Star projection for unknown types in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a list with star projection.

Kotlin

val list: List<[1]> = listOf("apple", "banana", "cherry")

Drag options to blanks, or click blank then click option'

A*

Bout Any

CAny

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?' instead of '*' for star projection.

Using 'Any' which is a specific type, not a star projection.

The star projection is represented by '*' in Kotlin generics to indicate an unknown type.

2fill in blank

medium

Complete the function parameter type using star projection.

Kotlin

fun printList(list: List<[1]>) {
    for (item in list) {
        println(item)
    }
}

Drag options to blanks, or click blank then click option'

A*

BAny

Cout Any

Din Any

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Any' which restricts the type to Any only.

Using 'out Any' or 'in Any' which are variance annotations, not star projections.

Using '*' as star projection allows the function to accept a list of any type.

3fill in blank

hard

Fix the error in the generic function using star projection.

Kotlin

fun <T> copy(from: Array<out [1]>, to: Array<[1]>) {
    for (i in from.indices) {
        to[i] = from[i]
    }
}

Drag options to blanks, or click blank then click option'

AAny

Bout Any

CT

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' inside function generic parameters which is invalid.

Using 'Any' which loses type safety.

The generic type parameter 'T' should be used consistently instead of star projection in this function.

4fill in blank

hard

Fill both blanks to create a map with star projections for keys and values.

Kotlin

val map: Map<[1], [2]> = mapOf("one" to 1, "two" to 2)

Drag options to blanks, or click blank then click option'

A*

BAny

Cout Any

Din Any

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Any' which is a specific type, not star projection.

Mixing variance annotations like 'out Any' or 'in Any' incorrectly.

Star projection '*' is used for both key and value types to indicate unknown types.

5fill in blank

hard

Fill all three blanks to declare a function with star projections and variance.

Kotlin

fun process(list: List<[1]>, producer: Producer<[2]>, consumer: Consumer<[3]>) {
    // function body
}

Drag options to blanks, or click blank then click option'

A*

Bout Any

Cin Any

DAny

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Any' instead of variance annotations for producer and consumer.

Using star projection in variance positions incorrectly.

Star projection '*' is used for the list, 'out Any' for producer (covariant), and 'in Any' for consumer (contravariant).

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