Collection size and emptiness checks in Kotlin - Mini Project: Build & Apply

Collection size and emptiness checks

📖 Scenario: You are managing a small library database. You want to keep track of books available in different genres. You will create a collection of books, check how many books are in the collection, and find out if the collection is empty or not.

🎯 Goal: Build a Kotlin program that creates a collection of books, sets a threshold for minimum books, checks the collection size, and verifies if the collection is empty.

📋 What You'll Learn

Create a list of books with exact titles

Create a variable for minimum book count threshold

Check the size of the book list using size

Check if the book list is empty using isEmpty()

Use exact variable names as instructed

💡 Why This Matters

🌍 Real World

Managing collections of items like books, products, or users is common in apps and databases. Checking size and emptiness helps control flow and decisions.

💼 Career

Understanding collection properties like size and emptiness is essential for database queries, data validation, and application logic in software development.

1

Create a list of books

Create a list called books with these exact book titles: "Kotlin Basics", "Advanced Kotlin", "Database Essentials".

Kotlin

# Create a list called books with the exact titles
# Your code here

Hint

Use listOf() to create a list with the exact book titles.

2

Set minimum book count threshold

Create an integer variable called minBooks and set it to 3.

Kotlin

val books = listOf("Kotlin Basics", "Advanced Kotlin", "Database Essentials")
# Create an integer variable minBooks and set it to 3
# Your code here

Hint

Use val to create a constant integer variable.

3

Check the size of the book list

Create an integer variable called bookCount and set it to the size of the books list using books.size.

Kotlin

val books = listOf("Kotlin Basics", "Advanced Kotlin", "Database Essentials")
val minBooks = 3
# Create an integer variable bookCount and set it to books.size
# Your code here

Hint

Use the size property of the list to get the number of items.

4

Check if the book list is empty

Create a boolean variable called isEmpty and set it to books.isEmpty() to check if the list is empty.

Kotlin

val books = listOf("Kotlin Basics", "Advanced Kotlin", "Database Essentials")
val minBooks = 3
val bookCount = books.size
# Create a boolean variable isEmpty and set it to books.isEmpty()
# Your code here

Hint

Use the isEmpty() function to check if the list has no items.

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()