Smart casts in when and if in Kotlin

Introduction

Smart casts let Kotlin automatically convert a variable to a specific type after you check it. This makes your code simpler and safer.

When you want to check a variable's type and then use it as that type without extra code.

When you use an if statement to check if a variable is a certain type and want to access its properties directly.

When you use a when expression to handle different types of a variable cleanly.

When you want to avoid writing manual casts and keep your code easy to read.

Syntax

Kotlin

if (variable is Type) {
    // variable is smart cast to Type here
}

when (variable) {
    is Type1 -> {
        // variable is smart cast to Type1 here
    }
    is Type2 -> {
        // variable is smart cast to Type2 here
    }
    else -> {
        // variable is not any of the above types
    }
}

Kotlin automatically casts the variable inside the if or when block after checking its type.

You don't need to use explicit casts like variable as Type inside these blocks.

Examples

Here, obj is checked if it is a String. Inside the if block, Kotlin lets you use obj as a String directly.

Kotlin

val obj: Any = "Hello"
if (obj is String) {
    println(obj.length) // smart cast to String
}

The when expression checks the type of obj. Each branch smart casts obj to the checked type.

Kotlin

val obj: Any = 123
when (obj) {
    is Int -> println(obj + 1) // smart cast to Int
    is String -> println(obj.length)
    else -> println("Unknown type")
}

Sample Program

This program uses a when expression with smart casts to handle different types of obj. It prints a description based on the type.

Kotlin

fun describe(obj: Any): String {
    return when (obj) {
        is Int -> "Integer: ${obj + 1}"
        is String -> "String of length ${obj.length}"
        else -> "Unknown type"
    }
}

fun main() {
    println(describe(10))
    println(describe("Kotlin"))
    println(describe(3.14))
}

Important Notes

Smart casts only work if the variable cannot change between the check and usage (like val or local variables).

If the variable is mutable or a property with a custom getter, Kotlin may not smart cast it.

Summary

Smart casts let you use variables as a specific type after checking their type.

They work automatically inside if and when blocks.

This feature makes your Kotlin code cleaner and safer.

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