Type checking with is operator in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the is operator do in Kotlin?

The is operator checks if a variable is of a specific type and returns true if it matches, otherwise false.

beginner

How do you check if a variable obj is a String in Kotlin?

Use if (obj is String) { ... }. This checks if obj is a String type.

intermediate

What happens to the variable inside the if block after checking with is?

Inside the if block, Kotlin smart casts the variable to the checked type automatically, so you can use it as that type without explicit casting.

beginner

How do you check if a variable is NOT a certain type using the is operator?

Use the negation operator: if (obj !is String) { ... } checks if obj is NOT a String.

intermediate

Can the is operator be used with nullable types?

Yes, but if the variable is null, is returns false because null is not an instance of any type.

What does obj is Int return if obj is 42?

A false

B true

C Throws an error

D Returns null

Since 42 is an Int, obj is Int returns true.

What is the result of obj !is String if obj is "hello"?

A false

B true

C Throws an error

D Returns null

Because obj is a String, obj !is String returns false.

Inside if (obj is String) { }, what type is obj treated as?

A String

B Nullable String

C Int

D Any

Kotlin smart casts obj to String inside the if block.

What does obj is String return if obj is null?

A Throws NullPointerException

B true

C false

D Returns null

Null is not an instance of any type, so is returns false.

Which operator checks if a variable is NOT a certain type?

A is

B not

C !==

D !is

The !is operator checks if a variable is NOT of a given type.

Explain how the is operator works in Kotlin and what happens to the variable inside the if block.

Think about how Kotlin lets you use the variable as the checked type without extra casting.

Describe how to check if a variable is NOT a certain type and what result you get if the variable is null.

Remember how null behaves with type checks and how to negate the check.

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