Type constraints with upper bounds in Kotlin - Mini Project: Build & Apply

Type Constraints with Upper Bounds in Kotlin

📖 Scenario: You are building a simple Kotlin program that works with shapes. Each shape can calculate its area. You want to write a function that accepts only shapes or their subclasses.

🎯 Goal: Create a generic function with a type constraint that accepts only objects of type Shape or its subclasses, and prints their area.

📋 What You'll Learn

Create an open class called Shape with an abstract function area() that returns a Double.

Create a subclass called Circle that inherits from Shape and implements area().

Create a generic function called printArea with a type parameter T constrained to Shape or its subclasses.

Inside printArea, call the area() function of the passed object and print the result.

💡 Why This Matters

🌍 Real World

Type constraints with upper bounds help ensure functions work only with compatible types, preventing errors and making code safer.

💼 Career

Understanding generics and type constraints is important for writing reusable and type-safe Kotlin code in professional Android or backend development.

1

Create the Shape class

Create an open class called Shape with an abstract function area() that returns a Double.

Kotlin

# Your code here

Need a hint?

Use open abstract class Shape and define abstract fun area(): Double inside it.

2

Create the Circle subclass

Create a class called Circle that inherits from Shape. Add a property radius of type Double and override the area() function to return the area of the circle using 3.14 * radius * radius.

Kotlin

open abstract class Shape {
    abstract fun area(): Double
}

# Your code here

Need a hint?

Use class Circle(val radius: Double) : Shape() and override area().

3

Create the generic function with upper bound

Create a generic function called printArea with a type parameter T that has an upper bound of Shape. The function should take a parameter shape of type T and print the area by calling shape.area().

Kotlin

open abstract class Shape {
    abstract fun area(): Double
}

class Circle(val radius: Double) : Shape() {
    override fun area(): Double {
        return 3.14 * radius * radius
    }
}

# Your code here

Need a hint?

Define fun <T : Shape> printArea(shape: T) and print the area.

4

Call the function and print the area

Create a Circle object with radius 5.0 and call printArea with this object. Print the output.

Kotlin

open abstract class Shape {
    abstract fun area(): Double
}

class Circle(val radius: Double) : Shape() {
    override fun area(): Double {
        return 3.14 * radius * radius
    }
}

fun  printArea(shape: T) {
    println("Area: ${shape.area()}")
}

# Your code here

Need a hint?

Create val circle = Circle(5.0) and call printArea(circle).