Smart casts in when and if in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable is a String using is.

Kotlin

fun checkType(x: Any) {
    if (x [1] String) {
        println("It's a string of length ${x.length}")
    }
}

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

A==

Bas

Cis

Din

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'is' for type checking.

Using 'as' which is for casting, not checking.

In Kotlin, is checks if a variable is of a certain type and smart casts it inside the block.

2fill in blank

medium

Complete the when expression to smart cast obj to Int.

Kotlin

fun describe(obj: Any): String = when (obj) {
    is [1] -> "Integer with value $obj"
    else -> "Unknown"
}

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

AInt

BString

CDouble

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing String or Double instead of Int.

Forgetting that is is needed for smart cast.

The when expression smart casts obj to Int when using is Int.

3fill in blank

hard

Fix the error by completing the if condition to smart cast value to String.

Kotlin

fun printLength(value: Any) {
    if (value [1] String) {
        println(value.length)
    }
}

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

Ais

B==

Cas

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which compares values, not types.

Using 'as' which is unsafe casting.

Using is smart casts value to String inside the if block, allowing access to length.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that includes only strings longer than 3 characters.

Kotlin

val words = listOf("cat", "house", "dog", "elephant")
val result = words.associateWith { it.length }.filter { it.key [1] 3 && it.key [2] String }
println(result)

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

A>

Bis

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for length comparison.

Using '==' instead of 'is' for type checking.

The filter keeps entries where the key is a String and its length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a when expression that smart casts and returns descriptions.

Kotlin

fun describeInput(input: Any): String = when {
    input [1] String -> "String of length ${input.length}"
    input [2] Int -> "Integer value $input"
    input [3] Boolean -> "Boolean value $input"
    else -> "Unknown type"
}

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

Ais

B==

C!=

Das

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' or '!=' instead of 'is' for type checking.

Using 'as' which casts but can throw exceptions.

Using is in when smart casts input to the checked type inside each branch.

Practice

1. What does Kotlin's smart cast feature do inside an if or when block?

easy

A. Disables type checking inside the block

B. Requires explicit casting of variables after type checks

C. Automatically treats a variable as a specific type after a type check

D. Converts all variables to strings inside the block

Solution

1. Step 1: Understand smart cast purpose

   Smart casts let Kotlin automatically treat a variable as a more specific type after you check its type.

2. Step 2: Apply to if/when blocks

   Inside if or when, Kotlin knows the variable's type and lets you use it without manual casting.

3. Final Answer:

   Automatically treats a variable as a specific type after a type check -> Option C

4. Quick Check:

   Smart cast = automatic type use after check [OK]

Hint: Smart casts auto-change type after check in if/when [OK]

Common Mistakes:

• Thinking manual cast is always needed
• Assuming smart casts disable type checks
• Believing variables convert to string automatically

2. Which of the following is the correct syntax to use smart casts in a when expression for variable obj?

easy

A. when obj { is String -> println(obj.length) else -> println("Not a string") }

B. when (obj) { obj is String -> println(obj.length) else -> println("Not a string") }

C. when (obj) { String -> println(obj.length) else -> println("Not a string") }

D. when (obj) { is String -> println(obj.length) else -> println("Not a string") }

Solution

1. Step 1: Check correct when syntax

   The when expression requires parentheses around the variable: when (obj).

2. Step 2: Use type check correctly

   The correct type check is is String without repeating variable name inside the block.

3. Final Answer:

   when (obj) { is String -> println(obj.length) else -> println("Not a string") } -> Option D

4. Quick Check:

   Correct when syntax uses parentheses and is [OK]

Hint: Use parentheses and 'is' keyword in when [OK]

Common Mistakes:

• Omitting parentheses around variable
• Using variable name inside when branches
• Using type name without 'is' keyword

3. What will be the output of this Kotlin code?

fun test(obj: Any) {
    when (obj) {
        is String -> println(obj.length)
        is Int -> println(obj + 1)
        else -> println("Unknown")
    }
}

test("Hello")
test(5)
test(3.14)

medium

A. 5
6
7

B. 5
6
Unknown

C. Hello
6
Unknown

D. 5
5
Unknown

Solution

1. Step 1: Analyze each call

   For test("Hello"), obj is String, so prints length 5.
   For test(5), obj is Int, so prints 5 + 1 = 6.
   For test(3.14), no matching type, prints "Unknown".

2. Step 2: Confirm output lines

   Outputs are:
   5
   6
   Unknown

3. Final Answer:

   5 6 Unknown -> Option B

4. Quick Check:

   Smart cast uses type to choose output [OK]

Hint: Match type in when to get output [OK]

Common Mistakes:

• Confusing string length with string itself
• Adding 1 to string instead of int
• Expecting default else to print number

4. Identify the error in this Kotlin code using smart casts:

fun check(obj: Any) {
    if (obj is String) {
        println(obj.length)
    } else if (obj is Int) {
        println(obj + 1)
    } else {
        println(obj.length)
    }
}

medium

A. Trying to access length on obj in else block without type check

B. Missing parentheses in if condition

C. Using else if instead of when

D. Smart casts do not work inside if blocks

Solution

1. Step 1: Review else block usage

   In the else block, obj can be any type, but code tries to access obj.length which is invalid for non-String types.

2. Step 2: Understand smart cast limits

   Smart casts only apply after type checks. Here, no type check before accessing length in else block causes error.

3. Final Answer:

   Trying to access length on obj in else block without type check -> Option A

4. Quick Check:

   Access property only after type check [OK]

Hint: Check type before accessing properties [OK]

Common Mistakes:

• Assuming else block has smart cast automatically
• Confusing else if with when
• Thinking smart casts fail inside if blocks

5. Given a list of Any objects, write a Kotlin function using when with smart casts to return a list of strings describing each element's type and value. What is the correct implementation?

fun describeList(items: List<Any>): List<String> {
    return items.map { item ->
        when (item) {
            is String -> "String of length ${'$'}{item.length}"
            is Int -> "Int with value ${'$'}item"
            else -> "Unknown type"
        }
    }
}

hard

A. Correct as is, smart casts allow using item.length and item directly

B. Needs explicit casts like (item as String).length and (item as Int)

C. Cannot use smart casts inside lambda expressions

D. Must use if-else instead of when for smart casts

Solution

1. Step 1: Check smart cast usage in when

   The when expression smart casts item to String or Int automatically after type check.

2. Step 2: Confirm lambda and map usage

   Inside the lambda, smart casts work normally, so accessing item.length and item is valid without explicit casts.

3. Final Answer:

   Correct as is, smart casts allow using item.length and item directly -> Option A

4. Quick Check:

   Smart casts work inside lambdas and when [OK]

Hint: Smart casts work inside lambdas and when blocks [OK]

Common Mistakes:

• Adding unnecessary explicit casts
• Thinking smart casts don't work in lambdas
• Replacing when with if-else unnecessarily