Collection size and emptiness checks in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What Kotlin function checks if a collection has no elements?

The isEmpty() function returns true if the collection contains no elements.

beginner

How do you check if a Kotlin collection has at least one element?

Use the isNotEmpty() function, which returns true if the collection contains one or more elements.

beginner

What property gives the number of elements in a Kotlin collection?

The size property returns the count of elements in the collection.

intermediate

Why is it better to use isEmpty() instead of size == 0 in Kotlin?

Using isEmpty() is clearer and can be more efficient because it may avoid counting all elements if the collection supports fast emptiness checks.

beginner

What will listOf(1, 2, 3).isNotEmpty() return?

It returns true because the list contains three elements.

Which Kotlin function checks if a collection is empty?

A isEmpty()

B isNotEmpty()

C hasElements()

D count()

The isEmpty() function returns true if the collection has no elements.

What does collection.size return in Kotlin?

A The last element of the collection

B The number of elements in the collection

C The first element of the collection

D True if collection is empty

size gives the count of elements in the collection.

Which function returns true if a Kotlin collection has one or more elements?

A isEmpty()

B hasElements()

C isNotEmpty()

D count()

isNotEmpty() returns true if the collection is not empty.

Why might isEmpty() be preferred over size == 0?

A It can be more efficient and clearer

B It is less readable

C It always returns false

D It counts elements twice

isEmpty() can be more efficient and expresses intent clearly.

What is the result of emptyList<Int>().isNotEmpty()?

A Throws an error

B true

C null

D false

An empty list returns false for isNotEmpty().

Explain how to check if a Kotlin collection is empty or not.

Think about functions that return true or false based on collection content.

Describe why using isEmpty() might be better than checking size == 0 in Kotlin collections.

Consider both code clarity and how the collection might be implemented.

Practice

1. Which Kotlin function checks if a collection has no elements?

easy

A. count()

B. isNotEmpty()

C. size()

D. isEmpty()

Solution

1. Step 1: Understand the purpose of isEmpty()

   This function returns true if the collection has zero elements.

2. Step 2: Compare with other functions

   isNotEmpty() checks for one or more elements, count() and size return the number of elements but do not return a boolean.

3. Final Answer:

   isEmpty() -> Option D

4. Quick Check:

   Check empty collection with isEmpty() = true [OK]

Hint: Use isEmpty() to check for no elements [OK]

Common Mistakes:

• Confusing isEmpty() with isNotEmpty()
• Using count() to check emptiness
• Calling size() as a function instead of property

2. Which of the following is the correct syntax to check if a list named items is not empty in Kotlin?

easy

A. if (items.isNotEmpty()) { ... }

B. if (items.count > 0) { ... }

C. if (items.size() > 0) { ... }

D. if (items.isEmpty) { ... }

Solution

1. Step 1: Identify correct function call syntax

   In Kotlin, isNotEmpty() is a function and requires parentheses.

2. Step 2: Check other options for syntax errors

   isEmpty without parentheses is invalid, size is a property, not a function, so size() is wrong, and count is a function, so count > 0 is invalid.

3. Final Answer:

   if (items.isNotEmpty()) { ... } -> Option A

4. Quick Check:

   Use isNotEmpty() with parentheses [OK]

Hint: Remember isNotEmpty() needs parentheses [OK]

Common Mistakes:

• Using property syntax without parentheses for functions
• Calling size as a function
• Using count without parentheses

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3, 4)
println(numbers.size)
println(numbers.isEmpty())
println(numbers.isNotEmpty())

medium

A. 4
true
false

B. 3
true
false

C. 4
false
true

D. Error: size is not a property

Solution

1. Step 1: Check the size of the list

   The list has 4 elements, so numbers.size returns 4.

2. Step 2: Evaluate emptiness checks

   numbers.isEmpty() returns false because the list is not empty.
   numbers.isNotEmpty() returns true because the list has elements.

3. Final Answer:

   4 false true -> Option C

4. Quick Check:

   Size = 4, empty? false, not empty? true [OK]

Hint: Remember size counts elements; isEmpty() returns false if not empty [OK]

Common Mistakes:

• Thinking size is zero-based index
• Confusing isEmpty() and isNotEmpty() results
• Assuming size is a function, causing error

4. Identify the error in this Kotlin code snippet:

val data = listOf<String>()
if (data.isEmpty) {
    println("No data")
}

medium

A. Missing parentheses after isEmpty

B. listOf<String>() cannot be empty

C. println syntax is incorrect

D. data should be mutable

Solution

1. Step 1: Check usage of isEmpty

   isEmpty is a function and must be called with parentheses: isEmpty().

2. Step 2: Verify other parts of the code

   listOf<String>() creates an empty list correctly, println syntax is correct, and mutability is not required for checking emptiness.

3. Final Answer:

   Missing parentheses after isEmpty -> Option A

4. Quick Check:

   Functions need parentheses even if no arguments [OK]

Hint: Always add () when calling functions like isEmpty() [OK]

Common Mistakes:

• Using property syntax for functions
• Thinking empty list creation is invalid
• Assuming mutability is needed for checks

5. You have a list of user names and want to print how many users are online only if the list is not empty. Which Kotlin code snippet correctly does this?

hard

A. if (users.isEmpty()) println("Online users: ${users.size}")

B. if (users.size > 0) println("Online users: ${users.size}")

C. if (users.count == 0) println("Online users: ${users.count}")

D. if (users.isNotEmpty) println("Online users: ${users.size}")

Solution

1. Step 1: Check condition for non-empty list

   Using users.size > 0 correctly checks if the list has one or more elements.

2. Step 2: Verify printing statement

   The print statement uses string interpolation to show the number of users online.

3. Step 3: Analyze incorrect options

   isEmpty() checks for empty, so if (users.isEmpty()) println("Online users: ${users.size}") prints when empty, which is wrong.
   count is a function, so count == 0 is invalid.
   isNotEmpty without parentheses is incorrect syntax.

4. Final Answer:

   if (users.size > 0) println("Online users: ${users.size}") -> Option B

5. Quick Check:

   Check size > 0 for non-empty list [OK]

Hint: Use size > 0 or isNotEmpty() with parentheses [OK]

Common Mistakes:

• Using isEmpty() to check for non-empty
• Calling count without parentheses
• Omitting parentheses on isNotEmpty()