| name | java-mvvm |
| description | Expert guidance on implementing MVVM in Java using Jetpack ViewModel and LiveData with unidirectional data flow. Use this when building screens, wiring Fragments/Activities to state holders, or reviewing MVVM code in Java. |
MVVM in Java with ViewModel + LiveData
Instructions
Use Jetpack ViewModel as the state holder and LiveData as the observable transport between ViewModel and Fragment/Activity. The view is dumb: it renders an immutable UI state and forwards events.
1. Immutable UI State
Model UI state as a single immutable record (Java 17). One state per screen — not one LiveData per field.
public record ArticleListUiState(
boolean loading,
List<Article> articles,
String errorMessage
) {
public static ArticleListUiState initial() {
return new ArticleListUiState(false, List.of(), null);
}
public ArticleListUiState loading() {
return new ArticleListUiState(true, articles, null);
}
public ArticleListUiState success(List<Article> items) {
return new ArticleListUiState(false, List.copyOf(items), null);
}
public ArticleListUiState failure(String message) {
return new ArticleListUiState(false, articles, message);
}
}
2. The ViewModel
@HiltViewModel
public class ArticleListViewModel extends ViewModel {
private final GetArticles getArticles;
private final MutableLiveData<ArticleListUiState> state =
new MutableLiveData<>(ArticleListUiState.initial());
private final ExecutorService io = Executors.newSingleThreadExecutor();
@Inject
public ArticleListViewModel(GetArticles getArticles) {
this.getArticles = getArticles;
refresh();
}
public LiveData<ArticleListUiState> state() {
return state;
}
public void refresh() {
state.setValue(requireState().loading());
io.execute(() -> {
try {
List<Article> items = getArticles.execute();
state.postValue(requireState().success(items));
} catch (Exception e) {
state.postValue(requireState().failure(e.getMessage()));
}
});
}
private ArticleListUiState requireState() {
return Objects.requireNonNull(state.getValue());
}
@Override
protected void onCleared() {
io.shutdownNow();
}
}
Key points:
setValue on the main thread, postValue from background threads.- Always shut down owned executors in
onCleared().
- Expose
LiveData<T>, not MutableLiveData<T>.
3. The Fragment
@AndroidEntryPoint
public class ArticleListFragment extends Fragment {
private FragmentArticleListBinding binding;
private ArticleListViewModel viewModel;
private ArticleAdapter adapter;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle s) {
binding = FragmentArticleListBinding.inflate(inflater, parent, false);
return binding.getRoot();
}
@Override
public void onViewCreated(View view, Bundle s) {
viewModel = new ViewModelProvider(this).get(ArticleListViewModel.class);
adapter = new ArticleAdapter();
binding.list.setAdapter(adapter);
binding.swipeRefresh.setOnRefreshListener(viewModel::refresh);
viewModel.state().observe(getViewLifecycleOwner(), this::render);
}
private void render(ArticleListUiState s) {
binding.swipeRefresh.setRefreshing(s.loading());
adapter.submitList(s.articles());
binding.error.setVisibility(s.errorMessage() == null ? View.GONE : View.VISIBLE);
binding.error.setText(s.errorMessage());
}
@Override
public void onDestroyView() {
super.onDestroyView();
binding = null;
}
}
4. Single-Shot Events
Use a SingleLiveEvent or Channel-like queue for navigation and snackbars so they are not re-delivered after rotation:
public final class Event<T> {
private final T content;
private boolean handled = false;
public Event(T content) { this.content = content; }
public T getIfNotHandled() {
if (handled) return null;
handled = true;
return content;
}
}
Expose as LiveData<Event<NavTarget>> and call getIfNotHandled() in the observer.
5. Two-Way Input with DataBinding (optional)
When form editing is heavy, enable DataBinding and bind MutableLiveData<String> directly to EditText via android:text="@={viewModel.query}". Otherwise stay on ViewBinding + TextWatcher and call viewModel.onQueryChanged(...).
Checklist
