Extension properties in Kotlin - Mini Project: Build & Apply

Using Extension Properties in Kotlin

📖 Scenario: Imagine you want to add a new way to get the full name of a person without changing the original Person class. Kotlin's extension properties let you do this easily.

🎯 Goal: Build a Kotlin program that uses an extension property to get the full name of a Person object by combining first and last names.

📋 What You'll Learn

Create a Person class with firstName and lastName properties

Create an extension property called fullName for the Person class

Use the fullName extension property to print the full name of a person

💡 Why This Matters

🌍 Real World

Extension properties let you add new features to existing classes, like adding a new way to get a full name, without changing the original code. This is useful when working with libraries or code you cannot modify.

💼 Career

Many Kotlin developers use extension properties to write cleaner and more readable code by extending functionality in a simple way. This skill is valuable for Android development and backend Kotlin projects.

1

Create the Person class

Create a Kotlin class called Person with two val properties: firstName and lastName, both of type String. Initialize them using the primary constructor.

Kotlin

class Person(val firstName: String, val lastName: String) {
    // Your code here
}

Hint

Use class Person(val firstName: String, val lastName: String) to define the class with properties.

2

Add an extension property for fullName

Create an extension property called fullName for the Person class that returns a String. It should combine firstName and lastName separated by a space.

Kotlin

class Person(val firstName: String, val lastName: String)

// Define the extension property fullName here
// Your code here

Hint

Use val Person.fullName: String get() = "$firstName $lastName" to create the extension property.

3

Create a Person object and use fullName

Create a variable called person of type Person with firstName set to "John" and lastName set to "Doe". Then create a variable called name and assign it the value of person.fullName.

Kotlin

class Person(val firstName: String, val lastName: String)

val Person.fullName: String
    get() = "$firstName $lastName"

// Create person and assign fullName to name
// Your code here

Hint

Create person with Person("John", "Doe") and assign person.fullName to name.

4

Print the full name

Write a println statement to print the value of the variable name.

Kotlin

class Person(val firstName: String, val lastName: String)

val Person.fullName: String
    get() = "$firstName $lastName"

val person = Person("John", "Doe")
val name = person.fullName

// Print the full name
// Your code here

Hint

Use println(name) to display the full name.

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