Safe casts with as? in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of safe cast with as? when cast succeeds?

Consider the following Kotlin code using safe cast as?. What will be printed?

Kotlin

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

A null

B Hello

C ClassCastException

D Compilation error

💡 Hint

Safe cast returns the object if it matches the type, else null.

The variable obj holds a String. Safe cast as? tries to cast obj to String. Since obj is already a String, the cast succeeds and str holds "Hello". So printing str outputs Hello.

❓ Predict Output

intermediate

What is the output of safe cast with as? when cast fails?

What will this Kotlin code print?

Kotlin

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

A null

B 123

C ClassCastException

D Compilation error

💡 Hint

Safe cast returns null if the object is not of the target type.

obj holds an Int, but we try to cast it safely to String. Since it is not a String, the safe cast returns null. So printing str outputs null.

🔧 Debug

advanced

Identify the error in unsafe cast without as? operator

What error will this Kotlin code produce at runtime?

Kotlin

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

A ClassCastException

B Compilation error

C null

D Prints 123

💡 Hint

Unsafe cast throws an exception if the cast is invalid.

The code uses unsafe cast as to cast an Int to String. This is invalid and causes a ClassCastException at runtime.

❓ Predict Output

advanced

What is the output when safe cast is used with a nullable type?

What will this Kotlin code print?

Kotlin

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

A ClassCastException

B Compilation error

C Empty string

D null

💡 Hint

Safe cast returns null if the object is null or not the target type.

obj is null. Safe cast as? returns null when casting null to any type. So str is null and printing it outputs null.

🧠 Conceptual

expert

Why use safe cast (as?) instead of unsafe cast (as)?

Which of the following best explains the advantage of using safe cast as? over unsafe cast as in Kotlin?

A Safe cast improves performance by skipping type checks.

B Safe cast always converts the object to the target type regardless of its actual type.

C Safe cast returns null instead of throwing an exception if the cast fails, preventing runtime crashes.

D Safe cast automatically converts null values to default instances of the target type.

💡 Hint

Think about what happens when a cast fails.

Safe cast as? returns null if the object cannot be cast to the target type, avoiding exceptions and crashes. Unsafe cast as throws ClassCastException on failure.

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