Type checking with is operator in Kotlin - Step-by-Step Execution

Concept Flow - Type checking with is operator

Start with variable

↓

Check: variable is Type?

No→Else branch or skip

Yes↓

Execute code for that type

↓

Continue or end

The program checks if a variable is a certain type using 'is'. If yes, it runs code for that type; if no, it skips or runs else.

Execution Sample

Kotlin

fun checkType(x: Any?) {
    if (x is String) {
        println("String of length ${x.length}")
    } else {
        println("Not a String")
    }
}

This function checks if x is a String and prints its length; otherwise, it prints 'Not a String'.

Execution Table

Step	Variable x	Condition (x is String)	Branch Taken	Output
1	Hello	true	if branch	String of length 5
2	123	false	else branch	Not a String
3	true	false	else branch	Not a String
4	Kotlin	true	if branch	String of length 6
5	-	false	else branch	Not a String

💡 All inputs checked; function ends after printing output.

Variable Tracker

Variable x	Start	After 1	After 2	After 3	After 4	After 5
x	Hello	Hello	123	true	Kotlin	-

Key Moments - 2 Insights

Why does the code inside 'if (x is String)' run only for some inputs?

Because the 'is' operator checks the actual type of x at runtime. Only when x is a String (rows 1 and 4) does the if branch run, as shown in the execution_table.

What happens if x is null? Does 'x is String' return true?

No, 'x is String' returns false if x is null, so the else branch runs (row 5). This is because null is not considered a String.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when x = 123?

A Not a String

B String of length 3

C Error

D No output

💡 Hint

Check row 2 in execution_table where x is 123 and condition is false.

At which step does the condition 'x is String' become true?

A Step 2

B Step 1

C Step 3

D Step 5

💡 Hint

Look at the Condition column in execution_table for true values.

If x was changed to an Int value 10, what would be the output?

A String of length 2

B Error

C Not a String

D No output

💡 Hint

Refer to how non-String types like true or 123 produce 'Not a String' in execution_table.

Concept Snapshot

Type checking with 'is' operator in Kotlin:
- Syntax: if (variable is Type) { ... }
- Checks if variable is of given Type at runtime
- Runs code inside if only if true
- Else branch runs if false
- Null is not considered any type

Full Transcript

This visual trace shows how Kotlin's 'is' operator checks a variable's type at runtime. The function checkType takes any value x. It tests if x is a String. If yes, it prints the string length. If no, it prints 'Not a String'. The execution table shows different inputs: 'Hello' and 'Kotlin' are Strings, so the if branch runs and prints length. Inputs like 123, true, and null are not Strings, so else branch runs. The variable tracker shows how x changes each step. Key moments clarify why only Strings pass the check and that null is not a String. The quiz tests understanding of outputs and condition results. This helps beginners see how type checking controls program flow in Kotlin.