Interfaces versus abstract classes

Now that we've covered both abstract classes and interfaces, you might be wondering what you should use when you are building your app. They look similar in some ways, but there are some important differences that we'll cover now, which will make it easier for you to choose which one to use.

We've already mentioned that Kotlin doesn't allow multiple inheritance. Take a look at this example:

abstract class InsureBase {
abstract fun insure()
}

abstract class CarBase {
abstract fun drive()
}

//compiler error, can't inherit from multiple classes
class InsurableCar: InsureBase(), CarBase() {
override fun insure() {
    }

override fun drive() {
    }
}

This won't compile, because we are trying to inherit from two classes.

Interfaces don't have that limitation; a class can implement as many interfaces as it wants. So, if we want to have a type that can be both driven and insured, we can have two interfaces that define that behavior and one concrete class that implements...