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.

Practice

1. What is the main purpose of extension functions in Kotlin?

easy

A. To create new classes that inherit from built-in types

B. To add new functions to existing types without modifying their source code

C. To override existing functions in built-in types

D. To convert one type to another automatically

Solution

1. Step 1: Understand extension functions concept

   Extension functions allow adding new functions to existing types without changing their original code.

2. Step 2: Compare options with concept

   Only To add new functions to existing types without modifying their source code correctly describes this purpose. Options B and C talk about inheritance and overriding, which are different concepts. To convert one type to another automatically is unrelated.

3. Final Answer:

   To add new functions to existing types without modifying their source code -> Option B

4. Quick Check:

   Extension functions add new behavior without changing original code [OK]

Hint: Remember: extensions add functions, not change classes [OK]

Common Mistakes:

• Confusing extension functions with inheritance
• Thinking extensions override existing methods
• Believing extensions convert types automatically

2. Which of the following is the correct syntax to define an extension function isEven for Int in Kotlin?

easy

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

B. fun isEven(): Boolean { return this % 2 == 0 }

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

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

Solution

1. Step 1: Recall extension function syntax

   Extension functions are declared as fun TypeName.functionName(). Here, Int.isEven() is correct.

2. Step 2: Check each option

   fun Int.isEven(): Boolean { return this % 2 == 0 } matches the correct syntax. fun isEven(): Boolean { return this % 2 == 0 } misses the receiver type. fun Int.isEven: Boolean = this % 2 == 0 misses parentheses. fun isEven(Int): Boolean = Int % 2 == 0 has wrong parameter syntax.

3. Final Answer:

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

4. Quick Check:

   Extension syntax = fun TypeName.functionName() [OK]

Hint: Extension syntax always starts with fun TypeName.functionName() [OK]

Common Mistakes:

• Omitting the receiver type before function name
• Missing parentheses after function name
• Using parameter instead of receiver for extension

3. What will be the output of this Kotlin code?

fun String.addExclamation() = this + "!"

fun main() {
    val word = "Hello"
    println(word.addExclamation())
}

medium

A. Hello

B. Hello!!

C. Compilation error

D. Hello!

Solution

1. Step 1: Understand the extension function behavior

   The extension function addExclamation adds an exclamation mark to the string.

2. Step 2: Trace the main function output

   Calling word.addExclamation() returns "Hello!" which is printed.

3. Final Answer:

   Hello! -> Option D

4. Quick Check:

   Extension adds "!" so output is "Hello!" [OK]

Hint: Extension functions act like normal methods on the type [OK]

Common Mistakes:

• Expecting original string unchanged
• Thinking extension adds multiple exclamations
• Assuming syntax error due to extension

4. Identify the error in this Kotlin extension function and fix it:

fun Int.isPositive {
    return this > 0
}

medium

A. Missing parentheses after function name; should be fun Int.isPositive()

B. Extension functions cannot return Boolean

C. The receiver type Int is invalid for extensions

D. The return statement is not allowed in extension functions

Solution

1. Step 1: Check function declaration syntax

   Extension functions must have parentheses after the function name, even if no parameters.

2. Step 2: Identify the fix

   Add parentheses: fun Int.isPositive(). Other options are incorrect because Boolean return is allowed, Int is valid receiver, and return statements are allowed.

3. Final Answer:

   Missing parentheses after function name; should be fun Int.isPositive() -> Option A

4. Quick Check:

   Extension functions need () after name [OK]

Hint: Always add () after extension function name, even if no parameters [OK]

Common Mistakes:

• Omitting parentheses after function name
• Thinking extensions can't return values
• Confusing receiver type validity

5. You want to create an extension function wordCount for String that returns the number of words separated by spaces. Which implementation correctly handles multiple spaces and empty strings?

hard

A. fun String.wordCount() = this.split(" ").size

B. fun String.wordCount() = this.count { it == ' ' }

C. fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size

D. fun String.wordCount() = this.length

Solution

1. Step 1: Understand the problem requirements

   We need to count words separated by spaces, ignoring extra spaces and empty strings.

2. Step 2: Analyze each option

   fun String.wordCount() = this.split(" ").size splits by space but counts empty strings if multiple spaces exist. fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size trims, splits, filters out empty strings, then counts, correctly handling multiple spaces and empty strings. fun String.wordCount() = this.length returns string length, unrelated. fun String.wordCount() = this.count { it == ' ' } counts spaces, not words.

3. Final Answer:

   fun String.wordCount() = this.trim().split(" ").filter { it.isNotEmpty() }.size -> Option C

4. Quick Check:

   Trim, split, filter empty = correct word count [OK]

Hint: Trim and filter empty strings after split to count words correctly [OK]

Common Mistakes:

• Counting empty strings as words
• Using string length instead of word count
• Counting spaces instead of words