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.

Practice

1. What does the is operator do in Kotlin?

easy

A. Checks if a variable is of a specific type

B. Changes the type of a variable

C. Creates a new variable

D. Deletes a variable

Solution

1. Step 1: Understand the purpose of is operator

   The is operator is used to check if a variable holds a value of a certain type.

2. Step 2: Differentiate from other options

   It does not change, create, or delete variables; it only checks type.

3. Final Answer:

   Checks if a variable is of a specific type -> Option A

4. Quick Check:

   Type check = A [OK]

Hint: Remember: is means 'is this type?' check [OK]

Common Mistakes:

• Thinking is changes the variable type
• Confusing is with variable creation
• Assuming is deletes variables

2. Which of the following is the correct syntax to check if obj is a String in Kotlin?

easy

A. if (obj instanceof String)

B. if (obj is String)

C. if (obj type String)

D. if (obj == String)

Solution

1. Step 1: Recall Kotlin syntax for type checking

   Kotlin uses is keyword to check types, not instanceof or other keywords.

2. Step 2: Match the correct syntax

   The correct syntax is if (obj is String).

3. Final Answer:

   if (obj is String) -> Option B

4. Quick Check:

   Correct syntax = C [OK]

Hint: Use is keyword, not Java's instanceof [OK]

Common Mistakes:

• Using Java syntax instanceof in Kotlin
• Writing obj type String which is invalid
• Comparing with == instead of type check

3. What will be the output of this Kotlin code?

fun checkType(x: Any) {
    if (x is Int) {
        println(x + 5)
    } else {
        println("Not an Int")
    }
}

checkType(10)
checkType("Hello")

medium

A. Error at runtime

B. 10
Not an Int

C. 15
Hello5

D. 15
Not an Int

Solution

1. Step 1: Analyze the first call checkType(10)

   10 is an Int, so inside the if block, x + 5 equals 15, printed as "15".

2. Step 2: Analyze the second call checkType("Hello")

   "Hello" is not an Int, so else block runs printing "Not an Int".

3. Final Answer:

   15 Not an Int -> Option D

4. Quick Check:

   Int check prints sum, else prints message = A [OK]

Hint: If is true, use variable as that type inside block [OK]

Common Mistakes:

• Assuming string "Hello" can be added to 5
• Expecting runtime error instead of else output
• Confusing output order

4. Identify the error in this Kotlin code snippet:

fun printLength(obj: Any) {
    if (obj is String) {
        println(obj.length())
    }
}

medium

A. No error, code is correct

B. Missing else block after if

C. Using parentheses with length property

D. Incorrect use of is operator

Solution

1. Step 1: Check usage of length on String

   In Kotlin, length is a property, not a function, so it should not have parentheses.

2. Step 2: Confirm other parts are correct

   The is operator is used correctly and else block is optional.

3. Final Answer:

   Using parentheses with length property -> Option C

4. Quick Check:

   Property access no parentheses = D [OK]

Hint: Remember: Kotlin properties have no parentheses [OK]

Common Mistakes:

• Adding parentheses to properties like length
• Thinking else block is mandatory
• Misusing is operator syntax

5. Given a list of Any elements, how can you create a new list containing only the strings using the is operator in Kotlin?

hard

A. val strings = list.filterIsInstance<String>()

B. val strings = list.filter { it is String }

C. val strings = list.map { if (it is String) it else null }

D. val strings = list.filter { it == String }

Solution

1. Step 1: Understand filtering by type

   To get only strings, Kotlin provides filterIsInstance<String>() which filters elements by type safely.

2. Step 2: Evaluate other options

   val strings = list.filter { it is String } filters by is String but keeps type as Any, not cast to String. val strings = list.map { if (it is String) it else null } maps to nullable list, not filtering. val strings = list.filter { it == String } compares with type itself, which is invalid.

3. Final Answer:

   val strings = list.filterIsInstance<String>() -> Option A

4. Quick Check:

   Use filterIsInstance for type filtering [OK]

Hint: Use filterIsInstance to get typed list quickly [OK]

Common Mistakes:

• Using filter without casting
• Mapping to nullable instead of filtering
• Comparing element to type object