Function references (::functionName) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function reference in Kotlin?

A function reference in Kotlin is a way to refer to a function by its name using the :: operator, allowing you to pass the function as a value or assign it to a variable.

beginner

How do you create a function reference for a top-level function named printMessage?

You create it by writing ::printMessage. This refers to the function itself without calling it.

beginner

What is the difference between printMessage() and ::printMessage in Kotlin?

printMessage() calls the function immediately, while ::printMessage creates a reference to the function without calling it.

intermediate

Can you use function references to pass functions as arguments in Kotlin? Give an example.

Yes. For example, listOf(1, 2, 3).forEach(::println) passes the println function as a reference to forEach.

intermediate

How do you reference a member function of a class in Kotlin?

You use the class name followed by :: and the function name, like <code>String::toUpperCase</code>. This creates a reference to the member function.

What does the Kotlin expression ::myFunction represent?

A A reference to the function named myFunction

B A call to the function myFunction

C A variable named myFunction

D An error in syntax

Using :: before a function name creates a reference to that function without calling it.

Which of the following is a correct way to pass a function reference to a higher-order function in Kotlin?

A list.forEach(::print)

B list.forEach(print())

C list.forEach(print)

D list.forEach(:print)

The correct syntax to pass a function reference is ::functionName, so ::print is correct.

What is the output of this Kotlin code?

fun greet() = println("Hello")
val ref = ::greet
ref()

A No output

B Hello

C ref

D Compilation error

Calling ref() calls the function greet(), which prints "Hello".

How do you reference a member function length of the String class in Kotlin?

A ::String.length

B length::String

C String::length

D String.length::

The correct syntax for referencing a member function is ClassName::functionName.

Which statement is true about function references in Kotlin?

A They are not supported in Kotlin.

B They immediately execute the function.

C They can only reference top-level functions.

D They allow functions to be passed as values.

Function references let you treat functions as values to pass or store without calling them.

Explain what a function reference (::functionName) is in Kotlin and how it differs from calling a function.

Think about how you can pass a function itself, not its result.

Describe how to use function references with higher-order functions in Kotlin with an example.

Consider how you can pass println directly to forEach.

Practice

1. What does the Kotlin syntax ::functionName represent?

easy

A. A call to the function named functionName

B. A comment about the function named functionName

C. A variable declaration named functionName

D. A reference to the function named functionName

Solution

1. Step 1: Understand the :: operator in Kotlin

   The :: operator is used to get a reference to a function, not to call it.

2. Step 2: Differentiate between calling and referencing

   Calling a function uses parentheses like functionName(), while referencing uses ::functionName.

3. Final Answer:

   A reference to the function named functionName -> Option D

4. Quick Check:

   Function reference = ::functionName [OK]

Hint: Remember :: means 'point to function', not call it [OK]

Common Mistakes:

• Confusing function reference (::) with function call (())
• Thinking :: declares a variable
• Assuming :: is a comment or annotation

2. Which of the following is the correct syntax to pass a function reference to map in Kotlin?

easy

A. list.map(::functionName)

B. list.map(:functionName)

C. list.map(functionName())

D. list.map(functionName)

Solution

1. Step 1: Identify how to pass function references in Kotlin

   Function references use the :: operator before the function name when passed as arguments.

2. Step 2: Check each option for correct syntax

   list.map(::functionName) uses ::functionName, which is the correct way to pass a function reference to map.

3. Final Answer:

   list.map(::functionName) -> Option A

4. Quick Check:

   Pass function reference with :: [OK]

Hint: Use :: before function name when passing as argument [OK]

Common Mistakes:

• Passing function call instead of reference (using parentheses)
• Omitting :: operator
• Using incorrect single colon : instead of double ::

3. What is the output of this Kotlin code?

fun square(x: Int) = x * x
val numbers = listOf(1, 2, 3)
val squares = numbers.map(::square)
println(squares)

medium

A. [1, 4, 9]

B. [square, square, square]

C. [1, 2, 3]

D. Compilation error

Solution

1. Step 1: Understand the function reference usage

   The ::square passes the square function to map, which applies it to each element.

2. Step 2: Calculate the result of applying square to each number

   Applying square to 1, 2, 3 gives 1, 4, 9 respectively.

3. Final Answer:

   [1, 4, 9] -> Option A

4. Quick Check:

   map(::square) squares each element [OK]

Hint: map(::function) applies function to each list item [OK]

Common Mistakes:

• Expecting original list instead of transformed list
• Confusing function reference with function call
• Thinking output is function names instead of results

4. Identify the error in this Kotlin code snippet:

fun greet(name: String) = "Hello, $name"
val greeter = greet
println(greeter("Alice"))

medium

A. Missing parentheses when assigning function reference

B. Cannot assign function directly without :: operator

C. Function greet must return Unit

D. No error, code runs fine

Solution

1. Step 1: Check how function references are assigned

   Assigning a function to a variable requires the :: operator to get a reference.

2. Step 2: Identify the missing :: operator

   The code assigns greet instead of ::greet, causing a type mismatch error.

3. Final Answer:

   Cannot assign function directly without :: operator -> Option B

4. Quick Check:

   Use :: to assign function reference [OK]

Hint: Use :: when assigning function to variable [OK]

Common Mistakes:

• Assigning function name without ::
• Adding parentheses which calls function instead of referencing
• Assuming function must return Unit

5. Given this Kotlin code, what will be the output?

fun isEven(n: Int) = n % 2 == 0
val numbers = listOf(1, 2, 3, 4, 5, 6)
val evenNumbers = numbers.filter(::isEven)
println(evenNumbers)

Choose the correct output and explain why function references are useful here.

hard

A. [true, false, true, false, true, false]

B. [1, 3, 5]

C. [2, 4, 6]

D. Compilation error due to filter usage

Solution

1. Step 1: Understand the function reference with filter

   The ::isEven passes the isEven function to filter, which keeps elements where the function returns true.

2. Step 2: Apply isEven to each number and filter

   Numbers 2, 4, and 6 are even, so they remain in the filtered list.

3. Step 3: Explain usefulness of function references

   Function references let us pass existing functions easily without writing extra lambdas.

4. Final Answer:

   [2, 4, 6] -> Option C

5. Quick Check:

   filter(::isEven) keeps even numbers [OK]

Hint: filter(::function) keeps items where function returns true [OK]

Common Mistakes:

• Confusing filter with map output
• Expecting boolean list instead of filtered list
• Not using :: for function reference