Collection size and emptiness checks in Kotlin - Step-by-Step Execution

Concept Flow - Collection size and emptiness checks

Start with a collection

↓

Check if collection is empty?

↓

Return true

↓

Return true/false

This flow shows how Kotlin checks if a collection is empty or has elements by using isEmpty(), isNotEmpty(), or size properties.

Execution Sample

Kotlin

val list = listOf(1, 2, 3)
println(list.isEmpty())
println(list.isNotEmpty())
println(list.size > 0)

This code checks if the list is empty or not and prints the results.

Execution Table

Step	Expression	Evaluation	Result
1	list.isEmpty()	Checks if list has no elements	false
2	list.isNotEmpty()	Checks if list has at least one element	true
3	list.size > 0	Checks if size is greater than zero	true
4	End	All checks done	Execution stops

💡 All checks completed, results show list is not empty.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
list	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
list.isEmpty()	N/A	false	false	false	false
list.isNotEmpty()	N/A	N/A	true	true	true
list.size	N/A	N/A	N/A	3	3
list.size > 0	N/A	N/A	N/A	true	true

Key Moments - 2 Insights

Why does list.isEmpty() return false but list.isNotEmpty() return true?

Because list has elements, isEmpty() returns false (not empty), and isNotEmpty() returns true (has elements). See execution_table rows 1 and 2.

Is checking list.size > 0 the same as list.isNotEmpty()?

Yes, both check if the collection has elements. list.size > 0 returns true if there is at least one element, same as isNotEmpty(). See execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of list.isEmpty() at step 1?

A false

B null

C true

D error

💡 Hint

Check the 'Result' column in execution_table row 1.

At which step does the code confirm the list has elements?

A Step 1

B Step 4

C Step 2

D Step 3

💡 Hint

Look for where isNotEmpty() returns true in execution_table.

If the list was empty, what would list.size > 0 return?

A true

B false

C null

D error

💡 Hint

Think about size of empty list and check execution_table logic.

Concept Snapshot

Kotlin collections have methods to check emptiness:
- isEmpty() returns true if no elements
- isNotEmpty() returns true if at least one element
- size gives number of elements
Use these to quickly check collection state.

Full Transcript

This visual execution shows how Kotlin checks collection emptiness. Starting with a list of three elements, isEmpty() returns false because the list has items. isNotEmpty() returns true confirming presence of elements. Checking size > 0 also returns true. Variables remain unchanged during checks. Key moments clarify why isEmpty and isNotEmpty return opposite booleans and confirm size > 0 is equivalent to isNotEmpty. The quiz tests understanding of these results referencing the execution table. This helps beginners see step-by-step how Kotlin evaluates collection size and emptiness.

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()