Data class copy and destructuring in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Kotlin data class?

A Kotlin data class is a special class used to hold data. It automatically provides functions like equals(), hashCode(), toString(), and copy() to make working with data easier.

beginner

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

The copy() function creates a new object with the same properties as the original, but you can change some properties by passing new values.

beginner

How do you destructure a Kotlin data class?

You can destructure a data class by declaring multiple variables in one line using parentheses, like: val (a, b) = dataObject. This extracts properties in order.

intermediate

What happens if you change a property in the copy() function?

Only the properties you specify in copy() change. The rest keep their original values.

beginner

Why is destructuring useful with data classes?

Destructuring lets you quickly get individual properties from an object without calling getters or accessing properties one by one.

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

A The original object unchanged

B A new object with the same or updated properties

C A string representation of the object

D A list of all properties

copy() returns a new object with the same properties, allowing you to update some if you want.

How do you extract properties from a data class using destructuring?

A val x = dataObject.x; val y = dataObject.y

B dataObject.getProperties()

C val (x, y) = dataObject

D dataObject.copy()

Destructuring uses val (x, y) = dataObject to assign properties to variables in one line.

Which of these is NOT automatically generated for a Kotlin data class?

A equals()

B hashCode()

C copy()

D finalize()

finalize() is not generated automatically; equals(), hashCode(), and copy() are.

If you call copy() without arguments, what happens?

A A new identical object is created

B An error occurs

C The original object is modified

D Nothing happens

copy() without arguments creates a new object identical to the original.

What order does destructuring follow when extracting properties?

A The order of properties declared in the data class

B Alphabetical order of property names

C Random order

D Order of property values

Destructuring extracts properties in the order they are declared in the data class.

Explain how the copy() function works in Kotlin data classes and why it is useful.

Think about making a similar object but with small changes.

Describe how destructuring declarations help when working with Kotlin data classes.

Imagine unpacking a box into separate items quickly.

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