Effective Kotlin Item 42: Respect the contract of equals

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

In Kotlin, every object extends Any, which has a few methods with well-established contracts. These methods are:

  • equals
  • hashCode
  • toString

Their contract is described in their comments and elaborated in the official documentation, and as I described in Item 32: Respect abstraction contracts, every subtype of a type with a contract should respect this contract. Mentioned methods have an important position in Kotlin, as they have been defined since the beginning of Java, and therefore many objects and functions depend on their contract. Breaking it will often lead to some objects or functions not working properly. This is why in the current and next items we will talk about overriding these functions and about their contracts. Let’s start with equals.


In Kotlin, there are two types of equality:

  • Structural equality - checked by the equals method or == operator (and its negated counterpart !=). a == b translates to a.equals(b) when a is not nullable, or otherwise to a?.equals(b) ?: (b === null).

  • Referential equality - checked by the === operator (and its negated counterpart !==), returns true when both sides point to the same object.

Since equals is implemented in Any, which is the superclass of every class, we can check the equality of any two objects. Although using operators to check equality is not allowed when objects are not of the same type:

open class Animal class Book Animal() == Book() // Error: Operator == cannot be // applied to Animal and Book Animal() === Book() // Error: Operator === cannot be // applied to Animal and Book

Objects either need to have the same type or one needs to be a subtype of another:

class Cat: Animal() Animal() == Cat() // OK, because Cat is a subclass of // Animal Animal() === Cat() // OK, because Cat is a subclass of // Animal

It is because it does not make sense to check equality of two objects of a different type. It will get clear when we will explain the contract of equals.

Why do we need equals?

The default implementation of equals coming from Any checks if another object is exactly the same instance. Just like the referential equality (===). It means that every object is unique by default:

class Name(val name: String) val name1 = Name("Marcin") val name2 = Name("Marcin") val name1Ref = name1 name1 == name1 // true name1 == name2 // false name1 == name1Ref // true name1 === name1 // true name1 === name2 // false name1 === name1Ref // true

Such behavior is useful for many objects. It is perfect for active elements, like a database connection, a repository, or a thread. However, there are objects where we need to represent equality differently. A popular alternative is a data class equality, that checks if all primary constructor properties are equal:

data class FullName(val name: String, val surname: String) val name1 = FullName("Marcin", "Moskała") val name2 = FullName("Marcin", "Moskała") val name3 = FullName("Maja", "Moskała") name1 == name1 // true name1 == name2 // true, because data are the same name1 == name3 // false name1 === name1 // true name1 === name2 // false name1 === name3 // false

Such behavior is perfect for classes that are represented by the data they hold, and so we often use the data modifier in data model classes or in other data holders.

Notice that data class equality also helps when we need to compare some, but not all properties. For instance when we want to skip cache or other redundant properties. Here is an example of an object representing date and time having properties asStringCache and changed that should not be compared by equality check:

class DateTime( /** The millis from 1970-01-01T00:00:00Z */ private var millis: Long = 0L, private var timeZone: TimeZone? = null ) { private var asStringCache = "" private var changed = false override fun equals(other: Any?): Boolean = other is DateTime && other.millis == millis && other.timeZone == timeZone //... }

The same can be achieved by using data modifier:

data class DateTime( private var millis: Long = 0L, private var timeZone: TimeZone? = null ) { private var asStringCache = "" private var changed = false //... }

Just notice that copy in such case will not copy those properties that are not declared in the primary constructor. Such behavior is correct only when those additional properties are truly redundant (the object will behave correctly when they will be lost).

Thanks to those two alternatives, default and data class equality, we rarely need to implement equality ourselves in Kotlin.

An example in which we might need to implement the equality is when just concrete a property should decide if objects are equal. For instance, a User class might have an assumption that two users are equal when their id is identical.

class User( val id: Int, val name: String, val surname: String ) { override fun equals(other: Any?): Boolean = other is User && other.id == id override fun hashCode(): Int = id }

As you can see, we implement equals ourselves when:

  • We need its logic to differ from the default one.
  • We need to compare only a subset of properties.
  • We do not want our object to be a data class or properties we need to compare are not in the primary constructor.

The contract of equals

This is how equals is described in its comments (Kotlin 1.3.11, formatted):

Indicates whether some other object is "equal to" this one. Implementations must fulfill the following requirements:

  • Reflexive: for any non-null value x, x.equals(x) should return true.

