Collection size and emptiness checks in Kotlin

Introduction

We check if a collection has items or is empty to decide what to do next in our program.

When you want to see if a list has any elements before processing it.

Before showing data from a collection, check if it is empty to show a message instead.

To avoid errors by making sure a collection is not empty before accessing its items.

When you want to count how many items are in a collection to display or use that number.

Syntax

Kotlin

collection.isEmpty()
collection.isNotEmpty()
collection.size
collection.count()

Use isEmpty() to check if the collection has no elements.

Use isNotEmpty() to check if the collection has one or more elements.

Examples

Check if the list has elements or not.

Kotlin

val list = listOf(1, 2, 3)
println(list.isEmpty())  // false
println(list.isNotEmpty())  // true

Check an empty list returns true for isEmpty.

Kotlin

val emptyList = emptyList<String>()
println(emptyList.isEmpty())  // true
println(emptyList.isNotEmpty())  // false

Get the number of items in the list using size or count.

Kotlin

val fruits = listOf("apple", "banana", "cherry")
println(fruits.size)  // 3
println(fruits.count())  // 3

Sample Program

This program checks if the list 'numbers' has items and prints the count or a message if empty.

Kotlin

fun main() {
    val numbers = listOf(10, 20, 30)
    if (numbers.isNotEmpty()) {
        println("The list has ${numbers.size} items.")
    } else {
        println("The list is empty.")
    }
}

Important Notes

Using isEmpty() and isNotEmpty() is clearer and safer than comparing size to zero.

Both size and count() return the number of elements; size is usually faster.

Checking emptiness helps avoid errors when accessing elements in a collection.

Summary

Use isEmpty() to check if a collection has no elements.

Use isNotEmpty() to check if it has one or more elements.

Use size or count() to find how many items are in the collection.