Fragments (Java)

A Fragment is a reusable portion of UI + behavior that you place inside an Activity (or another Fragment’s view). Fragments have their own lifecycle, participate in the Activity’s back stack, and make multi-pane/tablet UIs easier. Think of them as screens inside a screen.

When to Use Fragments

Activity vs Fragment (At a Glance)

Aspect	Activity	Fragment
Purpose	Top-level screen	Reusable UI block inside Activity
Lifecycle Owner	Yes	Yes (getViewLifecycleOwner() for Views)
Navigation	startActivity / NavHost	FragmentTransaction / NavController
Back Stack	System back stack	FragmentManager back stack
UI Container	Window DecorView	FragmentContainerView / any container

Create a Fragment (Java)

// HelloFragment.java
public class HelloFragment extends Fragment {

  public static HelloFragment newInstance(String name) {
    HelloFragment f = new HelloFragment();
    Bundle args = new Bundle();
    args.putString("name", name);
    f.setArguments(args);
    return f;
  }

  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_hello, container, false);
  }

  @Override
  public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    String name = getArguments() != null ? getArguments().getString("name") : "Guest";
    TextView tv = view.findViewById(R.id.tvHello);
    tv.setText("Hello, " + name);
  }

  @Override
  public void onDestroyView() {
    super.onDestroyView();
    // null out view references or viewBinding objects here
  }
}

Layout

<!-- res/layout/fragment_hello.xml -->
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">

  <TextView
      android:id="@+id/tvHello"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Hello Fragment"
      android:textSize="22sp" />
</FrameLayout>

Adding Fragments to an Activity

1) In XML with FragmentContainerView

<!-- res/layout/activity_host.xml -->
<androidx.fragment.app.FragmentContainerView
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

2) Dynamically via FragmentTransaction

// In HostActivity.java
Fragment f = HelloFragment.newInstance("Sonu");
getSupportFragmentManager()
  .beginTransaction()
  .replace(R.id.fragment_container, f)  // or add()
  .addToBackStack(null)               // optional: back navigates previous fragment
  .commit();

Back Stack & Transactions

API	Effect	Notes
addToBackStack(name)	Push transaction on stack	Back button will reverse it
commit()	Asynchronous	Safe for UI-thread usage
commitNow()	Synchronous	Do only when needed; must be on main thread
commitAllowingStateLoss()	May lose state	Last resort (e.g., during onStop); avoid if possible

Communication Patterns

1) Activity ↔ Fragment using interface (classic)

// In Fragment
public interface OnHelloClick {
  void onHello(String name);
}
private OnHelloClick callback;

@Override public void onAttach(@NonNull Context ctx) {
  super.onAttach(ctx);
  if (ctx instanceof OnHelloClick) callback = (OnHelloClick) ctx;
}

private void notifyHello(){ if(callback!=null) callback.onHello("Sonu"); }

2) Shared ViewModel (recommended for siblings)

// Shared ViewModel scoped to Activity
public class SharedVM extends ViewModel {
  public MutableLiveData<String> selected = new MutableLiveData<>();
}

// In both fragments
SharedVM vm = new ViewModelProvider(requireActivity()).get(SharedVM.class);
vm.selected.observe(getViewLifecycleOwner(), value -> { // update UI  });

DialogFragment

public class ConfirmDialog extends DialogFragment {
  @NonNull @Override
  public Dialog onCreateDialog(Bundle s) {
    return new AlertDialog.Builder(getActivity())
      .setTitle("Confirm")
      .setMessage("Proceed?")
      .setPositiveButton("Yes", (d,w) -> {/* ... */})
      .setNegativeButton("No", null)
      .create();
  }
}

Child Fragments

Fragments can host other fragments (tabs, pagers). Use getChildFragmentManager() for nested transactions.

RecyclerView inside Fragment

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle s) {
  return inflater.inflate(R.layout.fragment_list, container, false);
}
@Override
public void onViewCreated(@NonNull View v, Bundle s) {
  RecyclerView rv = v.findViewById(R.id.recycler);
  rv.setLayoutManager(new LinearLayoutManager(getContext()));
  rv.setAdapter(new ItemsAdapter());
}

State & Restoration

• Use setArguments(Bundle) for construction params (persisted automatically).
• Put UI state in ViewModel; release view references in onDestroyView().
• Let the system recreate fragments; avoid manual fragment caching in singletons.

Fragment Results (no direct interface)

// Sender (child fragment)
Bundle result = new Bundle();
result.putString("selected", "Item-42");
getParentFragmentManager().setFragmentResult("pick_key", result);

// Receiver (parent or host fragment/activity)
getSupportFragmentManager().setFragmentResultListener("pick_key", this, (req, bundle) -> {
  String id = bundle.getString("selected");
  // use id
});

FragmentFactory (advanced)

Inject dependencies into fragments via a custom FragmentFactory so the system uses your constructor.

public class MyFactory extends FragmentFactory {
  private final Repo repo;
  public MyFactory(Repo r){ repo = r; }
  @NonNull @Override
  public Fragment instantiate(@NonNull ClassLoader cl, @NonNull String className) {
    if(className.equals(HelloFragment.class.getName())) {
      return new HelloFragment(repo);
    }
    return super.instantiate(cl, className);
  }
}

Troubleshooting

Best Practices

• Use ViewBinding or findViewById and clear references in onDestroyView().
• Keep Fragments UI-focused; put business logic in ViewModels/repositories.
• Prefer FragmentContainerView over the old <fragment> tag.
• Share data across fragments with a shared ViewModel scoped to the Activity.
• For complex flows, use the Navigation Component (it manages back stack & arguments safely).

Hands-On Exercise

1. Create ListFragment (shows items) and DetailFragment (shows details).
2. On item click, navigate to DetailFragment with the selected ID in arguments.
3. Use a shared ViewModel to update a toolbar title based on the selected item.
4. Add transactions to the back stack and verify back navigation works.