Star projection for unknown types in Kotlin - Mini Project: Build & Apply

Star Projection for Unknown Types in Kotlin

📖 Scenario: Imagine you are building a library system that can hold collections of different types of items, like books, magazines, or DVDs. Sometimes, you don't know exactly what type of items a collection holds, but you still want to work with it safely.

🎯 Goal: You will create a Kotlin program that uses star projection to handle collections with unknown types. This helps you work with generic types safely when you don't know the exact type inside.

📋 What You'll Learn

Create a generic class called Box that holds a value of any type.

Create a list of Box objects with different types inside.

Use star projection Box<*> to handle the list with unknown types.

Print the contents of each Box safely using star projection.

💡 Why This Matters

🌍 Real World

Star projection helps when working with libraries or APIs where you don't know the exact generic type but still want to use the data safely.

💼 Career

Understanding star projection is important for Kotlin developers working with generics, especially in Android development and library design.

1

Create the generic class Box

Create a generic class called Box with a property value of type T. This class will hold any type of value.

Kotlin

class Box(val value: T) {
    // Your code here
}

Need a hint?

Use class Box<out T>(val value: T) to create a generic class with a value property.

2

Create a list of Box objects with different types

Create a list called boxes that contains Box objects holding these exact values: Box(123), Box("Hello"), and Box(45.6).

Kotlin

class Box(val value: T)

val boxes = listOf(
    // Your code here
)

Need a hint?

Use listOf(Box(123), Box("Hello"), Box(45.6)) to create the list.

3

Use star projection to handle unknown types

Create a variable called unknownBoxes of type List<Box<*>> and assign it the value of boxes. This uses star projection to handle boxes with unknown types.

Kotlin

class Box(val value: T)

val boxes = listOf(Box(123), Box("Hello"), Box(45.6))

val unknownBoxes: List> = boxes
// Your code here

Need a hint?

Use val unknownBoxes: List<Box<*>> = boxes to declare the variable with star projection.

4

Print the contents of each Box safely

Use a for loop with variable box to iterate over unknownBoxes. Inside the loop, print the value property of each box.

Kotlin

class Box(val value: T)

val boxes = listOf(Box(123), Box("Hello"), Box(45.6))

val unknownBoxes: List> = boxes

for (box in unknownBoxes) {
    // Your code here
}

Need a hint?

Use for (box in unknownBoxes) { println(box.value) } to print each value.