MainActivity_addViewModel.java

    // As class member
    private BooksViewModel model;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // Rest of method removed from example

        // Load our BooksViewModel or create a new one
        model = ViewModelProviders.of(this).get(BooksViewModel.class);
        // Listen for changes on the BooksViewModel
        model.getBooks().observe(this, new Observer<List<Book>>() {
            @Override
            public void onChanged(@Nullable List<Book> books) {
                // Called each time the BooksViewModel.books content changes
                updateUi(books);
            }
        });
    }