No. Currently the users type does not return any private_info and neither does the user_timeline type return any voting information.
No other API method allows for detecting a user's vote history except one question at a time. You would have to fetch every single question and test the downvoted and the upvoted properties.
If you limited your search ...
It's allowed with the caveat there's no guarantee it won't be merged, provided that you abide by the good faith criteria.
If you attempt to actually run something like the script in your previous post, for example, both accounts will likely be suspended without second thought. Having a separate account for testing doesn't shield you from the consequences of ...
You need to use a filter that returns those properties. The default filter does not.
Also, the API is at version 2.2. You should use that unless you've a very good reason not to.
So try, for example:
I don't have much experience with HttpClient but the problem does not seem to be caused by your code. Probably it is the lack of required parameters that causes problem. When I called this function in C#, I passed following parameters in order to make it work. I am not sure about "preview" parameter but I believe the site parameter is a must.
Hope this ...
I've written a userscript: Wilson confidence rating calculator. It's in its own post to have an application key, so that you can use it more than 300 times per day. (Note that showing the vote breakdown also calculates the Wilson rating for that specific post, but doesn't use the API.)
It's no different in the API than it is for the main site. From the FAQ:
you may change your vote within a five minute window from the time of the first vote you cast on that post. Note that after voting and undoing your vote 30 times, it will also be locked in.