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.