It’s been a long discussion so easy to miss parts 
In one of my replies above, I have given a modified source to test the connection. So in short we were testing the connection so far.
Now, what you see is not the connection error, but instead, it is one of database/collection/document errors. That means (at least to me) you have corrected connection problems and now the rest is history 
From now on, you need to check documentation on how to do CRUD operation. I had a commented line in the same post above. You can try that one for simple testing.
//stmt.execute("use corpus_test;"); // use this or embed after hostname "localhost:27017/corpus_test"
PS: here in my code, “corpus_test” becomes both your auth database and work database. so heads up if you change auth and work databases.