Safe casts with as? in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely cast 'obj' to String using 'as?'.

Kotlin

val str = obj [1] String

Drag options to blanks, or click blank then click option'

Aas

Bis

Cas?

Dto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' which throws an exception if the cast fails.

Using 'is' which checks type but does not cast.

Using 'to' which is not a cast operator.

The safe cast operator as? tries to cast the object and returns null if it fails, avoiding exceptions.

2fill in blank

medium

Complete the code to safely cast 'item' to Int and assign to 'number'.

Kotlin

val number: Int? = item [1] Int

Drag options to blanks, or click blank then click option'

Aas?

Bis

Cas

Dto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' which throws an exception if cast fails.

Using 'is' which only checks type but does not cast.

Using 'to' which is not a cast operator.

Using as? safely casts 'item' to Int or returns null if it cannot be cast.

3fill in blank

hard

Fix the error in the code by replacing the cast operator with a safe cast.

Kotlin

val text = data [1] String

Drag options to blanks, or click blank then click option'

Aas?

Bis

Cas

Dto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' which throws exceptions on failure.

Using 'is' which does not cast but only checks type.

Replacing as with as? prevents exceptions by returning null if the cast fails.

4fill in blank

hard

Fill both blanks to create a safe cast and check if the result is null.

Kotlin

val result = value [1] String
if (result [2] null) {
    println("Cast succeeded")
}

Drag options to blanks, or click blank then click option'

Aas?

B==

C!=

Das

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' instead of 'as?' causing exceptions.

Using '==' instead of '!=' to check for null.

Use as? for safe cast and != to check if the cast did not return null.

5fill in blank

hard

Fill all three blanks to safely cast 'input' to Double, check if not null, and print the value.

Kotlin

val number = input [1] Double
if (number [2] null) {
    println("Number is: [3]")
}

Drag options to blanks, or click blank then click option'

Aas?

B!=

Cnumber

Das

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'as' instead of 'as?'.

Checking '==' null instead of '!=' null.

Printing a wrong variable name.

Use as? for safe cast, != to check not null, and print the variable number.

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