Kotlin Inheritance: Open for Extension, Closed by Default
In Kotlin, classes are final by default. Think of inheritance as an opt-in feature you enable with the open keyword. Use it to create specialized versions of a base class, like a Student from a Person.
Why it exists
Inheritance enables code reuse and polymorphism by letting one class acquire the properties and methods of another. However, unrestricted inheritance can create fragile, tightly-coupled systems. Kotlin's design makes inheritance an intentional act to encourage more robust code.
The mental model
Think of Kotlin classes as sealed boxes. By default, you can't alter or extend them. To allow inheritance, you must explicitly mark a class as open. This "final by default" philosophy promotes composition over inheritance and makes code easier to reason about, as a class's behavior isn't changed by a subclass unless it was explicitly designed for it.
How it works
All Kotlin classes implicitly inherit from Any, which provides equals(), hashCode(), and toString(). To inherit from another class, declare the supertype after a colon: class Derived : Base(). The base class and any members you wish to override must be marked open. The derived class must call the base class constructor. To override a member, you must use the override keyword. This overridden member is itself open for further subclassing unless you mark it final override. You can override a val property with a var, but not the reverse.
When to use it
Use inheritance for clear "is-a" relationships where you need to share implementation, not just a contract. A Circle "is-a" Shape. It's also necessary when a framework requires it, like extending android.view.View. Before creating a class hierarchy, always consider if an abstract class or an interface (which are open by default) would be a better fit.
When not to use it
Avoid inheritance if an interface or composition is a better fit. If classes only share a common contract but not a common implementation, use an interface. For "has-a" relationships, like a Car that has an Engine, prefer composition by holding an Engine instance rather than inheriting from it. Kotlin's final default nudges you to consider these alternatives first.
One canonical example
To create a specialized Student class from a Person class, you must mark Person and its introduce method as open. The Student class then uses override to provide its own implementation.
open class Person(val name: String) {
open fun introduce() { println("Hello, my name is $name.") }
}
class Student(name: String, val school: String) : Person(name) {override fun introduce() { println("Hi, I'm name, and I study at school.") }
}
Interview question
What is the primary reason Kotlin classes are final by default?
- a.To encourage explicit design for inheritance and promote composition over inheritance.Correct
- b.To optimize runtime performance by allowing the compiler to inline method calls more aggressively.
- c.To ensure that all class properties are immutable unless explicitly marked as mutable.
- d.To prevent developers from creating circular dependencies in class hierarchies.
Why? this is the answer
The card states that Kotlin's design makes inheritance an intentional act to encourage more robust code and promotes composition over inheritance. Option A directly captures these core reasons. Option C is incorrect as 'final by default' relates to inheritance, not the mutability of properties, which is controlled by 'val' and 'var'.
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Read the original → kotlinlang.org
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