How to Access Google Calendar REST API v3 with Java

If you just need to access a particular set of calendars, I would create a service account and share the necessary calendars with that account.

To do so:

1. Create a "Service Account" in this Cloud Console (it is found under "Web application" / "Certificate").
2. Download the private key and store in a safe place.
3. Take note of the email address associated with the service account.
4. Share (via Calendar user interface) any necessary calendars with this email address.
5. Install Google API Java Client libraries (https://developers.google.com/api-client-library/java/apis/).

Then you should be able to use the following code:

import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.json.gson.GsonFactory;
import java.io.File;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import java.util.Arrays;
import com.google.api.services.calendar.Calendar;

GoogleCredential credentials = new GoogleCredential.Builder().setTransport(GoogleNetHttpTransport.newTrustedTransport())
  .setJsonFactory(new GsonFactory())
  .setServiceAccountId("<service account email address>@developer.gserviceaccount.com")
  .setServiceAccountScopes(Arrays.asList("https://www.googleapis.com/auth/calendar.readonly"))
  .setServiceAccountPrivateKeyFromP12File(new File("<private key for service account in P12 format>-privatekey.p12"))
.build();
Calendar client = new Calendar.Builder(GoogleNetHttpTransport.newTrustedTransport(), new GsonFactory(), credentials).build();
client.<do calendar stuff>.execute();

If instead you are a domain administrator who needs to access calendars for all Google Apps accounts that are part of your domain without consent from individual users, then instead of step 4 above:

1. Take note of the client ID associated with the service account. This can be found in the client_secrets.json file - typically in the form 1234.apps.googleusercontent.com.
2. Authorize this client to make requests on behalf of users in your organization. See https://support.google.com/a/answer/162106?hl=en for steps - use whichever scopes you will be requesting later.

You should now be able to write code like the following:

import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.json.gson.GsonFactory;
import java.io.File;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import java.util.Arrays;
import com.google.api.services.calendar.Calendar;

GoogleCredential credentials = new GoogleCredential.Builder().setTransport(GoogleNetHttpTransport.newTrustedTransport())
  .setJsonFactory(new GsonFactory())
  .setServiceAccountId("<service account email address>@developer.gserviceaccount.com")
  .setServiceAccountScopes(Arrays.asList("https://www.googleapis.com/auth/calendar"))
  .setServiceAccountPrivateKeyFromP12File(new File("<private key for service account in P12 format>-privatekey.p12"))
  .setServiceAccountUser("<domain user whose data you need>@yourdomain.com")
.build();
Calendar client = new Calendar.Builder(GoogleNetHttpTransport.newTrustedTransport(), new GsonFactory(), credentials).build();
client.<do calendar stuff as that user>()

In addition to the steps mentioned in @aeijdenberg's answer, the Service Account now needs to explicitly accept the shared calendar by adding it to its CalendarList via CalendarList.insert. See:

Service Accounts don't accept automatically shared calendars anymore [148804709] https://issuetracker.google.com/issues/148804709