Welcome back! This week we continue deepening our understanding of Kotlin objects. Today, we begin by describing a feature of Kotlin objects called companion objects. But first, let's introduce one new piece of object syntax...
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.
is equivalent to this:
The example above is one use of
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
Let's go through it together:
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:
Even though they share an implementation of
doubleName, instances each act like they have their own
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:
Companion object methods cannot access instance variables. Let's look at why, and the differences between class and instance methods:
In Kotlin, we can create methods that are not associated with any class, simply by declaring them outside a class body:
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 objects can also declare fields, just like instances:
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.
This is extremely rare, and very easy to get wrong.
So much so that we won't bother demonstrating how to do it!