Extension properties in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an extension property isEven for Int that returns true if the number is even.

Kotlin

val Int.isEven: Boolean
    get() = this [1] 2 == 0

Drag options to blanks, or click blank then click option'

A*

B/

C+

D%

Attempts:

3 left

💡 Hint

Common Mistakes

Using division (/) instead of modulus (%)

Using multiplication (*) or addition (+) operators

The modulus operator (%) returns the remainder. If the remainder when divided by 2 is 0, the number is even.

2fill in blank

medium

Complete the code to declare an extension property lastChar for String that returns the last character.

Kotlin

val String.lastChar: Char
    get() = this[1]

Drag options to blanks, or click blank then click option'

A.last()

B[0]

C.first()

D[length - 1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 0 which returns the first character

Using length as an index which is out of bounds

The last() function returns the last character of the string.

3fill in blank

hard

Fix the error in the extension property that returns the first half of a string.

Kotlin

val String.firstHalf: String
    get() = this.substring(0, this.length [1] 2)

Drag options to blanks, or click blank then click option'

A+

B/

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition (+) or subtraction (-) instead of division

Using multiplication (*) which increases length

To get the first half, divide the length by 2 to get the midpoint index.

4fill in blank

hard

Fill the blank to create an extension property isPalindrome for String that checks if the string reads the same forwards and backwards.

Kotlin

val String.isPalindrome: Boolean
    get() = this [1] this.reversed()

Drag options to blanks, or click blank then click option'

A==

B!=

C&&

D||

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality (!=) which gives wrong result

Using logical AND (&&) or OR (||) which are not comparison operators

To check palindrome, compare the string with its reversed version using equality (==).

5fill in blank

hard

Fill all three blanks to create an extension property wordCount for String that returns the number of words separated by spaces.

Kotlin

val String.wordCount: Int
    get() = this.[1](" ").[2] [3] 0

Drag options to blanks, or click blank then click option'

Asplit

Bsize

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using minus (-) instead of plus (+) which can reduce count incorrectly

Using wrong functions like join or length

Split the string by spaces, get the size of the resulting list, and add 0 (no change) to get the count.

Practice

1. What is an extension property in Kotlin?

easy

A. A property that replaces an existing property in a class

B. A property that stores data inside the original class

C. A property that can only be used inside the class it is declared

D. A property added to an existing class without modifying its source code

Solution

1. Step 1: Understand extension properties concept

   Extension properties allow adding new properties to classes without changing their original code.

2. Step 2: Identify correct description

   "A property added to an existing class without modifying its source code" correctly describes this as adding properties externally without modifying the class.

3. Final Answer:

   A property added to an existing class without modifying its source code -> Option D

4. Quick Check:

   Extension property = added externally [OK]

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

Common Mistakes:

• Thinking extension properties store data inside the class
• Confusing extension properties with member properties
• Believing extension properties can replace existing ones

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

easy

A. fun Int.isEven() = this % 2 == 0

B. val Int.isEven get() = this % 2 == 0

C. var Int.isEven = this % 2 == 0

D. val isEven: Int get() = this % 2 == 0

Solution

1. Step 1: Recall extension property syntax

   Extension properties use val/var Type.propertyName get() = ... without a body for setter if read-only.

2. Step 2: Check options

   val Int.isEven get() = this % 2 == 0 correctly uses val Int.isEven get() = this % 2 == 0. var Int.isEven = this % 2 == 0 wrongly uses var without setter. fun Int.isEven() = this % 2 == 0 is a function, not a property. val isEven: Int get() = this % 2 == 0 misses receiver type.

3. Final Answer:

   val Int.isEven get() = this % 2 == 0 -> Option B

4. Quick Check:

   Extension property syntax = val Type.prop get() [OK]

Hint: Extension properties need a getter, no setter if val [OK]

Common Mistakes:

• Using var without setter for extension property
• Writing extension property as a function
• Omitting receiver type before property name

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

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

fun main() {
    val word = "hello"
    println(word.firstChar)
}

medium

A. firstChar

B. hello

C. h

D. Compilation error

Solution

1. Step 1: Understand extension property behavior

   The extension property firstChar returns the first character of the string using this[0].

2. Step 2: Trace the code output

   For word = "hello", word.firstChar returns 'h', which is printed.

3. Final Answer:

   h -> Option C

4. Quick Check:

   Extension property returns first char = h [OK]

Hint: Extension property uses receiver object like member [OK]

Common Mistakes:

• Expecting full string instead of first character
• Confusing property with function call
• Thinking extension properties cause errors

4. Identify the error in this Kotlin extension property declaration:

var String.lastChar: Char
    get() = this[length - 1]
    set(value) { this[length - 1] = value }

medium

A. You cannot assign to a character in a String

B. The getter syntax is incorrect

C. Extension properties cannot have setters

D. The property must be declared inside the class

Solution

1. Step 1: Review extension property setter rules

   Extension properties can have setters, but they cannot modify the original object if it is immutable.

2. Step 2: Check String mutability

   Strings in Kotlin are immutable; you cannot assign to this[length - 1]. This causes a compilation error.

3. Final Answer:

   You cannot assign to a character in a String -> Option A

4. Quick Check:

   Strings immutable = cannot set chars [OK]

Hint: Strings are immutable; extension setters can't modify them [OK]

Common Mistakes:

• Assuming extension properties can store data
• Thinking extension setters always cause errors
• Confusing syntax errors with immutability issues

5. You want to add an extension property isAdult to the Person class that returns true if age is 18 or more, otherwise false. Given data class Person(val name: String, val age: Int), which is the correct extension property implementation?

hard

A. val Person.isAdult get() = age >= 18

B. var Person.isAdult get() = age >= 18; set(value) { age = if(value) 18 else 0 }

C. fun Person.isAdult() = age >= 18

D. val isAdult: Person get() = age >= 18

Solution

1. Step 1: Define read-only extension property

   Since age is a val, we can only create a read-only extension property with a getter.

2. Step 2: Check options for correct syntax and logic

   val Person.isAdult get() = age >= 18 correctly defines val Person.isAdult get() = age >= 18. var Person.isAdult get() = age >= 18; set(value) { age = if(value) 18 else 0 } tries to set age which is immutable. fun Person.isAdult() = age >= 18 is a function, not a property. val isAdult: Person get() = age >= 18 has wrong syntax and receiver placement.

3. Final Answer:

   val Person.isAdult get() = age >= 18 -> Option A

4. Quick Check:

   Read-only extension property with getter = val Person.isAdult get() = age >= 18 [OK]

Hint: Use val with getter for read-only extension properties [OK]

Common Mistakes:

• Trying to create setter for immutable property
• Confusing extension property with extension function
• Incorrect syntax for receiver type in property