Equality (== structural vs === referential) in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of structural vs referential equality

What is the output of this Kotlin code snippet?

Kotlin

data class Person(val name: String)

fun main() {
    val p1 = Person("Alice")
    val p2 = Person("Alice")
    println(p1 == p2)
    println(p1 === p2)
}

A

false
false

B

false
true

C

true
true

D

true
false

💡 Hint

Remember that '==' checks structural equality and '===' checks if both variables point to the same object.

In Kotlin, '==' calls the equals() method, which for data classes compares properties (structural equality). '===' checks if both references point to the same object (referential equality). Since p1 and p2 have the same data but are different objects, p1 == p2 is true and p1 === p2 is false.

❓ Predict Output

intermediate

Equality with mutable objects

What will this Kotlin code print?

Kotlin

class Box(var value: Int)

fun main() {
    val b1 = Box(10)
    val b2 = Box(10)
    println(b1 == b2)
    println(b1 === b2)
}

A

true
true

B

true
false

C

false
false

D

false
true

💡 Hint

Default '==' for classes without overriding equals compares references.

Since Box is a regular class without overriding equals(), b1 == b2 compares references, which are different objects, so it returns false. Also, b1 === b2 is false because they are different objects.

🔧 Debug

advanced

Why does this equality check fail?

Consider this Kotlin code. Why does the equality check print false even though the objects have the same data?

Kotlin

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

fun main() {
    val p1 = Point(1, 2)
    val p2 = Point(1, 2)
    println(p1 == p2)
}

A Because Point class does not override equals(), so '==' compares references which are different.

B Because '==' always compares references in Kotlin, regardless of class.

C Because the data in p1 and p2 are different types.

D Because the println statement is missing parentheses.

💡 Hint

Check if the class overrides equals() or is a data class.

Regular classes in Kotlin do not override equals() by default. So p1 == p2 compares references, which are different objects, resulting in false. To compare data, use a data class or override equals().

🧠 Conceptual

advanced

Difference between == and === in Kotlin

Which statement correctly describes the difference between '==' and '===' in Kotlin?

A '==' checks structural equality by calling equals(), '===' checks referential equality (same object).

B '==' checks referential equality, '===' checks structural equality.

C '==' and '===' both check structural equality but in different ways.

D '==' and '===' are interchangeable and mean the same.

💡 Hint

Think about how Kotlin compares objects by content vs by reference.

In Kotlin, == calls the equals() method to check if two objects have the same content (structural equality). The === operator checks if two references point to the exact same object (referential equality).

❓ Predict Output

expert

Output with nullable types and equality

What is the output of this Kotlin code?

Kotlin

data class User(val id: Int)

fun main() {
    val u1: User? = User(5)
    val u2: User? = null
    println(u1 == u2)
    println(u1 === u2)
    println(u2 == null)
    println(u2 === null)
}

A

false
false
false
false

B

false
false
true
true

C

true
false
true
false

D

true
true
true
true

💡 Hint

Remember how Kotlin handles nulls with '==' and '==='.

u1 == u2 is false because u1 is a User object and u2 is null. u1 === u2 is false because they are not the same reference. u2 == null is true because u2 is null. u2 === null is also true because both references are null.