Activity concept in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when an Activity is launched?

Consider an Android app where MainActivity starts SecondActivity. What is the visible result after calling startActivity(Intent(this, SecondActivity::class.java))?

Android Kotlin

val intent = Intent(this, SecondActivity::class.java)
startActivity(intent)

A The screen shows SecondActivity UI, and MainActivity is paused in the background.

B The screen remains on MainActivity, and SecondActivity runs in the background.

C The app closes and returns to the home screen.

D Both MainActivity and SecondActivity UI appear side by side.

💡 Hint

Think about what startActivity does to the current and new activity.

When startActivity is called, the new activity (SecondActivity) is brought to the front and shown. The previous activity (MainActivity) is paused but still in memory.

❓ lifecycle

intermediate

Which lifecycle method is called when an Activity becomes visible?

In Android, when an Activity moves from not visible to visible on screen, which lifecycle method is called?

A onStart()

B onCreate()

C onResume()

D onPause()

💡 Hint

Visibility means the activity is about to be seen but not yet interactive.

onStart() is called when the activity becomes visible but before it gains focus. onResume() is called after onStart() when the activity is ready for user interaction.

❓ navigation

advanced

What is the effect of calling finish() inside an Activity?

If you call finish() inside an Activity, what happens next?

Android Kotlin

finish()

A The current Activity is paused but remains visible.

B The current Activity is destroyed and removed from the back stack, returning to the previous Activity.

C The app closes completely.

D Nothing happens until the user presses back.

💡 Hint

Think about what finish() means for the activity lifecycle and navigation stack.

Calling finish() ends the current activity and removes it from the back stack, so the previous activity is shown.

📝 Syntax

advanced

Identify the correct way to start an Activity with data in Kotlin

Which code snippet correctly starts DetailActivity passing a String extra with key "user" and value "Alice"?

A

val intent = Intent(this, DetailActivity::class.java)
intent.addExtra("user", "Alice")
startActivity(intent)

B startActivity(Intent(this, DetailActivity::class.java).putExtra(user = "Alice"))

C

val intent = Intent(this, DetailActivity::class.java).apply { putExtra("user", "Alice") }
startActivity(intent)

D

val intent = Intent(this, DetailActivity::class.java)
intent.put("user", "Alice")
startActivity(intent)

💡 Hint

Check the correct method name and syntax for adding extras to an Intent.

putExtra(key, value) is the correct method to add data to an Intent. The other options use invalid method names or syntax.

🔧 Debug

expert

Why does this Activity crash on launch?

Given this Activity code, why does the app crash immediately when launched? class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) val button = findViewById

Android Kotlin

class MainActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    val button = findViewById<Button>(R.id.myButton)
    button.setOnClickListener {
      Toast.makeText(this, "Clicked", Toast.LENGTH_SHORT).show()
    }
  }
}

A The onCreate method is missing a call to super.onStart(), causing lifecycle errors.

B Toast.makeText requires applicationContext instead of this as context, causing a crash.

C setContentView is called after findViewById, so button is null.

D The layout activity_main.xml does not contain a view with id myButton, so findViewById returns null causing a NullPointerException.

💡 Hint

Check if the button view exists in the layout before accessing it.

If findViewById returns null because the view id is missing, calling setOnClickListener on null causes a crash (NullPointerException).

Practice

1. What is the main role of an Activity in an Android app?

easy

A. It stores app data permanently.

B. It handles network requests only.

C. It manages background tasks without UI.

D. It represents a single screen with a user interface.

Solution

1. Step 1: Understand Activity purpose

   An Activity is designed to show a screen where users interact.

2. Step 2: Compare options

   Only It represents a single screen with a user interface. correctly describes an Activity as a UI screen.

3. Final Answer:

   It represents a single screen with a user interface. -> Option D

4. Quick Check:

   Activity = Screen UI [OK]

Hint: Remember: Activity = one screen in app [OK]

Common Mistakes:

• Confusing Activity with data storage
• Thinking Activity runs without UI
• Mixing Activity with background services

2. Which method is used to set up the UI when an Activity starts?

easy

A. onResume()

B. onStart()

C. onCreate()

D. onPause()

Solution

1. Step 1: Identify lifecycle methods

   onCreate() is called when the Activity is first created to set up UI.

2. Step 2: Eliminate other methods

   onStart() and onResume() happen later; onPause() is for pausing.

3. Final Answer:

   onCreate() -> Option C

4. Quick Check:

   UI setup = onCreate() [OK]

Hint: UI setup always starts in onCreate() [OK]

Common Mistakes:

• Choosing onStart() or onResume() for UI setup
• Confusing onPause() as setup method
• Not knowing Activity lifecycle order

3. What will happen when this Activity code runs?

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

medium

A. The screen shows the layout defined in activity_main.xml.

B. The app shows a blank screen.

C. Nothing happens because onCreate is empty.

D. The app crashes due to missing layout.

Solution

1. Step 1: Analyze onCreate method

   It calls super.onCreate and sets content view to activity_main layout.

2. Step 2: Understand setContentView effect

   This loads and displays the UI from activity_main.xml on screen.

3. Final Answer:

   The screen shows the layout defined in activity_main.xml. -> Option A

4. Quick Check:

   setContentView loads layout = screen shows layout [OK]

Hint: setContentView shows UI layout on screen [OK]

Common Mistakes:

• Assuming onCreate does nothing
• Thinking app crashes without error
• Confusing blank screen with missing layout

4. Identify the error in this Activity code:

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

medium

A. setContentView called before super.onCreate()

B. No error, code is correct

C. Missing call to super.onCreate()

D. onCreate method should not be overridden

Solution

1. Step 1: Check order of calls in onCreate

   super.onCreate() must be called before setContentView to initialize Activity properly.

2. Step 2: Identify mistake

   Here, setContentView is called first, which is incorrect and can cause issues.

3. Final Answer:

   setContentView called before super.onCreate() -> Option A

4. Quick Check:

   super.onCreate() first, then setContentView [OK]

Hint: Always call super.onCreate() before setContentView [OK]

Common Mistakes:

• Calling setContentView before super.onCreate()
• Forgetting to override onCreate
• Not calling super.onCreate() at all

5. You want to start a new Activity called DetailActivity from MainActivity. Which code snippet correctly starts the new Activity?

hard

A. startActivity(DetailActivity::class.java)

B. val intent = Intent(this, DetailActivity::class.java)
startActivity(intent)

C. Intent intent = new Intent(DetailActivity)
startActivity(intent)

D. val intent = Intent(DetailActivity)
startActivity()

Solution

1. Step 1: Understand Intent creation

   Intent requires context (this) and target Activity class (DetailActivity::class.java).

2. Step 2: Check startActivity usage

   startActivity(intent) correctly starts the new Activity with the Intent.

3. Final Answer:

   val intent = Intent(this, DetailActivity::class.java) startActivity(intent) -> Option B

4. Quick Check:

   Intent(this, TargetClass) + startActivity(intent) [OK]

Hint: Use Intent(this, TargetActivity::class.java) to start Activity [OK]

Common Mistakes:

• Omitting context in Intent constructor
• Calling startActivity without Intent
• Using wrong syntax for Intent creation