You can avoid null if you try

TL;DR: Don't use null for real places

Problems

Coupling
Unexpected Results

Solutions

Model the unknown location with polymorphism

Context

Null Island is a fictional place, which is located at the coordinates 0°N 0°E, at the intersection of the Prime Meridian and the Equator in the Atlantic Ocean.

The name "Null Island" comes from the fact that this location represents the point where a lot of GPS systems place any data that has missing or invalid location coordinates.

In reality, there is no landmass at this location, and it is actually in the middle of the ocean.

This point has become a popular reference for geographic information systems (GIS) and mapping software, as it serves as a way to filter out errors in location data.

Sample Code

Wrong

class Person(val name: String, val latitude: Double, val longitude: Double)

fun main() {
    val people = listOf(
        Person("Alice", 40.7128, -74.0060), // New York City
        Person("Bob", 51.5074, -0.1278), // London
        Person("Charlie", 48.8566, 2.3522), // Paris
        Person("Tony Hoare", 0.0, 0.0) // Null Island
    )

    for (person in people) {
        if (person.latitude == 0.0 && person.longitude == 0.0) {
            println("${person.name} lives on Null Island!")
        } else {
            println("${person.name} lives at (${person.latitude}, ${person.longitude}).")
        }
    }
}

Right

abstract class Location {
    abstract fun calculateDistance(other: Location): Double
}

class Coordinates(val latitude: Double, val longitude: Double) : Location() {
    override fun calculateDistance(other: Location): Double {    
        val earthRadius = 6371.0 // kilometers
        val latDistance = Math.toRadians(latitude - other.latitude)
        val lngDistance = Math.toRadians(longitude - other.longitude)
        val a = sin(latDistance / 2) * sin(latDistance / 2) +
                cos(Math.toRadians(latitude)) * cos(Math.toRadians(other.latitude)) *
                sin(lngDistance / 2) * sin(lngDistance / 2)
        val c = 2 * atan2(sqrt(a), sqrt(1 - a))
        return earthRadius * c
    }
}

class UnknownLocation : Location() {
    override fun calculateDistance(other: Location): Double {
        throw IllegalArgumentException("Cannot calculate distance from unknown location.")
    }
}

class Person(val name: String, val location: Location)

fun main() {
    val people = listOf(
        Person("Alice", Coordinates(40.7128, -74.0060)), // New York City
        Person("Bob", Coordinates(51.5074, -0.1278)), // London
        Person("Charlie", Coordinates(48.8566, 2.3522)), // Paris
        Person("Tony", UnknownLocation()) // Unknown location
    )

    val rio = Coordinates(-22.9068, -43.1729) // Rio de Janeiro coordinates

    for (person in people) {
        try {
            val distance = person.location.calculateDistance(rio)
            println("${person.name} is ${distance} kilometers from Rio de Janeiro.")
        } catch (e: IllegalArgumentException) {
            println("${person.name} is at an unknown location.")
        }
    }
}

Detection

[X] Semi-Automatic

We can check for special numbers used as nulls

Conclusion

Don't use Null to represent real objects

Relations

Code Smell 12 - Null

Maxi Contieri ・ Oct 31 '20

#codenewbie #tutorial #oop

Code Smell 126 - Fake Null Object

Maxi Contieri ・ Apr 2 '22

More Info

https://hackernoon.com/null-the-billion-dollar-mistake-8t5z32d6

https://en.wikipedia.org/wiki/Null_Island

Disclaimer

Code Smells are my opinion.

The billion dollar mistake of having null in the language. And since JavaScript has both null and undefined, it's the two billion dollar mistake.

Anders Hejlsberg

Software Engineering Great Quotes

Maxi Contieri ・ Dec 28 '20

#codenewbie #programming #quotes #software

This article is part of the CodeSmell Series.

How to Find the Stinky parts of your Code

Maxi Contieri ・ May 21 '21

#codenewbie #tutorial #codequality #beginners

Code Smell 208 - Null Island