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.