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.

Practice

1. What does the star projection * represent in Kotlin generics?

easy

A. A type that can only be used with primitive data types

B. A specific generic type that must be defined explicitly

C. A wildcard that allows writing but restricts reading

D. An unknown generic type that allows safe reading but restricts writing

Solution

1. Step 1: Understand star projection meaning

   The star projection * means an unknown generic type in Kotlin.

2. Step 2: Analyze its usage

   It allows safe reading from generic collections but restricts writing to avoid type errors.

3. Final Answer:

   An unknown generic type that allows safe reading but restricts writing -> Option D

4. Quick Check:

   Star projection = unknown generic type with safe read [OK]

Hint: Star projection means unknown type, safe read only [OK]

Common Mistakes:

• Thinking star projection allows writing to generics
• Confusing star projection with specific generic types
• Assuming star projection only works with primitives

2. Which of the following is the correct syntax to declare a list with star projection in Kotlin?

easy

A. val list: List<Any> = listOf(1, 2, 3)

B. val list: List<*> = listOf(1, 2, 3)

C. val list: List<> = listOf(1, 2, 3)

D. val list: List = listOf(1, 2, 3)

Solution

1. Step 1: Recall star projection syntax

   Star projection uses * inside angle brackets to represent unknown type.

2. Step 2: Check each option

   val list: List<*> = listOf(1, 2, 3) uses List<*> which is correct syntax for star projection.

3. Final Answer:

   val list: List<*> = listOf(1, 2, 3) -> Option B

4. Quick Check:

   Star projection syntax = List<*> [OK]

Hint: Star projection syntax always uses * inside angle brackets [OK]

Common Mistakes:

• Leaving angle brackets empty like List<>
• Using List<Any> which is not star projection
• Confusing star projection with out-projection

3. What will be the output of the following Kotlin code?

fun printFirst(list: List<*>) {
    println(list[0])
}

fun main() {
    val numbers: List<Int> = listOf(10, 20, 30)
    printFirst(numbers)
}

medium

A. 10

B. Compilation error

C. null

D. Runtime exception

Solution

1. Step 1: Understand star projection usage in function

   The function accepts List<*>, meaning unknown generic type, safe to read elements.

2. Step 2: Analyze the call with List<Int>

   Passing List<Int> is valid; reading list[0] returns 10 safely.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Reading from List<*> returns element safely [OK]

Hint: Reading from List<*> works, writing does not [OK]

Common Mistakes:

• Assuming star projection causes compile error
• Expecting null instead of actual element
• Thinking runtime exception occurs on read

4. Identify the error in the following Kotlin code using star projection:

fun addElement(list: MutableList<*>, element: Any) {
    list.add(element)
}

medium

A. Element type Any is incompatible with list

B. Syntax error in function declaration

C. Cannot add element to MutableList<*> due to unknown type

D. No error, code compiles and runs fine

Solution

1. Step 1: Understand star projection with MutableList

   MutableList<*> means unknown generic type; writing is restricted to avoid type errors.

2. Step 2: Analyze the add operation

   Trying to add element causes compile error because type is unknown and unsafe to write.

3. Final Answer:

   Cannot add element to MutableList<*> due to unknown type -> Option C

4. Quick Check:

   Writing to MutableList<*> causes error [OK]

Hint: Cannot write to star-projected mutable collections [OK]

Common Mistakes:

• Thinking add works with MutableList<*>
• Confusing syntax error with type error
• Assuming Any type is always accepted

5. Given a function that accepts a list with star projection, how can you safely copy elements from List<*> to a MutableList<Any> in Kotlin?

hard

A. Use a loop to cast each element to Any and add to MutableList<Any>

B. Directly assign List<*> to MutableList<Any>

C. Use star projection on both lists

D. Use MutableList<*> instead of MutableList<Any>

Solution

1. Step 1: Understand star projection restrictions

   List<*> allows safe reading but elements are of unknown type, so direct assignment to MutableList<Any> is unsafe.

2. Step 2: Safely copy elements

   Use a loop to read each element and cast to Any before adding to MutableList<Any>, ensuring type safety.

3. Final Answer:

   Use a loop to cast each element to Any and add to MutableList<Any> -> Option A

4. Quick Check:

   Copy with loop and cast = safe transfer [OK]

Hint: Copy elements with loop and cast from List<*> to MutableList<Any> [OK]

Common Mistakes:

• Trying direct assignment between incompatible generic types
• Using star projection on mutable list incorrectly
• Ignoring type casting when copying elements