Safe casts with as? in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It tries to cast a value to a specified type. If the cast is not possible, it returns null instead of throwing an exception.

beginner

How is as? different from the regular as cast in Kotlin?

as throws a ClassCastException if the cast fails, while as? returns null safely without crashing the program.

beginner

What will be the result of val x: String? = obj as? String if obj is an Int?

The result will be null because obj cannot be safely cast to String.

intermediate

Why is using as? useful in Kotlin programming?

It helps avoid program crashes by safely handling type casts that might fail, making code more robust and easier to maintain.

beginner

Write a simple Kotlin code snippet using as? to safely cast an Any type to String.

val obj: Any = 123 val str: String? = obj as? String println(str) // prints null because obj is not a String

What does val x = obj as? String return if obj is an Int?

A null

B Throws ClassCastException

C The integer value

D Empty string

as? returns null if the cast fails instead of throwing an exception.

Which operator in Kotlin safely casts a value and returns null if it fails?

A as?

B as

C is

D !!

as? is the safe cast operator that returns null on failure.

What happens if you use as to cast an incompatible type?

A Returns null

B Returns default value

C Throws ClassCastException

D Compiles but ignores cast

The regular as operator throws a ClassCastException if the cast is invalid.

Which of these is a benefit of using as??

A Forces non-null values

B Improves performance by skipping checks

C Automatically converts types

D Avoids program crashes on bad casts

as? avoids crashes by returning null instead of throwing exceptions.

If val y = obj as? String and y is null, what does that mean?

A The cast succeeded

B <code>obj</code> was not a String

C <code>obj</code> was a String

D The program crashed

Null means the cast failed because obj was not of type String.

Explain how the safe cast operator as? works in Kotlin and why it is useful.

Think about what happens when a cast might fail.

Write a Kotlin example using as? to safely cast an Any variable to a String and handle the null case.

Try to cast and then check if the result is null before using it.

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