Classes Workbook (Part 1)

Practice problems for A Kotlin Class Is Mostly Its Header. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

declarations and constructors

1. The smallest class

Declare an empty class Person and create an instance.

class Person

val p = Person()

2. Properties in the header

Declare Point with a read-only x and y of type Int in its primary constructor.

class Point(val x: Int, val y: Int)

3. A default property value

Declare User(name: String) with a var active property that defaults to true.

class User(val name: String, var active: Boolean = true)

4. An init block

Declare Account(balance: Int) that throws IllegalArgumentException at construction if balance is negative.

class Account(val balance: Int) {
    init {
        require(balance >= 0) { "balance must be non-negative" }
    }
}

5. Construct with named arguments

Given class Server(val host: String, val port: Int = 80, val secure: Boolean = false), construct one for "example.com" that is secure, keeping the default port.

Server(host = "example.com", secure = true)

6. A secondary constructor

Add a secondary constructor to Point that takes a single Int and uses it for both x and y.

class Point(val x: Int, val y: Int) {
    constructor(both: Int) : this(both, both)
}

properties and encapsulation

7. A computed property

Add a read-only area property to Rectangle(val w: Int, val h: Int) computed from its sides.

class Rectangle(val w: Int, val h: Int) {
    val area: Int
        get() = w * h
}

8. Read-public, write-private

Declare Counter with an Int property value that anyone can read but only the class can change, plus an increment() method.

class Counter {
    var value: Int = 0
        private set

    fun increment() { value++ }
}

inheritance

9. Classes are final by default

Make Animal extendable and give it an open speak() returning "..."; then have Dog override it to return "woof".

open class Animal {
    open fun speak() = "..."
}

class Dog : Animal() {
    override fun speak() = "woof"
}

10. An abstract class

Declare an abstract Shape with an abstract area(): Double, and a Circle(val r: Double) that implements it.

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

class Circle(val r: Double) : Shape() {
    override fun area() = Math.PI * r * r
}

11. Make it extendable

This doesn’t compile. Fix the declaration so Derived can extend Base:

class Base
class Derived : Base()

open class Base
class Derived : Base()

Back to the lesson, A Kotlin Class Is Mostly Its Header, or on to the next one: class types.