Error Encyclopedia/Kotlin/ClassCastException: X cannot be cast to Y

🅺KotlinMed. SeverityCategory: Type Safety

ClassCastException: X cannot be cast to Y

Unsafe cast operator (as) fails due to incompatible runtime object type.

What the Error Means

Kotlin's unsafe cast operator `as` throws a `ClassCastException` if the target object cannot be cast to the specified type.

Why It Happens

Casting `anyValue as String` when `anyValue` is an `Int` or a non-matching type.

Code Comparison & Resolution

CastErr.kt

Broken

CastErr.kt

1fun main() {
2    val data: Any = 100
3    val text: String = data as String // Throws ClassCastException!
4    println(text)
5}

CastErrFixed.kt

Fixed

CastErrFixed.kt

1fun main() {
2    val data: Any = 100
3 
4    // Fix 1: Safe cast operator (as?) returns null on failure
5    val text: String? = data as? String
6    println("Text: $text") // Outputs: Text: null
7 
8    // Fix 2: Smart casting with 'is' check
9    if (data is String) {
10        println("Length: ${data.length}") // Kotlin smart-casts data to String
11    } else {
12        println("Data is not a String.")
13    }
14}

compiler_diagnostic_engine.log

Interactive Diagnostic Sandbox

Simulate standard system builds to trigger compiler trace records and track memory crashes locally.

Deep Dive Explanation

`data` holds an integer (`100`). Unsafely forcing `data as String` fails at runtime.

Step-by-Step Interactive Resolution Checklist

0 of 2 completed

Use safe cast operator `as?` which returns `null` instead of throwing an exception.

Use `is` type check for Kotlin's automatic smart casting.

Proactivity & Tips to Avoid This Error

Avoid raw unsafe `as` casts on dynamic objects.

← Previous ErrorUninitializedPropertyAccessException: lateinit property X has not been initialized Next Error →Type mismatch: inferred type is X but Y was expected

Back to all Kotlin errors

Error Encyclopedia/Kotlin/ClassCastException: X cannot be cast to Y

🅺KotlinMed. SeverityCategory: Type Safety

ClassCastException: X cannot be cast to Y

Unsafe cast operator (as) fails due to incompatible runtime object type.

What the Error Means

Kotlin's unsafe cast operator `as` throws a `ClassCastException` if the target object cannot be cast to the specified type.

Why It Happens

Casting `anyValue as String` when `anyValue` is an `Int` or a non-matching type.

Code Comparison & Resolution

CastErr.kt

Broken

CastErr.kt

1fun main() {
2    val data: Any = 100
3    val text: String = data as String // Throws ClassCastException!
4    println(text)
5}

CastErrFixed.kt

Fixed

CastErrFixed.kt

1fun main() {
2    val data: Any = 100
3 
4    // Fix 1: Safe cast operator (as?) returns null on failure
5    val text: String? = data as? String
6    println("Text: $text") // Outputs: Text: null
7 
8    // Fix 2: Smart casting with 'is' check
9    if (data is String) {
10        println("Length: ${data.length}") // Kotlin smart-casts data to String
11    } else {
12        println("Data is not a String.")
13    }
14}

compiler_diagnostic_engine.log

Interactive Diagnostic Sandbox

Simulate standard system builds to trigger compiler trace records and track memory crashes locally.

Deep Dive Explanation

`data` holds an integer (`100`). Unsafely forcing `data as String` fails at runtime.

Step-by-Step Interactive Resolution Checklist

0 of 2 completed

Use safe cast operator `as?` which returns `null` instead of throwing an exception.

Use `is` type check for Kotlin's automatic smart casting.

Proactivity & Tips to Avoid This Error

Avoid raw unsafe `as` casts on dynamic objects.

← Previous ErrorUninitializedPropertyAccessException: lateinit property X has not been initialized Next Error →Type mismatch: inferred type is X but Y was expected

Back to all Kotlin errors