Extensions on built-in types in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an extension function in Kotlin?

An extension function lets you add new functions to existing classes without changing their source code. It looks like a normal function but is called as if it belongs to the class.

beginner

How do you declare an extension function for the String class that returns the first character?

You write:

fun String.firstChar(): Char = this[0]

This adds a function called firstChar() to all String objects.

intermediate

Can extension functions access private members of the class they extend?

No, extension functions cannot access private or protected members of the class. They work like regular functions with the receiver object passed in, so only public members are accessible.

intermediate

What happens if an extension function has the same signature as a member function?

The member function always wins. The extension function is ignored when calling on an instance of the class.

intermediate

Write an extension property for List<Int> that returns the sum of all elements.

You can write:

val List<Int>.sumOfElements: Int get() = this.sum()

This adds a read-only property to List<Int>.

How do you call an extension function on a String named 'greet'?

A greet("Hello")

B "Hello".greet()

C String.greet("Hello")

D call greet on "Hello"

Extension functions are called like normal member functions on the object.

Can extension functions modify the original object they extend?

A Only if the object is mutable

B No, they cannot modify the original object

C Yes, they can change private fields

D Yes, but only for built-in types

Extension functions can modify the object if it is mutable, like a MutableList, but they cannot access private fields.

What keyword is used to declare an extension function in Kotlin?

A override

B extend

C fun

D extension

Extension functions are declared with the fun keyword, followed by the receiver type.

If a class has a member function and an extension function with the same name and parameters, which one is called?

A Compilation error

B Extension function

C Both are called

D Member function

Member functions have higher priority than extension functions.

Which of these is a valid extension property declaration for Int?

A val Int.isEven get() = this % 2 == 0

B var Int.isEven = this % 2 == 0

C fun Int.isEven() = this % 2 == 0

D property Int.isEven get() = this % 2 == 0

Extension properties must be declared with val or var and use a getter; var is not allowed without backing field.

Explain what extension functions are and how they help when working with built-in types in Kotlin.

Think about how you can add new abilities to existing classes.

Describe the difference between member functions and extension functions when they have the same name.

Consider what happens when Kotlin looks for a function to call.