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.

Practice

1. What is true about an abstract class in Kotlin?

easy

A. It cannot be instantiated directly and may contain abstract methods.

B. It can be instantiated like any other class.

C. It must not contain any methods with a body.

D. It is the same as an interface.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes cannot create objects directly because they are incomplete by design.

2. Step 2: Check method types in abstract classes

   Abstract classes can have both abstract methods (without body) and concrete methods (with body).

3. Final Answer:

   It cannot be instantiated directly and may contain abstract methods. -> Option A

4. Quick Check:

   Abstract class = no direct objects + can have abstract methods [OK]

Hint: Abstract classes can't create objects directly [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated
• Believing abstract classes can't have method bodies
• Confusing abstract classes with interfaces

2. Which of the following is the correct way to declare an abstract method in a Kotlin abstract class?

easy

A. abstract fun calculate(): Int

B. fun abstract calculate(): Int

C. fun calculate() abstract: Int

D. abstract calculate fun(): Int

Solution

1. Step 1: Recall Kotlin syntax for abstract methods

   In Kotlin, abstract methods are declared with the abstract keyword before fun, followed by the method signature without a body.

2. Step 2: Check each option's syntax

   Only abstract fun calculate(): Int matches the correct syntax: abstract fun calculate(): Int. Others have incorrect keyword order or punctuation.

3. Final Answer:

   abstract fun calculate(): Int -> Option A

4. Quick Check:

   abstract fun method() = correct syntax [OK]

Hint: abstract keyword comes before fun in method declaration [OK]

Common Mistakes:

• Placing abstract after fun
• Adding a method body to abstract methods
• Incorrect keyword order

3. What will be the output of this Kotlin code?

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

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

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

medium

A. Animal

B. sound

C. Bark

D. Compilation error

Solution

1. Step 1: Understand class inheritance and method override

   The abstract class Animal has an abstract method sound(). The subclass Dog overrides it to return "Bark".

2. Step 2: Analyze main function output

   Creating a Dog object and calling sound() prints "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Overridden method returns "Bark" [OK]

Hint: Overridden abstract method returns subclass value [OK]

Common Mistakes:

• Expecting abstract class method to run
• Confusing method name with output
• Thinking abstract class can be instantiated

4. Identify the error in this Kotlin code:

abstract class Vehicle {
    abstract fun drive()
}

class Car : Vehicle() {
    fun drive() {
        println("Driving")
    }
}

medium

A. drive method in Vehicle should have a body

B. Vehicle class cannot be abstract

C. Car class cannot inherit from Vehicle

D. Missing override keyword in Car's drive method

Solution

1. Step 1: Check method overriding rules in Kotlin

   When a subclass implements an abstract method, it must use the override keyword.

2. Step 2: Identify missing override keyword

   The Car class defines drive() without override, causing a compilation error.

3. Final Answer:

   Missing override keyword in Car's drive method -> Option D

4. Quick Check:

   Override keyword required for abstract method implementation [OK]

Hint: Use override keyword when implementing abstract methods [OK]

Common Mistakes:

• Omitting override keyword
• Adding body to abstract method in base class
• Misunderstanding inheritance rules

5. Given this abstract class and subclass, what will be the output?

abstract class Shape {
    abstract fun area(): Double
    fun description() = "Shape area is ${area()}"
}

class Circle(val radius: Double) : Shape() {
    override fun area() = 3.14 * radius * radius
}

fun main() {
    val c = Circle(2.0)
    println(c.description())
}

hard

A. Shape area is 6.28

B. Shape area is 12.56

C. Shape area is 4.0

D. Compilation error due to abstract method

Solution

1. Step 1: Calculate area in Circle subclass

   Area = π * radius² = 3.14 * 2.0 * 2.0 = 12.56

2. Step 2: Call description method from Shape

   Description returns string with area value: "Shape area is 12.56".

3. Final Answer:

   Shape area is 12.56 -> Option B

4. Quick Check:

   Abstract method overridden, description uses area() [OK]

Hint: description calls overridden area() method [OK]

Common Mistakes:

• Forgetting to override abstract method
• Miscomputing area formula
• Expecting abstract class method to run alone