Safe casts with as? in Kotlin - Time & Space Complexity

Time Complexity: Safe casts with as?

O(n)

Understanding Time Complexity

We want to understand how long it takes to perform safe casts using as? in Kotlin.

Specifically, how does the time grow when casting different objects?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun safeCastExample(obj: Any): String? {
    return obj as? String
}

fun main() {
    val items = listOf(1, "hello", 3.14, "world")
    for (item in items) {
        println(safeCastExample(item))
    }
}
    

This code tries to safely cast each item in a list to a String using as?.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Safe cast check obj as? String inside the loop.
• How many times: Once per item in the list, so as many times as the list size.

How Execution Grows With Input

Each item requires one safe cast check. So if the list doubles in size, the total checks double too.

Input Size (n)	Approx. Operations
10	10 safe cast checks
100	100 safe cast checks
1000	1000 safe cast checks

Pattern observation: The number of operations grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to safely cast all items grows linearly with the number of items.

Common Mistake

[X] Wrong: "Safe casts are instant and do not depend on input size."

[OK] Correct: Each safe cast must check the type of the object, so more items mean more checks and more time.

Interview Connect

Understanding how safe casts scale helps you reason about type checks in real apps, showing you can think about performance even in simple operations.

Self-Check

"What if we replaced the list with a nested list and tried safe casting on inner elements? How would the time complexity change?"

Practice

1.

What does the safe cast operator as? do in Kotlin?

easy

A. It converts a value to a string representation.

B. It forces a cast and throws an exception if it fails.

C. It tries to cast a value to a type and returns null if it fails.

D. It checks if a value is null without casting.

Solution

1. Step 1: Understand the safe cast operator

   The as? operator attempts to cast a value to a specified type safely.

2. Step 2: Behavior on failure

   If the cast is not possible, instead of throwing an error, it returns null.

3. Final Answer:

   It tries to cast a value to a type and returns null if it fails. -> Option C

4. Quick Check:

   Safe cast = returns null on failure [OK]

Hint: Safe cast returns null if cast fails, no crash [OK]

Common Mistakes:

• Confusing safe cast with forced cast
• Thinking it throws an exception on failure
• Assuming it converts to string automatically

2.

Which of the following is the correct syntax for a safe cast in Kotlin?

val x = obj ___ String

easy

A. as?

B. to

C. is

D. as

Solution

1. Step 1: Identify safe cast syntax

   In Kotlin, the safe cast operator is as? which tries to cast safely.

2. Step 2: Match syntax to code

   The code val x = obj as? String attempts to cast obj to String safely.

3. Final Answer:

   as? -> Option A

4. Quick Check:

   Safe cast syntax = as? [OK]

Hint: Safe cast uses as? with question mark [OK]

Common Mistakes:

• Using as without question mark causes exception
• Confusing is (type check) with as? (cast)
• Using to which is for conversions, not casting

3.

What will be the output of this Kotlin code?

val obj: Any = 123
val str: String? = obj as? String
println(str)

medium

A. null

B. ClassCastException

C. 123

D. "123"

Solution

1. Step 1: Analyze the cast attempt

   Variable obj holds an integer 123, but we try to cast it to String using safe cast as?.

2. Step 2: Safe cast behavior

   Since obj is not a String, the safe cast returns null instead of throwing an error.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Safe cast fails = null output [OK]

Hint: Safe cast returns null if types mismatch [OK]

Common Mistakes:

• Expecting ClassCastException instead of null
• Thinking it converts integer to string automatically
• Confusing safe cast with forced cast

4.

Identify the error in this Kotlin code snippet and fix it:

val obj: Any = "Hello"
val num: Int = obj as? Int
println(num)

medium

A. Replace as? with as

B. Change val num: Int to val num: Int?

C. Cast obj to String instead of Int

D. No error, code runs fine

Solution

1. Step 1: Analyze variable types and cast

   The safe cast obj as? Int returns null if cast fails, so the result is nullable Int?.

2. Step 2: Fix type mismatch

   Declaring num as non-nullable Int causes a type mismatch error. Changing it to Int? fixes this.

3. Final Answer:

   Change val num: Int to val num: Int? -> Option B

4. Quick Check:

   Safe cast returns nullable type [OK]

Hint: Safe cast result must be nullable type [OK]

Common Mistakes:

• Using non-nullable type for safe cast result
• Replacing safe cast with forced cast causing exceptions
• Ignoring type mismatch errors

5.

You have a list of Any objects: listOf(1, "two", 3.0, "four"). You want to create a new list containing only the strings using safe casts. Which Kotlin code correctly does this?

hard

A. val strings = list.mapNotNull { it as String? }

B. val strings = list.map { it as String }

C. val strings = list.filter { it is String }

D. val strings = list.map { it as? String }.filterNotNull()

Solution

1. Step 1: Understand the goal

   We want to safely cast each element to String and keep only successful casts.

2. Step 2: Analyze options

   val strings = list.map { it as? String }.filterNotNull() uses safe cast as? which returns null for non-strings, then filters out null values with filterNotNull(). This correctly produces a list of strings.

3. Step 3: Why others are incorrect

   val strings = list.map { it as String } forces cast and will throw exception on non-strings. val strings = list.filter { it is String } filters by type but does not cast, so type is still Any. val strings = list.mapNotNull { it as String? } tries to cast with forced cast inside mapNotNull which will throw exception.

4. Final Answer:

   val strings = list.map { it as? String }.filterNotNull() -> Option D

5. Quick Check:

   Safe cast + filterNotNull = filtered strings [OK]

Hint: Use as? with filterNotNull to keep only successful casts [OK]

Common Mistakes:

• Using forced cast causing exceptions
• Filtering type without casting
• Misusing mapNotNull with forced cast