Extension functions in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct extension function syntax

Which option correctly defines an extension function isEven for Int that returns true if the number is even?

A extension fun Int.isEven(): Boolean { return this % 2 == 0 }

B fun Int.isEven: Boolean = this % 2 == 0

C fun isEven(this Int): Boolean = this % 2 == 0

D fun Int.isEven(): Boolean = this % 2 == 0

💡 Hint

Remember, extension functions use the syntax: fun ReceiverType.functionName() { ... }

Option D correctly uses the Kotlin extension function syntax with the receiver type before the function name and a proper function body returning a Boolean.

❓ ui_behavior

intermediate

Extension function effect on UI component

Given this extension function for TextView in Android Kotlin:

fun TextView.makeRed() { this.setTextColor(android.graphics.Color.RED) }

What will happen when you call myTextView.makeRed() in an activity?

A The text color of myTextView changes to red

B The background color of myTextView changes to red

C The app crashes with a NullPointerException

D Nothing happens because extension functions cannot modify UI

💡 Hint

Extension functions can call existing methods on the receiver object.

The extension function calls setTextColor on the TextView instance, changing its text color to red.

❓ lifecycle

advanced

Extension function and lifecycle impact

Consider an extension function on Activity that shows a toast message:

fun Activity.showWelcome() { Toast.makeText(this, "Welcome!", Toast.LENGTH_SHORT).show() }

What is a potential lifecycle-related issue when calling showWelcome() inside onCreate() of an activity?

A The toast may not show if the activity is not fully created yet

B The toast will cause a memory leak if called in onCreate

C The extension function will crash because Toast requires a Fragment

D There is no issue; the toast always shows correctly

💡 Hint

Consider when UI elements are ready during the activity lifecycle.

If called too early, the UI might not be ready to display the toast properly, so the toast might not appear as expected.

❓ navigation

advanced

Extension function for navigation

You have this extension function for Fragment to navigate to another fragment:

fun Fragment.goToDetails() { findNavController().navigate(R.id.detailsFragment) }

What happens if you call goToDetails() in a fragment that is not attached to a NavController?

A Navigates to detailsFragment successfully

B Causes a compile-time error

C Throws an IllegalStateException at runtime

D Does nothing silently

💡 Hint

findNavController() requires the fragment to be attached to a NavHost.

If the fragment is not attached to a NavController, findNavController() throws an IllegalStateException at runtime.

🔧 Debug

expert

Debugging extension function unexpected behavior

Given this extension function:

fun String.removeFirstChar(): String = this.substring(1)

What happens if you call "".removeFirstChar() (empty string)?

A Returns an empty string

B Throws StringIndexOutOfBoundsException

C Returns null

D Returns the original empty string

💡 Hint

Check what substring(1) does on an empty string.

Calling substring(1) on an empty string causes StringIndexOutOfBoundsException because index 1 is out of range.