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Companion Objects

class Math {
companion object {
const val PI = 3.141597
fun pow(base: Int, exponent: Int): Int {
assert(exponent >= 0) { "No support for negative exponents" }
var result = 1
for (i in 0 until exponent) {
result *= base
}
return result
}
}
}
println(Math.PI)
println(Math.pow(2, 3))

As we continue, we begin by describing a feature of Kotlin objects called companion objects. We’ll also introduce introduce one new piece of object syntax.

Warm Up Debugging Challenge
Warm Up Debugging Challenge

But first, let’s warm up with another graded debugging challenge!

this
this

this is a keyword in Kotlin that you can use in your instance methods. It always refers to the current instance that is executing the method.

So this:

class Course(var number: String) {
fun changeNumber(newNumber: String) {
number = newNumber
}
}

is equivalent to this:

class Course(var number: String) {
fun changeNumber(newNumber: String) {
this.number = newNumber
}
}

The example above is one use of this. However, we’ll usually just go the first route, and choose parameter names that don’t conflict with our instance variable names. This helps avoid mistakes.

However, there is one place where we do and will use this. Let’s go through it together:

class Course(var number: String)

Companion Objects
Companion Objects

Up until now the properties and methods that we’ve been establishing on our Kotlin objects are instance properties and methods. Meaning each instance of an class has its own:

class Person(val name: String) {
fun doubleName(): String {
return name + name
}
}
val first = Person("Geoff")
val second = Person("Chuchu")
println(first.name)
println(second.name)
println(first.doubleName())

Even though they share an implementation of doubleName, instances each act like they have their own doubleName method.

However, Kotlin also allows us to create methods that are provided by the class and can be accessed without an instance. We do this using something called a companion object. Let’s see how:

class Person(val name: String)

Companion v. Instance
Companion v. Instance

Companion object methods cannot access instance variables. Let’s look at why, and the differences between class and instance methods:

class Person(val name: String) {
companion object {
fun getName(): String {
return name
}
}
}
val student = Person("You")
println(student.getName())

Uses for Companion Objects
Uses for Companion Objects

In Kotlin, we can create methods that are not associated with any class, simply by declaring them outside a class body:

class Person(val name: String)
fun greetPerson(person: Person) {
println("Hello ${person.name}!")
}
val geoff = Person("Geoff")
greetPerson(geoff)

This works completely fine, and limits the degree to which we need to use companion objects. So if you have a method that doesn’t need an instance, you can either declare it outside the class or in a companion object.

Companion Fields
Companion Fields

Companion objects can also declare fields, just like instances:

class Person(val name: String) {
companion object {
val typicalGreeting = "Hello"
}
}
println(Person.typicalGreeting)

This can be a good place to put constant values, as shown above, particularly if, like a method, they really belong with the class that defines the companion object.

You can also define non-constant (i.e., var) properties on companion objects. However. This is extremely rare, and very easy to get wrong. So much so that we won’t bother demonstrating how to do it!

More Practice

Need more practice? Head over to the practice page.