Type checking with is operator in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable obj is a String.

Kotlin

if (obj [1] String) {
    println("It's a string")
}

Drag options to blanks, or click blank then click option'

A==

Bequals

Cinstanceof

Dis

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'is' for type checking.

Using 'instanceof' which is Java syntax, not Kotlin.

The is operator in Kotlin checks if an object is of a specific type.

2fill in blank

medium

Complete the code to check if value is NOT an Int.

Kotlin

if (value ![1] Int) {
    println("Not an integer")
}

Drag options to blanks, or click blank then click option'

Ais

Attempts:

3 left

💡 Hint

Common Mistakes

Placing the exclamation mark after 'is' instead of before.

Using '!=' instead of '!is' for type checking.

The correct syntax to check if a value is NOT a type is to use !is.

3fill in blank

hard

Fix the error in the code to check if item is a Double.

Kotlin

if (item [1] Double) {
    println("It's a double")
}

Drag options to blanks, or click blank then click option'

A==

Bequals

Cis

Dinstanceof

Attempts:

3 left

💡 Hint

Common Mistakes

Using Java's 'instanceof' instead of Kotlin's 'is'.

Using '==' which compares values, not types.

Use the is operator to check the type in Kotlin.

4fill in blank

hard

Fill both blanks to check if data is a String and print its length.

Kotlin

if (data [1] String) {
    println(data[2]length)
}

Drag options to blanks, or click blank then click option'

Ais

B==

C?.

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'is' for type checking.

Using '.' or '?.' instead of '!!' which is appropriate after type check.

Use is to check type and non-null assertion operator !! to access length safely after type check.

5fill in blank

hard

Fill all three blanks to check if input is NOT a List and print a message.

Kotlin

if (input [1] [2] [3]) {
    println("Input is not a list")
}

Drag options to blanks, or click blank then click option'

Ais

BList

C)

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the exclamation mark to negate the check.

Not closing the if condition with a parenthesis.

Use '!is' to check NOT a type, specify 'List', and close the if condition with ')'.

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