Here is an example with the relevant imports
From the docs (https://developers.google.com/admin-sdk/directory/v1/reference/users/list)
As an account administrator, you can also use the my_customer alias to represent your account's customerId
import java.io.IOException;
import java.util.List;
import com.google.api.services.admin.directory.Directory;
import com.google.api.services.admin.directory.model.User;
import com.google.api.services.admin.directory.model.Users;
public static List<User> listUser(Directory directory) throws IOException {
Users users = directory.users().list().setCustomer("my_customer").execute();
return users.getUsers();
}