Abstract classes and methods in Kotlin - Step-by-Step Execution

Concept Flow - Abstract classes and methods

Define abstract class

↓

Declare abstract method(s)

↓

Create subclass

↓

Implement abstract method(s)

↓

Instantiate subclass and use methods

An abstract class defines a template with abstract methods that subclasses must implement before creating objects.

Execution Sample

Kotlin

abstract class Animal {
    abstract fun sound(): String
}

class Dog : Animal() {
    override fun sound() = "Bark"
}

fun main() {
    val dog = Dog()
    println(dog.sound())
}

Defines an abstract class Animal with an abstract method sound, then Dog implements sound and prints its result.

Execution Table

Step	Action	Evaluation	Result
1	Define abstract class Animal with abstract method sound()	Class Animal created with abstract method	No instance can be created from Animal
2	Define subclass Dog inheriting Animal	Dog class created	Dog must implement sound()
3	Implement sound() in Dog returning "Bark"	sound() overridden	Dog is concrete and instantiable
4	Create instance dog of Dog	dog = Dog()	dog object created
5	Call dog.sound()	dog.sound() returns "Bark"	Output: Bark

💡 Execution ends after printing the dog's sound.

Variable Tracker

Variable	Start	After Step 4	After Step 5
dog	undefined	Dog instance created	Dog instance unchanged

Key Moments - 3 Insights

Why can't we create an instance of the abstract class Animal?

Because Animal has an abstract method without implementation (see execution_table step 1), Kotlin prevents creating objects from it to ensure all abstract methods are implemented.

What happens if Dog does not implement the abstract method sound()?

Dog would remain abstract and cannot be instantiated (see execution_table step 2 and 3). Kotlin requires all abstract methods to be implemented in concrete subclasses.

Why do we override sound() in Dog?

To provide the actual behavior for the abstract method declared in Animal (see execution_table step 3), making Dog a concrete class that can be used.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when dog.sound() is called at step 5?

A "Meow"

B "Bark"

C Compilation error

D No output

💡 Hint

Refer to execution_table row 5 where dog.sound() returns "Bark".

At which step is the Dog class confirmed to be instantiable?

A Step 3

B Step 4

C Step 1

D Step 2

💡 Hint

Check execution_table step 3 where sound() is overridden making Dog concrete.

If we remove the override of sound() in Dog, what changes in the execution table?

A dog.sound() returns null

B dog object is created but sound() throws error

C Dog becomes abstract and cannot be instantiated at step 4

D No change, program runs normally

💡 Hint

Refer to key_moments about implementing abstract methods and execution_table steps 2 and 3.

Concept Snapshot

abstract class ClassName {
    abstract fun methodName(): ReturnType
}

class SubClass : ClassName() {
    override fun methodName() = implementation
}

- Abstract classes cannot be instantiated.
- Abstract methods have no body and must be overridden.
- Subclasses must implement all abstract methods to be concrete.

Full Transcript

This example shows how Kotlin uses abstract classes and methods. First, an abstract class Animal is defined with an abstract method sound. This method has no body, so Animal cannot be instantiated. Then, a subclass Dog inherits Animal and overrides the sound method to return "Bark". Because Dog implements all abstract methods, it becomes a concrete class that can be instantiated. Finally, we create a Dog object and call its sound method, which prints "Bark". This flow ensures that abstract classes provide a template, and subclasses provide specific behavior before objects are created.