Smart casts in when and if in Kotlin - Step-by-Step Execution

Concept Flow - Smart casts in when and if

Start

↓

Check type with if/when

↓

Smart cast variable

↓

Use variable as casted type

↓

End

The program checks a variable's type using if or when, then automatically treats it as that type inside the block.

Execution Sample

Kotlin

fun demo(x: Any) {
  if (x is String) {
    println(x.length)
  } else {
    println("Not a string")
  }
}

Checks if x is a String, then uses x as String inside if block without explicit cast.

Execution Table

Step	Condition	Condition Result	Smart Cast Applied	Action	Output
1	x is String	True	x as String	Print x.length	5
2	x is String	False	No cast	Print "Not a string"	Not a string

💡 Execution ends after printing output based on type check.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	Any ("Hello")	Smart cast to String	No cast	Any/String depending on branch

Key Moments - 2 Insights

Why can we use x.length without casting inside the if block?

Because the condition 'x is String' is true at step 1, Kotlin smart casts x to String automatically inside that block, as shown in the execution_table.

What happens if the condition 'x is String' is false?

At step 2, the condition is false, so no smart cast happens and x remains Any, so we cannot use String properties safely.

Visual Quiz - 3 Questions

Look at the execution table, what is the smart cast applied at step 1?

A No cast

B x as Int

C x as String

D x as Any

💡 Hint

Check the 'Smart Cast Applied' column at step 1 in the execution_table.

At which step does the program print "Not a string"?

A Step 2

B Both steps

C Step 1

D Never

💡 Hint

Look at the 'Output' column in the execution_table for each step.

If x was an Int, what would happen to the smart cast in the if block?

A x would be smart cast to String

B No smart cast would happen

C x would be smart cast to Int

D Program would crash

💡 Hint

Refer to the 'Condition Result' column in the execution_table and how smart casts depend on it.

Concept Snapshot

Smart casts let Kotlin treat a variable as a specific type after checking with if or when.
Syntax: if (x is Type) { use x as Type }
No explicit cast needed inside the block.
If condition false, no cast happens.
Smart casts improve safety and readability.

Full Transcript

This example shows how Kotlin uses smart casts in if and when statements. When the program checks if variable x is a String, and the condition is true, Kotlin automatically treats x as a String inside the if block. This means you can use String properties like length without casting. If the condition is false, no smart cast happens, and x remains its original type. The execution table shows two steps: one where x is a String and smart cast applies, printing the length; and one where x is not a String, so it prints "Not a string". This helps beginners understand how Kotlin safely narrows types without extra code.

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