Extension functions in Android 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. It helps you extend functionality easily.

beginner

How do you declare an extension function in Kotlin?

You write the receiver type followed by a dot and the function name. For example: fun String.shout() = this.uppercase() + "!"

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 as a parameter.

intermediate

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

The member function always takes precedence over the extension function when called on an instance.

beginner

Why are extension functions useful in Android development?

They let you add helpful functions to Android classes like View or Context without subclassing, making code cleaner and easier to read.

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

A text.extensionFunction()

B extensionFunction(text)

C String.extensionFunction(text)

D call extensionFunction on String

Extension functions are called like regular functions on the object, e.g., text.extensionFunction().

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

A extend

B fun

C override

D newfun

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

Can extension functions modify the state of the object they extend?

A Yes, always

B No, they cannot modify state

C Only if the object is mutable

D Only if declared inside the class

Extension functions can modify the object if it is mutable, like a mutable list.

What is the receiver in an extension function?

A The parameter list

B The function name

C The return type

D The class being extended

The receiver is the type of the object the extension function is adding functionality to.

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

A Member function

B It causes a compile error

C Both are called

D Extension function

Member functions have higher priority and are called instead of extension functions.

Explain what an extension function is and how it can be declared in Kotlin.

Think about adding new functions to existing classes without changing them.

Describe the rules about access and precedence when using extension functions in Kotlin.

Consider what happens if a class already has a function with the same name.

Practice

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

easy

A. To override existing functions in a class

B. To create a new subclass from an existing class

C. To add new functions to existing classes without modifying them

D. To delete functions from a class

Solution

1. Step 1: Understand extension functions

   Extension functions let you add new functions to classes without changing their original code.

2. Step 2: Compare options

   Only To add new functions to existing classes without modifying them correctly describes this behavior; others describe subclassing, overriding, or deleting, which are different concepts.

3. Final Answer:

   To add new functions to existing classes without modifying them -> Option C

4. Quick Check:

   Extension function = Add new functions [OK]

Hint: Extension functions add features without changing original class [OK]

Common Mistakes:

• Confusing extension functions with inheritance
• Thinking extension functions override existing methods
• Believing extension functions can remove class methods

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

easy

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

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

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

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

Solution

1. Step 1: Check extension function syntax

   Extension functions are declared with fun ClassName.functionName(), so fun Int.isEven() is correct.

2. Step 2: Validate return type and body

   fun Int.isEven(): Boolean { return this % 2 == 0 } correctly uses parentheses and return type with a proper function body.

3. Final Answer:

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

4. Quick Check:

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

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

Common Mistakes:

• Omitting the receiver type before function name
• Missing parentheses after function name
• Incorrect return type declaration

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

fun String.addHello() = "Hello, $this!"

fun main() {
  val name = "Anna"
  println(name.addHello())
}

medium

A. Hello, Anna!

B. AnnaHello, !

C. Hello, !

D. Compilation error

Solution

1. Step 1: Understand the extension function

   The function addHello() adds "Hello, " before the string and an exclamation mark after.

2. Step 2: Apply function to variable

   Calling name.addHello() where name is "Anna" returns "Hello, Anna!".

3. Final Answer:

   Hello, Anna! -> Option A

4. Quick Check:

   Extension adds prefix and suffix correctly [OK]

Hint: Extension functions run like normal methods on instances [OK]

Common Mistakes:

• Misreading string interpolation syntax
• Assuming extension modifies original string
• Expecting output without exclamation mark

4. Identify the error in this Kotlin extension function code:

fun Int.isPositive() {
  return this > 0
}

medium

A. Function must declare a return type Boolean

B. The function body should not use return

C. The keyword 'this' cannot be used in extension functions

D. Extension function cannot be declared on Int

Solution

1. Step 1: Check function return type

   The function returns a Boolean expression but does not declare a return type, which is required.

2. Step 2: Confirm correct syntax

   Adding : Boolean after the function name fixes the error.

3. Final Answer:

   Function must declare a return type Boolean -> Option A

4. Quick Check:

   Return type needed for non-Unit functions [OK]

Hint: Declare return type when function returns a value [OK]

Common Mistakes:

• Omitting return type for Boolean-returning functions
• Thinking 'this' is disallowed in extensions
• Confusing Unit functions with value-returning ones

5. You want to add an extension function toTitleCase() to String that capitalizes the first letter of each word. Which implementation correctly achieves this?

hard

A. fun String.toTitleCase() = this.toUpperCase()

B. fun String.toTitleCase() = this.lowercase()

C. fun String.toTitleCase() = this.replaceFirstChar { it.uppercase() }

D. fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() }

Solution

1. Step 1: Understand title case requirement

   Title case means capitalizing the first letter of each word, not the whole string or just the first letter.

2. Step 2: Analyze options

   fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() } splits the string by spaces, capitalizes each word, then joins them back, correctly implementing title case.

3. Final Answer:

   fun String.toTitleCase() = split(" ").joinToString(" ") { it.capitalize() } -> Option D

4. Quick Check:

   Title case = capitalize each word [OK]

Hint: Split words, capitalize each, then join for title case [OK]

Common Mistakes:

• Using uppercase() on whole string instead of each word
• Capitalizing only first character of entire string
• Ignoring spaces between words