Kotlin : 100 Interview Questions and Answers PART15

71. The difference between “==” and “===” operators in Kotlin

In Kotlin, == and === are different:

== → Structural Equality

Checks if two values are equal (like .equals() in Java).

val a = "Bind"
val b = "Bind"
println(a == b)  // ✅ true → because values are equal

Kotlin internally calls:

a?.equals(b) ?: (b == null)

=== → Referential Equality

Checks if two references point to the same object in memory.

val a = "Bind"
val b = "Bind"
println(a === b)  // May be true due to string interning

val x = String("Bind".toCharArray())
val y = String("Bind".toCharArray())
println(x == y)   // ✅ true → values equal
println(x === y)  // ❌ false → different objects in memory

72. The purpose of the operator modifier in Kotlin

In Kotlin, the operator modifier allows you to define or overload operators (like +, -, [], in, ==, etc.) for your custom types (classes).

Why Use operator?

It enables natural, readable syntax when working with custom objects by letting you write:

val sum = a + b  // instead of a.plus(b)

Example: Overloading + Operator

data class Point(val x: Int, val y: Int) {
    operator fun plus(other: Point): Point {
        return Point(x + other.x, y + other.y)
    }
}

val p1 = Point(1, 2)
val p2 = Point(3, 4)
val result = p1 + p2  // Calls operator fun plus()

Common Operators You Can Overload

73. Explain the use of the @JvmOverloads annotation in Kotlin

The @JvmOverloads annotation is used in Kotlin to automatically generate overloaded methods for Java interoperability when calling Kotlin code from Java.

Problem It Solves

Kotlin supports default parameter values, but Java does not. So if you write this in Kotlin:

fun greet(name: String, message: String = "Hello") {
    println("$message, $name!")
}

You can call:

greet("John")           // OK in Kotlin
greet("John", "Hi")     // OK

But from Java, you must pass all parameters, since Java doesn’t understand default values.

Solution with @JvmOverloads

@JvmOverloads
fun greet(name: String, message: String = "Hello") {
    println("$message, $name!")
}

This generates the following overloads for Java:

greet("John");              // Uses default message
greet("John", "Hi");        // Uses provided message

When to Use @JvmOverloads

74. The difference between lateinit and lazy initialization in Kotlin

75. The purpose of the const modifier in Kotlin

In Kotlin, the const modifier is used to declare compile-time constants.

What is a const in Kotlin?

• - const means the value is known at compile-time, not runtime.
• - It is used with val (not var)
• - Must be top-level or in an object or companion object
• - Can only hold primitive types (Int, String, etc.)

Syntax:

const val PI = 3.14

Not Allowed Inside Functions

fun example() {
    // ❌ This will cause a compile error
    const val name = "Bind the Logic"
}

Allowed Locations

const val APP_NAME = "BindTheLogic" // ✅ Top-level

object Config {
    const val VERSION = "1.0"       // ✅ In object
}

class MyClass {
    companion object {
        const val TAG = "MyClassTag" // ✅ In companion object
    }
}

Why Use const?

val vs const val