  • Symmetric: for any non-null values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

  • Transitive: for any non-null values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.

  • Consistent: for any non-null values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.

  • Never equal to null: for any non-null value x, x.equals(null) should return false.

Additionally, we expect equals, toString and hashCode to be fast. It is not a part of the official contract, but it would be highly unexpected to wait a few seconds to check if two elements are equal.

All those requirements are important. They are assumed from the beginning, also in Java, and so now many objects depend on those assumptions. Don’t worry if they sound confusing right now, we’ll describe them in detail.

  • Object equality should be reflexive, meaning that x.equals(x) returns true. Sounds obvious, but this can be violated. For instance, someone might want to make a Time object that can also represent the current time, and compares milliseconds:
// DO NOT DO THIS! class Time( val millisArg: Long = -1, val isNow: Boolean = false ) { val millis: Long get() = if (isNow) System.currentTimeMillis() else millisArg override fun equals(other: Any?): Boolean = other is Time && millis == other.millis } val now = Time(isNow = true) now == now // Sometimes true, sometimes false List(100000) { now }.all { it == now } // Most likely false

Notice that here the result is inconsistent, so it also violates the last principle.

When an object is not equal to itself, it might not be found in most collections even if it is there when we check using the contains method. It will not work correctly in most unit test assertions either.

val now1 = Time(isNow = true) val now2 = Time(isNow = true) assertEquals(now1, now2) // Sometimes passes, sometimes not

When the result is not constant, we cannot trust it. We can never be sure if the result is correct or is it just a result of inconsistency.

How should we improve it? A simple solution is checking separately if the object represents the current time and if not, then whether it has the same timestamp. Though it is a typical example of tagged class, and as described in Item 40: Prefer class hierarchies to tagged classes, it would be even better to use class hierarchy instead:

sealed class Time data class TimePoint(val millis: Long): Time() object Now: Time()
  • Object equality should be symmetric, meaning that the result of x == y and y == x should always be the same. It can be easily violated when in our equality we accept objects of a different type. For instance, let’s say that we implemented a class to represent complex numbers and made its equality accept Double:
class Complex( val real: Double, val imaginary: Double ) { // DO NOT DO THIS, violates symmetry override fun equals(other: Any?): Boolean { if (other is Double) { return imaginary == 0.0 && real == other } return other is Complex && real == other.real && imaginary == other.imaginary } }

The problem is that Double does not accept equality with Complex. Therefore, the result depends on the order of the elements:

Complex(1.0, 0.0).equals(1.0) // true 1.0.equals(Complex(1.0, 0.0)) // false

Lack of symmetry means, for instance, unexpected results on collections contains or on unit tests assertions.

val list = listOf<Any>(Complex(1.0, 0.0)) list.contains(1.0) // Currently on the JVM this is false, // but it depends on the collection’s implementation // and should not be trusted to stay the same

When equality is not symmetric, and it is used by another object, we cannot trust the result because it depends on whether this object compares x to y or y to x. This fact is not documented, and it is not a part of the contract as object creators assume that both should work the same (they assume symmetry). It can also change at any moment - creators during some refactorization might change the order of those values. If your object is not symmetric, it might lead to unexpected and really hard to debug errors in your implementation. This is why when we implement equals, we should always consider symmetry.

The general solution is that we should not accept equality between different classes. I’ve never seen a case where it would be reasonable. Notice that in Kotlin similar classes are not equal to each other. 1 is not equal to 1.0, and 1.0 is not equal to 1.0F. Those are different types, and they are not even comparable. In Kotlin we cannot use the == operator between two different types that do not have a common superclass other than Any:

Complex(1.0, 0.0) == 1.0 // ERROR
  • Object equality should be transitive, meaning that for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. The biggest problem with transitivity is when we implement different kinds of equality that check a different subtype of properties. For instance, let’s say that we have Date and DateTime defined this way:
open class Date( val year: Int, val month: Int, val day: Int ) { // DO NOT DO THIS, symmetric but not transitive override fun equals(o: Any?): Boolean = when (o) { is DateTime -> this == o.date is Date -> o.day == day && o.month == month && o.year == year else -> false } // ... } class DateTime( val date: Date, val hour: Int, val minute: Int, val second: Int ) : Date(date.year, date.month, date.day) { // DO NOT DO THIS, symmetric but not transitive override fun equals(o: Any?): Boolean = when (o) { is DateTime -> o.date == date && o.hour == hour && o.minute == minute && o.second == second is Date -> date == o else -> false } // ... }

