Star Projection in Kotlin

Introduction

Star Projection is a fundamental concept every Kotlin developer should understand. Generics let you write type-safe containers and algorithms that work with many types while keeping compile-time checks.

Star projection (*) is used when generic type argument is unknown. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Star projection (*) is used when generic type argument is unknown.
• It provides safe access with limited operations.
• Useful when type details are not important.

Syntax

List<*>

Star Projection in Kotlin Example Program in Kotlin

fun printSize(list: List<*>) {
    println("Size: ${list.size}")
}

fun main(args: Array<String>) {
    printSize(listOf(1, 2, 3))
    printSize(listOf("A", "B"))
}

Sample Output

Size: 3
Size: 2

When to use

Use generics when building reusable containers, parsers, or algorithms that must work with more than one type.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. The println("Size: ${list.size}") statement writes a line to the console — this produces part of the sample output below.

3. Star projection (*) is used when generic type argument is unknown.

4. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Understand the core idea: star projection (*) is used when generic type argument is unknown.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Star projection (*) is used when generic type argument is unknown.
• It provides safe access with limited operations.
• Useful when type details are not important.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.