How to use google java api?

I want to use google java api (https://developers.google.com/api-client-library/java/), but I don't know how to use google java api.

I found this site (https://github.com/processing/processing/wiki/How-to-Install-a-Contributed-Library), but my try is failed.

How to use google java api?

I'm sorry for my English. English is not my first language

Example : https://github.com/google/google-api-java-client-samples/blob/master/oauth2-cmdline-sample/src/main/java/com/google/api/services/samples/oauth2/cmdline/OAuth2Sample.java