Smart casts in when and if in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a smart cast in Kotlin?

A smart cast is when Kotlin automatically converts a variable to a more specific type after checking its type with conditions like if or when, so you don't need to cast it manually.

beginner

How does Kotlin use smart casts inside an if statement?

Inside an if block, if you check a variable's type (e.g., if (x is String)), Kotlin automatically treats x as that type inside the block without needing a manual cast.

intermediate

Explain smart casts with a when expression.

When you use when to check a variable's type, Kotlin smart casts the variable to the matched type inside each branch, so you can use it directly as that type.

intermediate

Why can't Kotlin smart cast a variable if it is mutable (var) and can change?

If a variable is mutable (var) and can be changed elsewhere, Kotlin can't guarantee its type stays the same, so it won't smart cast it to avoid errors.

beginner

Show a simple Kotlin example using smart casts in an if statement.

fun printLength(obj: Any) { if (obj is String) { // obj is smart cast to String here println(obj.length) } else { println("Not a string") } }

What does Kotlin do after you check if (x is String) inside the if block?

A Throws an error if you use x as String

B Automatically treats x as String without manual cast

C Requires you to cast x manually

D Changes the type of x permanently

Kotlin smart casts x to String inside the if block after the type check.

Can Kotlin smart cast a var variable inside an if statement?

A Only if the variable is null

B Yes, always

C No, if the variable can change

D Only if the variable is a primitive type

Kotlin won't smart cast mutable variables if they can be changed elsewhere.

In a when expression, how does Kotlin handle smart casts?

A It smart casts the variable to the matched type in each branch

B It never smart casts in when expressions

C It requires manual casting in each branch

D It only smart casts if the variable is a String

Kotlin smart casts the variable to the matched type inside each when branch.

Which of these is a reason Kotlin might NOT smart cast a variable?

A The variable is immutable (val)

B The variable is a local variable

C The variable is checked with is operator

D The variable is mutable (var) and can be changed

If the variable is mutable and can change, Kotlin avoids smart casting to prevent errors.

What is the benefit of smart casts in Kotlin?

A You write less code by avoiding manual casts

B It makes the program slower

C It requires more manual type checks

D It disables type safety

Smart casts let you use variables as their checked type without extra casting code.

Explain how Kotlin smart casts work inside an if statement with a type check.

Think about how Kotlin treats the variable after confirming its type.

Describe why Kotlin might not smart cast a mutable variable and how this affects your code.

Consider what happens if the variable can change after the check.

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