Kotlin Interview Preparation

# CHAPTER 28

Kotlin Interview Preparation #

1. Chapter Introduction #

2. Core Concepts & Syntax #

Q2. Explain the difference between val and var. *Answer:* var declares a mutable variable, meaning its value can be reassigned. val declares a read-only (immutable) variable. Its reference cannot be reassigned once initialized. Modern Kotlin strongly encourages using val to prevent side effects and create thread-safe code.

Q3. What is Type Inference? *Answer:* Type Inference is the compiler's ability to deduce the data type of a variable from the assigned value at compile time (e.g., val score = 100 is inferred as an Int). This removes the need for explicit type declarations like int score = 100.

3. Null Safety #

Q5. Explain the Elvis Operator (?:). *Answer:* The Elvis operator provides a default fallback value if an expression evaluates to null. For example, val name = user?.name ?: "Guest". If user?.name is null, the variable is assigned "Guest".

Q6. What is the !! operator and why is it discouraged? *Answer:* The Not-Null Assertion operator (!!) forces the compiler to treat a nullable type as non-nullable. If the variable happens to be null at runtime, it will crash the application with an NPE. It defeats Kotlin's null safety mechanism and should generally be avoided.

4. Object-Oriented Programming #

Q8. What is a Data Class? *Answer:* A data class is a concise way to create classes primarily used for holding data. By prepending the data keyword, the compiler automatically generates equals(), hashCode(), toString(), copy(), and componentN() functions, saving dozens of lines of boilerplate code.

Q9. What is a Sealed Class? *Answer:* A sealed class is an advanced Enum. It restricts class hierarchies by ensuring all subclasses are defined within the same file. This allows the compiler to know every possible state, providing exhaustive checking when used with a when expression (e.g., ensuring you handle Loading, Success, and Error network states without needing an else branch).

5. Advanced Functions #

Q11. What is a Higher-Order Function? *Answer:* A function that either takes another function as a parameter, returns a function, or both. They are the backbone of functional programming in Kotlin, allowing for powerful collection operations like .map and .filter.

Q12. What does the it keyword represent? *Answer:* Inside a Lambda expression that takes exactly ONE parameter, it is an implicit name for that parameter. It saves developers from having to explicitly declare the parameter name (e.g., list.filter { it > 5 }).

6. Concurrency (Coroutines) #

Q14. What does the suspend keyword do? *Answer:* It marks a function as a Coroutine function. It indicates that the function can pause its execution (yield the thread) and resume later. suspend functions can only be called from within a coroutine or another suspend function.

Q15. Explain launch vs async. *Answer:* Both are coroutine builders. launch is "fire and forget"—it starts a background task and returns a Job object, but does not return a result. async is used when you expect a result; it returns a Deferred object, and you must call .await() to retrieve the data.

7. Common Coding Challenge: FizzBuzz #

fun fizzBuzz() {
    for (i in 1..100) {
        when {
            i % 15 == 0 -> println("FizzBuzz")
            i % 3 == 0 -> println("Fizz")
            i % 5 == 0 -> println("Buzz")
            else -> println(i)
        }
    }
}

8. Summary #

9. Next Chapter Recommendation #