it cal also be written as "three fifty" as per the description?

Doesn't this go into an infinite loop if I put in ) or any negative number?
if n == 0 then we call 0 + triangular(-1) and so on and so forth.

Fails for empty string.

Doesn't work for negative input!

In the description, step 4 is still missing the S for siblings.

i.e.
4. FLAMES --> 1>F 2>L 3>A 4>M 5>E 6>S

6>S is missing

For java the feedback on the test is flipped. When you return 6 it prints:

expected:<6> but was:<8>