override

Learn to redefine methods with override.

override

To redefine an open method of the base class in a subclass, you use the override modifier. This is the standard way to add subclass-specific behavior.

Example: class Circle : Shape() { override fun area(): Double { return 3.14 * r * r } }. If area is marked open in Shape, the subclass overrides it with override. Forgetting override is a compile error; Kotlin avoids accidental method hiding. Overridden methods are themselves open by default.

The open + override rule

To redefine a method in a subclass with override, the same method must be open in the base class. If it is not open, override fails to compile.

Example: open class Shape { open fun area() = 0.0 }; the subclass can write override fun area() = .... If area is not open in Shape, the subclass override is a compile error. Writing final override blocks further overriding down the hierarchy.

Exercise The Shape base class and the Circle subclass

Define an open class named Shape with a primary constructor taking val name: String. Add an open fun area(): Double = 0.0 method. Define a subclass named Circle with a primary constructor taking name: String, val radius: Double that inherits from Shape(name). Its override fun area(): Double returns 3.14 * radius * radius. In main create a Circle("Disk", 2.0) instance and print area().

Show example solution

open class Shape(val name: String) {
    open fun area(): Double = 0.0
}

class Circle(name: String, val radius: Double) : Shape(name) {
    override fun area(): Double {
        return 3.14 * radius * radius
    }
}

fun main() {
    val c = Circle("Disk", 2.0)
    println(c.area())
}

Test yourself

Match the override pattern

open fun area()

Method opened in base
override fun area()

Redefinition in subclass
: Shape()

Inheritance and constructor
return 3.14 * r * r

New body
A subclass writes <code>override fun area()</code> but <code>area</code> is not <code>open</code> in the base. What happens?
- A compile error is raised
- The code compiles normally
- The method is hidden at runtime
- The base body runs instead
Show answer

A compile error is raised — A non-open method cannot be overridden; the compiler rejects it.

Complete the override method

class Circle : Shape() {
    ____ fun area(): Double {
        return 3.14 * r * r
    }
}

Show answer

class Circle : Shape() {
    override fun area(): Double {
        return 3.14 * r * r
    }
}

Next lesson: super

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