Extension function syntax in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an extension function in Kotlin?

An extension function lets you add a new function to an existing class without changing its source code or inheriting from it.

beginner

How do you declare an extension function for the String class named <code>shout</code> that returns the string in uppercase?

You write <code>fun String.shout(): String { return this.uppercase() }</code>. The <code>String</code> before the dot is the class being extended.

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 only work with the public API.

beginner

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

The keyword <code>fun</code> is used, followed by the class name, a dot, and the function name.

beginner

How do you call an extension function on an object?

You call it just like a normal function on the object, for example: "hello".shout().

Which of the following is the correct syntax to declare an extension function double for Int that returns the number multiplied by 2?

A extension fun Int.double() = this * 2

B fun double(Int) = Int * 2

C fun Int.double() = this * 2

D fun double() Int = this * 2

The correct syntax is fun ClassName.functionName() = ... so fun Int.double() = this * 2 is right.

Can extension functions override existing member functions of a class?

A No, they cannot override member functions.

B Only for open classes.

C Only if marked with override keyword.

D Yes, they replace the member functions.

Extension functions do not override member functions; if a member function exists, it is called instead.

What does the this keyword refer to inside an extension function?

A The caller of the function.

B The extension function itself.

C The global object.

D The class being extended instance.

Inside an extension function, this refers to the instance of the class being extended.

Which of these is NOT true about Kotlin extension functions?

A They can access private members of the class.

B They are declared with fun keyword.

C They can add new functions to existing classes.

D They do not modify the original class.

Extension functions cannot access private or protected members of the class.

How do you call an extension function greet on a String object "World"?

A greet("World")

B "World".greet()

C String.greet("World")

D call greet on "World"

Extension functions are called like normal functions on the object: "World".greet()

Explain how to write and use an extension function in Kotlin.

Think about adding a new function to a class without changing it.

What are the limitations of Kotlin extension functions compared to member functions?

Consider what extension functions cannot do inside the class.