There’s an official Google+ Java API, but it’s limited to read only access to your Google+ profile by OAuth tokens.
Google itself claims that they do not want the g+ stream to be writable by “non-humans”. Twitter was subject to “massive tweet spam” beginning at the time they decided to provide tweet functionalty in their API…
Just to check. It’s quick and dirty, but it works: Google+: Java API post on stream (PoC)