Extension properties in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an extension property in Kotlin?

An extension property allows you to add a new property to an existing class without modifying its source code. It behaves like a regular property but is defined outside the class.

intermediate

Can extension properties have backing fields in Kotlin?

No, extension properties cannot have backing fields. They must be defined with a getter (and optionally a setter) that computes the value.

beginner

How do you define a read-only extension property for the String class that returns the first character?

You define it like this: <br>

val String.firstChar: Char<br>    get() = this[0]

intermediate

Why can't extension properties access private members of the class they extend?

Because extension properties are defined outside the class, they do not have access to the class's private members. They only use the public API.

intermediate

What happens if you define an extension property with the same name as a member property?

The member property always takes precedence. The extension property will be ignored when accessed through an instance of the class.

Which of the following is true about Kotlin extension properties?

A They can have backing fields.

B They can only have getters and setters without backing fields.

C They can modify private members of the class.

D They require modifying the original class source code.

Extension properties cannot have backing fields; they must use getters and setters that compute values.

How do you declare a read-only extension property named 'lastChar' for String?

A fun String.lastChar() = this.last()

B val lastChar: String get() = this.last()

C var String.lastChar: Char = this.last()

D val String.lastChar: Char get() = this.last()

Extension properties must specify the receiver type before the property name.

If a class has a member property 'size' and you define an extension property 'size', which one is used when accessing 'size' on an instance?

A The member property

B The extension property

C Both are used alternately

D It causes a compile error

Member properties always take precedence over extension properties with the same name.

Can extension properties access private properties of the class they extend?

A No, they cannot access private members

B Only if declared in the same package

C Only if the class is open

D Yes, always

Extension properties are defined outside the class and cannot access private members.

What is the main benefit of using extension properties?

A To access private members of a class

B To override existing properties

C To add new properties without changing the original class

D To create new classes

Extension properties let you add new properties to classes without modifying their source code.

Explain what extension properties are in Kotlin and how they differ from regular properties.

Think about how you add new properties without changing the original class.

Describe the limitations of extension properties in Kotlin.

Consider what extension properties cannot do compared to regular properties.