Effective Kotlin Item 44: Respect the contract of compareTo

This is a chapter from the book Effective Kotlin. You can find it on LeanPub or Amazon.

The compareTo method is not in the Any class. It is an operator in Kotlin that translates into the mathematical comparison signs:

obj1 > obj2 // translates to obj1.compareTo(obj2) > 0 obj1 < obj2 // translates to obj1.compareTo(obj2) < 0 obj1 >= obj2 // translates to obj1.compareTo(obj2) >= 0 obj1 <= obj2 // translates to obj1.compareTo(obj2) <= 0

It is also located in the Comparable<T> interface. When an object implements this interface, or when it has an operator method named compareTo with one parameter, it means that this object has a natural order. Such an order needs to be:

  • Antisymmetric, meaning if a >= b and b >= a then a == b. Therefore, there is a relation between comparison and equality, and they need to be consistent with each other.

  • Transitive, meaning if a >= b and b >= c then a >= c. Similarly, when a > b and b > c then a > c. This property is important, because without it, sorting of elements might take literally forever in some sorting algorithms.

  • Connex, meaning there must be a relationship between every two elements. So either a >= b, or b >= a. In Kotlin, it is guaranteed by typing system for compareTo, because it returns Int, and every Int is either positive, negative or zero. This property is important, because if there is no relationship between two elements, we cannot use classic sorting algorithms like quicksort or insertion sort. Instead, we need to use one of the special algorithms for partial orders, like topological sorting.

Do we need a compareTo?

In Kotlin we rarely implement compareTo ourselves. We get more freedom by specifying the order on a case by case basis, than by assuming one global natural order. For instance, we can sort a collection using sortedBy and provide a key, that is comparable. So in the example below, we sort users by their surname:

class User(val name: String, val surname: String) val names = listOf<User>(/*...*/) val sorted = names.sortedBy { it.surname }

What if we need a more complex comparison than just by a key? For that, we can use the sortedWith function that sorts elements using a comparator. This comparator can be produced using a function compareBy. So in the following example, we sort users by comparing them by their surname, and if they match, we compare them by their name:

val sorted = names .sortedWith(compareBy({ it.surname }, { it.name }))

Surely, we might make User implement Comparable<User>, but what order should it choose? Is any truly natural for this type? We might need to sort them by any property. When this is not absolutely clear, it is better to not make such objects comparable.

String has a natural order, which is an alphanumerical order, and so it implements Comparable<String>. This fact is very useful, because we often do need to sort text alphanumerically. However, it also has its downsides. For instance, we can compare two strings using a comparision sign, which seems highly unintuitive. Most people seeing comparison sign between two strings will be rather confused.

// DON'T DO THIS! print("Kotlin" > "Java") // true

Surely there are objects with a clear natural order. Units of measure, date and time are all perfect examples. Although if you are not sure about whether your object has a natural order, it is better to use comparators instead. If you use a few of them often, you can place them in the companion object of your class:

class User(val name: String, val surname: String) { // ... companion object { val DISPLAY_ORDER = compareBy(User::surname, User::name) } } val sorted = names.sortedWith(User.DISPLAY_ORDER)

Implementing compareTo

When we do need to implement compareTo ourselves, we have top-level functions that can help us. If all you need is to compare two values, you can use the compareValues function:

class User( val name: String, val surname: String ): Comparable<User> { override fun compareTo(other: User): Int = compareValues(surname, other.surname) }

If you need to use more values, or if you need to compare them using selectors, use compareValuesBy:

class User( val name: String, val surname: String ) : Comparable<User> { override fun compareTo(other: User): Int = compareValuesBy(this, other, { it.surname }, { it.name } ) }

This function helps us create most comparators we might need. If you need to implement some with a special logic, remember that it should return:

  • 0 if the receiver and other are equal
  • a positive number if the receiver is greater than other
  • a negative number if the receiver is smaller than other

Once you did that, don't forget to verify that your comparison is antisymmetric, transitive and connex.