Data class copy and destructuring in Kotlin - Mini Project: Build & Apply

Data class copy and destructuring

📖 Scenario: You are managing a small library system. Each book has a title, author, and year of publication.

🎯 Goal: You will create a data class for books, make a copy of a book with a changed year, and use destructuring to get the book details.

📋 What You'll Learn

Create a data class called Book with title, author, and year properties

Create an instance of Book called originalBook with exact values

Create a copy of originalBook called updatedBook with the year changed

Use destructuring to get title, author, and year from updatedBook

Print the destructured values in the format: Title: [title], Author: [author], Year: [year]

💡 Why This Matters

🌍 Real World

Data classes and copying are useful when you want to keep original data safe but create modified versions, like updating book info without changing the original record.

💼 Career

Understanding data classes, copying, and destructuring is important for Kotlin developers working on apps that manage data efficiently and clearly.

1

Create the Book data class and originalBook instance

Create a data class called Book with title (String), author (String), and year (Int) properties. Then create a variable called originalBook and set it to Book("The Hobbit", "J.R.R. Tolkien", 1937).

Kotlin

// Create the data class Book and the originalBook instance
// Your code here

Hint

Use data class keyword to create the class. Then create originalBook with the exact values.

2

Create a copy of originalBook with a new year

Create a variable called updatedBook by copying originalBook and changing the year to 1951 using the copy function.

Kotlin

data class Book(val title: String, val author: String, val year: Int)

val originalBook = Book("The Hobbit", "J.R.R. Tolkien", 1937)

// Create updatedBook by copying originalBook with year 1951
// Your code here

Hint

Use originalBook.copy(year = 1951) to create the new book.

3

Destructure updatedBook into variables

Use destructuring to create variables title, author, and year from updatedBook.

Kotlin

data class Book(val title: String, val author: String, val year: Int)

val originalBook = Book("The Hobbit", "J.R.R. Tolkien", 1937)

val updatedBook = originalBook.copy(year = 1951)

// Destructure updatedBook into title, author, and year
// Your code here

Hint

Use val (title, author, year) = updatedBook to get the values.

4

Print the destructured book details

Print the book details using println in this exact format: Title: [title], Author: [author], Year: [year].

Kotlin

data class Book(val title: String, val author: String, val year: Int)

val originalBook = Book("The Hobbit", "J.R.R. Tolkien", 1937)

val updatedBook = originalBook.copy(year = 1951)

val (title, author, year) = updatedBook

// Print the book details in the required format
// Your code here

Hint

Use println with an f-string style using $ to insert variables.

Practice

1.

What does the copy() function do in a Kotlin data class?

easy

A. Changes the original object directly.

B. Deletes the original object and creates a new one.

C. Creates a new object with the same properties, optionally changing some.

D. Converts the object to a string representation.

Solution

1. Step 1: Understand the purpose of copy()

   The copy() function in Kotlin data classes creates a new instance with the same property values.

2. Step 2: Recognize optional property changes

   You can specify new values for some properties while copying, leaving others unchanged.

3. Final Answer:

   Creates a new object with the same properties, optionally changing some. -> Option C

4. Quick Check:

   copy() = new object with changes [OK]

Hint: Remember: copy() clones with optional changes [OK]

Common Mistakes:

• Thinking copy() modifies the original object
• Confusing copy() with toString()
• Assuming copy() deletes the original

2.

Which of the following is the correct syntax to copy a Kotlin data class Person and change only the age property?

data class Person(val name: String, val age: Int)

easy

A. val newPerson = person.copy(age = 30)

B. val newPerson = person.copy(30)

C. val newPerson = copy(person, age = 30)

D. val newPerson = person.copy(name = "John")

Solution

1. Step 1: Recall the copy() syntax

   The copy() function uses named arguments to change specific properties.

2. Step 2: Identify correct usage

   Using person.copy(age = 30) changes only the age, keeping name same.

3. Final Answer:

   val newPerson = person.copy(age = 30) -> Option A

4. Quick Check:

   copy() uses named args for changes [OK]

Hint: Use named arguments inside copy() to change properties [OK]

Common Mistakes:

• Passing values without property names
• Using copy() as a standalone function
• Changing wrong property by mistake

3.

What is the output of this Kotlin code?

data class Point(val x: Int, val y: Int)

fun main() {
    val p1 = Point(5, 10)
    val (a, b) = p1
    val p2 = p1.copy(y = 20)
    println("a = $a, b = $b")
    println(p2)
}

medium

A. a = 5, b = 20
Point(x=5, y=10)

B. a = 5, b = 10
Point(x=5, y=20)

C. a = 10, b = 5
Point(x=5, y=20)

D. a = 5, b = 10
Point(x=5, y=10)

Solution

1. Step 1: Understand destructuring of p1

   Destructuring assigns a = p1.x = 5 and b = p1.y = 10.

2. Step 2: Understand copy() usage

   p2 is a copy of p1 with y changed to 20, so Point(x=5, y=20).

3. Final Answer:

   a = 5, b = 10 Point(x=5, y=20) -> Option B

4. Quick Check:

   Destructuring + copy() = expected values [OK]

Hint: Destructure original, copy with changed property [OK]

Common Mistakes:

• Mixing up destructured values
• Assuming copy changes original
• Printing wrong variable

4.

Find the error in this Kotlin code snippet:

data class User(val name: String, val age: Int)

fun main() {
    val user1 = User("Alice", 25)
    val (n, a) = user1.copy(age = 30)
    println("Name: $n, Age: $a")
}

medium

A. Variable names in destructuring must match property names.

B. Destructuring cannot be done on copy() result.

C. copy() requires all properties to be specified.

D. No error; code runs and prints updated age.

Solution

1. Step 1: Check copy() usage

   The copy() function is correctly called with age = 30.

2. Step 2: Check destructuring

   Destructuring works on the copy result, assigning n = "Alice" and a = 30.

3. Final Answer:

   No error; code runs and prints updated age. -> Option D

4. Quick Check:

   copy() + destructuring works fine [OK]

Hint: copy() returns data class instance usable for destructuring [OK]

Common Mistakes:

• Thinking destructuring fails on copy() result
• Believing copy() needs all properties
• Confusing variable names with property names

5.

Given this data class and list, how do you create a new list of Person objects with age increased by 1, using copy() and destructuring?

data class Person(val name: String, val age: Int)

val people = listOf(Person("Tom", 20), Person("Sue", 25))

hard

A. people.map { it.copy(age = it.age + 1) }

B. people.forEach { it.copy(age = it.age + 1) }

C. people.copy { age += 1 }

D. people.map { (n, a) -> Person(n, a + 1) }

Solution

1. Step 1: Understand list transformation

   We want to create a new list with each person's age increased by 1.

2. Step 2: Use copy() inside map()

   Using map with it.copy(age = it.age + 1) creates new objects with updated age.

3. Final Answer:

   people.map { it.copy(age = it.age + 1) } -> Option A

4. Quick Check:

   map + copy() = new list with changed ages [OK]

Hint: Use map with copy() to update properties in list [OK]

Common Mistakes:

• Using forEach which returns Unit, not a list
• Trying to copy the whole list directly
• Ignoring destructuring or copy() usage