Abstract classes and methods in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract class in Kotlin?

An abstract class in Kotlin is a class that cannot be instantiated directly. It is designed to be a base class for other classes and can contain abstract methods that must be implemented by subclasses.

beginner

What is an abstract method in Kotlin?

An abstract method is a method declared inside an abstract class without a body. Subclasses must provide an implementation for this method.

beginner

Can you create an instance of an abstract class directly in Kotlin?

No, you cannot create an instance of an abstract class directly. You must create a subclass that implements all abstract methods and then instantiate that subclass.

beginner

How do you declare an abstract class and an abstract method in Kotlin? Provide a simple example.

Use the keyword 'abstract' before the class and method. Example:<br><pre>abstract class Animal {
    abstract fun sound()
}

class Dog : Animal() {
    override fun sound() {
        println("Woof")
    }
}</pre>

intermediate

Why use abstract classes and methods?

Abstract classes and methods help define a common template for related classes. They enforce that subclasses implement specific behaviors, promoting code organization and reuse.

Which keyword is used to declare an abstract class in Kotlin?

A abstract

B open

C final

D sealed

The keyword 'abstract' is used to declare an abstract class in Kotlin.

Can an abstract class in Kotlin have non-abstract methods?

A Only if the methods are private

B No, all methods must be abstract

C Only if the class is also open

D Yes, it can have both abstract and non-abstract methods

An abstract class can have both abstract methods (without body) and non-abstract methods (with body).

What happens if a subclass does not implement all abstract methods?

A The subclass becomes abstract automatically

B The code compiles without errors

C The subclass can be instantiated normally

D The program crashes at runtime

If a subclass does not implement all abstract methods, it must be declared abstract itself.

Which of the following is true about abstract methods in Kotlin?

A They must have a body

B They cannot be overridden

C They have no body and must be overridden

D They are private by default

Abstract methods have no body and must be overridden in subclasses.

Can you instantiate an abstract class directly?

A Yes, like any other class

B No, you must instantiate a subclass

C Only if it has no abstract methods

D Only inside the same package

Abstract classes cannot be instantiated directly; you must instantiate a subclass that implements all abstract methods.

Explain what an abstract class and an abstract method are in Kotlin and why they are useful.

Think about how abstract classes help organize code and enforce rules for subclasses.

Write a simple Kotlin example showing an abstract class with one abstract method and a subclass that implements it.

Use 'abstract' keyword and 'override' keyword.