I did the WHV and as stated above:
Most people will get a job in a restaurant/Bar or as English teacher. There are some people who did other Jobs, however they spoke Japanese. I dont say that it is impossible to get a job in your field, but its generally gonna be difficult. Here, as said in numerous other Threads: Why should they hire a foreigner if a Japanese can do the Job as well? Of course if you speak Japanese, English and maybe a 3rd language that might be a good reason, but otherwise its gonna be problematic.
The Working Holiday Visa is meant to help you to live and travel in Japan for a year, and not to find a longterm work. However your chances in getting somewhere are better than all the other people without this Visa.
You can stay in Japan legally for 1 year, you can get a small job to supply yourself while searching for a "real job",....... and you are allowed to do nearly any kind of work, so you could talk a interested company into a trial-period because they wouldent have to bother with a Visa. And after they know that you are such a good worker
they will hire you for sure
Anyway, good luck with that!
Oh, and there might be the chance for you to get a Visa for South Korea and Singapore as well (as German citicen i can)