Smart casts in when and if in Kotlin - Time & Space Complexity

Time Complexity: Smart casts in when and if

O(n)

Understanding Time Complexity

We want to understand how the time needed to run code with smart casts in when and if changes as input grows.

How does checking types and branching affect the work done?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun describe(obj: Any): String {
    return when (obj) {
        is String -> "String of length ${obj.length}"
        is Int -> "Integer value $obj"
        else -> "Unknown type"
    }
}

fun checkAndPrint(list: List) {
    for (item in list) {
        if (item is String) {
            println("String: ${item.length}")
        } else {
            println("Not a string")
        }
    }
}
    

This code uses smart casts inside when and if to check types and then access properties safely.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping over each item in the list in checkAndPrint.
• How many times: Once for each element in the input list.

How Execution Grows With Input

Each item in the list is checked once to see if it is a String, then printed accordingly.

Input Size (n)	Approx. Operations
10	About 10 type checks and prints
100	About 100 type checks and prints
1000	About 1000 type checks and prints

Pattern observation: The work grows directly with the number of items; doubling items doubles work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the number of items checked.

Common Mistake

[X] Wrong: "Smart casts make the code run faster than just checking types manually, so time complexity is less than linear."

[OK] Correct: Smart casts help with safe access but do not reduce the need to check each item once, so the time still grows with input size.

Interview Connect

Understanding how type checks and smart casts affect performance shows you can reason about code efficiency and safety together, a useful skill in real projects.

Self-Check

What if we replaced the for loop with a recursive function? How would the time complexity change?

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