KotlinCS 124 LogoJava

Anonymous Classes

interface Adder {
fun addTo(value: Int): Int
}
val addOne = object : Adder {
override fun addTo(value: Int) = value + 1
}
val addEight = object : Adder {
override fun addTo(value: Int) = value + 8
}
println(addOne.addTo(1))
println(addEight.addTo(1))
println(addOne.addTo(8))

This lesson ventures into interesting uncharted territory. Until now, our classes have needed to have names. But Kotlin doesn’t actually require this! Let’s explore anonymous classes and their uses…

Please note that the next two lessons are on fairly advanced topics. You will see and need to understand code that uses these ideas, but testing on them will be limited.

Warm Up Debugging Challenge
Warm Up Debugging Challenge

But… hold on! Let’s warm up with another graded debugging challenge!

Anonymous Classes
Anonymous Classes

We’ve seen how to define what is called a named class. This should be familiar to us by now:

// Person is a named class
class Person

However, Kotlin also allows us to define so-called anonymous classes. Let’s go through an example carefully:

open class Person {
open fun getType(): String {
return "Person"
}
}
val person = Person()
val student = object : Person() {
override fun getType(): String {
return "Student"
}
}
println(person.getType())
println(student.getType())

In contrast to named classes, anonymous classes:

Capturing Variables
Capturing Variables

Anonymous classes can capture the value of variables that are available when they are created. This can be extremely useful, even if the results are a bit spooky. Let’s look at an example:

open class Person {
open fun getType(): String {
return "Person"
}
}
// Show variable capture

Uses for Anonymous Classes
Uses for Anonymous Classes

We can create anonymous classes in Kotlin. Cool! But… so what!? What problems can these classes solve?

Surprisingly, anonymous classes turn out to be common and quite powerful. Let’s look at an example.

Imagine that we want to count the number of elements in an array of Ints that meet some condition. The condition could be that the element was positive, or negative, or odd, or even, or divisible by three, or whatever.

One approach would be to write separate methods for each thing we would want to count:

fun countArrayPositive(values: Array<Int>): Int {
var count = 0
for (value in values) {
if (value >= 0) {
count++
}
}
return count
}
fun countArrayNegative(values: Array<Int>): Int {
var count = 0
for (value in values) {
if (value < 0) {
count++
}
}
return count
}
fun countArrayEven(values: Array<Int>): Int {
var count = 0
for (value in values) {
if (value % 2 == 0) {
count++
}

Wow, this is getting tedious—and we’ve only done three! Imagine if we had a bunch of different conditions we needed to handle… But they are all very similar. There must be a better way.

Let’s see how to rewrite the code above using an anonymous class in a way that makes the counting logic completely flexible.

fun countArrayPositive(values: Array<Int>): Int {
var count = 0
for (value in values) {
if (value >= 0) {
count++
}
}
return count
}
fun countArrayNegative(values: Array<Int>): Int {
var count = 0
for (value in values) {
if (value < 0) {
count++
}
}
return count
}
val array = arrayOf(1, 2, 5)
println(countArrayPositive(array))

Practice: Bracket an Anonymous Class

Created By: Geoffrey Challen
/ Version: 2020.10.0

Declare a method create. create takes a single Int parameter and returns an anonymous object that implements the Bracket interface:

The returned object should implement top so that it returns the passed Int and bottom so that it returns the passed Int * -1. So, for example:

Practice: String Both Ways Anonymous Class

Created By: Geoffrey Challen
/ Version: 2021.10.0

Declare a method create. create takes a single String parameter and returns an anonymous object that implements the IBothWays interface:

So, for example:

Homework: Which Hemisphere Anonymous Class

Created By: Geoffrey Challen
/ Version: 2022.10.0

Create a method named create that accepts a Position object and returns an anonymous object that implements the IWhichHemisphere interface:

The return values of isNorthern and isSouthern are determined by the latitude value on the passed Position object, which you can retrieve as a Double latitude property. Here's how your class should work:

However, note that the latitude values of the passed Position object can change after it is passed to your method! So you should not precompute the results of isNorthern and isSouthern, but rather calculate them in your anonymous class when the corresponding methods are called.

We define any position with a positive latitude as being in the Northern Hemisphere, and with a negative latitude as being in the Southern Hemisphere. Meaning, for the purposes of this problem, points on the equator are in neither hemisphere.

CS People: Shoshana Zuboff
CS People: Shoshana Zuboff

Shoshana Zuboff is an author and professor at the Harvard Business School, where she was the first tenured female professor. She may be best known for coining the term “surveillance capitalism”, and for her book “The Age of Surveillance Capitalism”, which outlines a global system of behavioral modification based on the collection and mining of data about our private lives.

In the short video below Shoshana Zuboff defines surveillance capitalism and discusses how it mirrors and differs from previous eras of capitalism:

More Practice

Need more practice? Head over to the practice page.