Star projection for unknown types in Kotlin

Introduction

Star projection helps you work with generic types when you don't know the exact type. It lets you use a generic class safely without specifying the type.

When you want to accept any type of generic object without caring about its specific type.

When you call a function that uses generics but you don't know or need the exact type.

When you want to read from a generic collection but don't want to write to it.

When you want to avoid type errors while working with unknown generic types.

Syntax

Kotlin

val list: List<*> = listOf(1, "two", 3.0)

The star * means "some unknown type".

You can read elements as Any? but cannot add elements safely.

Examples

You can get elements as Any? but cannot add new elements.

Kotlin

val anyList: List<*> = listOf(1, 2, 3)
val first: Any? = anyList[0]

This function accepts any list regardless of its element type.

Kotlin

fun printList(list: List<*>) {
    for (item in list) {
        println(item)
    }
}

You can use star projection to hold mixed types safely.

Kotlin

val mixedList: List<*> = listOf("hello", 42, 3.14)
println(mixedList.size)

Sample Program

This program shows how to use star projection to print lists of unknown types safely.

Kotlin

fun printUnknownList(list: List<*>) {
    for (item in list) {
        println(item)
    }
}

fun main() {
    val numbers = listOf(1, 2, 3)
    val words = listOf("apple", "banana")
    val mixed = listOf(1, "two", 3.0)

    printUnknownList(numbers)
    printUnknownList(words)
    printUnknownList(mixed)
}

Important Notes

You cannot add elements to a collection with star projection because the type is unknown.

Star projection is useful for reading data safely when you don't know the exact generic type.

Summary

Star projection * means an unknown generic type.

It allows safe reading but restricts writing to generic collections.

Use it when you want to work with generics without specifying exact types.