The problem with the above implementation is that when we compare two DateTime, we check more properties than when we compare DateTime and Date. Therefore, two DateTime with the same day, but a different time, will not be equal to each other, but they’ll both be equal to the same Date. As a result, their relation is not transitive:

val o1 = DateTime(Date(1992, 10, 20), 12, 30, 0) val o2 = Date(1992, 10, 20) val o3 = DateTime(Date(1992, 10, 20), 14, 45, 30) o1 == o2 // true o2 == o3 // true o1 == o3 // false, so equality is not transitive

Notice that here the restriction to compare only objects of the same type didn’t help because we’ve used inheritance. Such inheritance violates the Liskov substitution principle, and should not be used. In this case, use composition instead of inheritance (Item 36: Prefer composition over inheritance). When you do, do not compare two objects of different types. These classes are perfect examples of objects holding data and representing them this way is a good choice:

data class Date( val year: Int, val month: Int, val day: Int ) data class DateTime( val date: Date, val hour: Int, val minute: Int, val second: Int ) val o1 = DateTime(Date(1992, 10, 20), 12, 30, 0) val o2 = Date(1992, 10, 20) val o3 = DateTime(Date(1992, 10, 20), 14, 45, 30) o1.equals(o2) // false o2.equals(o3) // false o1 == o3 // false o1.date.equals(o2) // true o2.equals(o3.date) // true o1.date == o3.date // true
  • Equality should be consistent, meaning that the method invoked on two objects should always return the same result, unless one of those objects was modified. For immutable objects, the result should be always the same. In other words, we expect equals to be a pure function (do not modify the state of an object) for which result always depends only on input and state of its receiver. We’ve seen the Time class, that violated this principle. This rule was also famously violated in java.net.URL.equals().

  • Never equal to null: for any non-null value x, x.equals(null) must return false. It is important because null should be unique, and no object should be equal to it.

Problem with equals in java.net.URL

One example of a really poorly designed equals is the one from java.net.URL. Equality of two java.net.URL objects depends on a network operation, as two hosts are considered equivalent if both hostnames can be resolved into the same IP addresses. Take a look at the following example:

import java.net.URL fun main() { val enWiki = URL("https://en.wikipedia.org/") val wiki = URL("https://wikipedia.org/") println(enWiki == wiki) }

Should it return true or false? According to the contract, it should be true, but the result is inconsistent. In normal conditions, it should print true because their IP address is resolved as the same, although if you have your internet turned off, it will print false. You can check it yourself. This is a big mistake! Equality should not be network dependent.

Here are the most important problems with this solution:

  • This behavior is inconsistent. For instance, two URLs could be equal when a network is available and unequal when it is not. Also, the network may change. The IP address for a given hostname varies over time and by the network. Two URLs could be equal on some networks and unequal on others.

  • The network may be slow, and we expect equals and hashCode to be fast. A typical problem is when we check if a URL is present in a list. Such an operation would require a network call for each element on the list. Also, on some platforms, like Android, network operations are prohibited on the main thread. As a result, even adding to a set of URLs needs to be started on a separate thread.

  • The defined behavior is known to be inconsistent with virtual hosting in HTTP. Equal IP addresses do not imply equal content. Virtual hosting permits unrelated sites to share an IP address. This method could report two otherwise unrelated URLs to be equal because they're hosted on the same server.

In Android, this problem was fixed in Android 4.0 (Ice Cream Sandwich). Since that release, URLs are only equal if their hostnames are equal. When we use Kotlin/JVM on other platforms, it is recommended to use java.net.URI instead of java.net.URL.

Implementing equals

I recommend against implementing equals yourself unless you have a good reason. Instead, use the default one or data class equality. If you do need custom equality, always consider if your implementation is reflexive, symmetric, transitive, and consistent. Make such class final, or beware that subclasses should not change how equality behaves. It is hard to make custom equality and support inheritance at the same time. Some even say it is impossible1. This is one of the reasons why data classes are final.


As Effective Java by Joshua Bloch, third edition claims in Item 10: Obey the general contract when overriding equals: "There is no way to extend an instantiable class and add a value component while preserving the equals contract, unless you’re willing to forgo the benefits of object-oriented abstraction".