What the interviewer is testing

The interviewer is checking whether you understand ownership and responsibility rather than simply comparing APIs or lifecycle callbacks.

A good answer distinguishes an Activity's responsibility for the window and system entry point from a Fragment's responsibility for a portion of UI inside that Activity. It should also reflect modern Android, where many applications use one Activity and compose their screens with Fragments or Jetpack Compose.

❌ Common mistake

If they ask you to elaborate

A common modern architecture is a single Activity that hosts navigation. Compose-first applications often use a single Activity and may not use Fragments at all, while many production applications still rely on Fragments for modular navigation and reusable UI.

When should you use each?

Code example

class MainActivity : AppCompatActivity(R.layout.activity_main) {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        if (savedInstanceState == null) {
            supportFragmentManager.commit {
                setReorderingAllowed(true)
                // Activity owns the window and container.
            replace(R.id.fragmentContainer, HomeFragment())
            }
        }
    }
}

class HomeFragment : Fragment(R.layout.fragment_home)

The Activity owns the window and the container. HomeFragment contributes one portion of the UI inside that container. The savedInstanceState == null check prevents adding the same Fragment again after configuration changes because the FragmentManager automatically restores existing Fragments.

Follow-up questions

What separates a senior answer

Key takeaways

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