First, the example is using Imgur API v2 which is old and unsupported. You should be using API v3.
Also note that:
For public read-only and anonymous resources, such as getting image info, looking up user comments, etc. all you need to do is send an authorization header with your client_id in your requests.
from docs at https://api.imgur.com/oauth2 -- so you don't really need OAuth for what you're doing.
There is some example Imgur API code that might help you, listed at https://api.imgur.com/ -- the Android example might be more relevant to you, since it uses Java, but unsurprisingly it comes with all the overhead of an Android project, compared with a plain Java application.