How to Check if List Contains Element in Kotlin

In Kotlin, you can check if a list contains an element using the contains() function or the in keyword. Both return a Boolean value indicating if the element is present in the list.

Syntax

The two common ways to check if a list contains an element are:

• list.contains(element): Calls the contains() function on the list.
• element in list: Uses the in keyword for a more readable check.

Both return true if the element is found, otherwise false.

kotlin

val list = listOf("apple", "banana", "cherry")

// Using contains()
val hasBanana = list.contains("banana")

// Using 'in' keyword
val hasOrange = "orange" in list

Example

This example shows how to check if a list contains certain fruits using both contains() and in. It prints the results to the console.

kotlin

fun main() {
    val fruits = listOf("apple", "banana", "cherry")

    if (fruits.contains("banana")) {
        println("Banana is in the list.")
    } else {
        println("Banana is not in the list.")
    }

    if ("orange" in fruits) {
        println("Orange is in the list.")
    } else {
        println("Orange is not in the list.")
    }
}

Output

Banana is in the list.
Orange is not in the list.

Common Pitfalls

Some common mistakes when checking list membership include:

• Using == to compare the whole list instead of checking elements.
• Forgetting that contains() and in are case-sensitive for strings.
• Trying to check for an element in a null list without safe calls.

Always ensure the list is not null or use safe calls like list?.contains(element).

kotlin

val list: List<String>? = null

// Unsafe call - will cause error if list is null
// val hasApple = list.contains("apple") // Error

// Safe call to avoid error
val hasAppleSafe = list?.contains("apple") ?: false

Quick Reference

Method	Description	Example
contains()	Checks if list contains element	list.contains("item")
in keyword	Checks if element is in list	"item" in list
Safe call	Avoids null pointer when list may be null	list?.contains("item") ?: false

Key Takeaways

Use contains() or in keyword to check if a list contains an element.

Both methods return a Boolean: true if found, false otherwise.

Remember that string checks are case-sensitive.

Use safe calls to avoid errors when the list might be null.

Avoid comparing the whole list with == when checking for an element.