public class MainActivity extends AppCompatActivity {

    DateViewModel model;
    private TextView dateTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        model = ViewModelProviders.of(this).get(DateViewModel.class);
        dateTextView = findViewById(R.id.dateTextView);

        Toast.makeText(this, model.getCurrentDate().getValue(), Toast.LENGTH_LONG).show();

        final Observer<String> dateObserver = new Observer<String>() {
            @Override
            public void onChanged(@Nullable String date) {
                dateTextView.setText(date);
            }
        };

        model.getCurrentDate().observe(this, dateObserver);
    }

    public void onButtonClicked(View view) {
        model.getCurrentDate().setValue(new Date().toString());
    }
}