java-mvvm

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

• Exactly one LiveData<UiState> per screen. No bag-of-LiveData.
• State class is immutable (record or final fields with defensive copies).
• Fragment observes with getViewLifecycleOwner(), never this.
• binding field is nulled in onDestroyView().
• Background work runs on an ExecutorService or WorkManager — never on the main thread.
• Single-shot events use an Event wrapper or SingleLiveEvent.