CH 3 - Classes and objects

Question 1

what is a class

Answer 1

A class in Kotlin is a blueprint for creating objects. It defines the properties (data) and methods (behaviors) that its object instances will have.

Question 2

What is the difference between a class and an object instance in Kotlin?

Answer 2

A class is the blueprint, while an object instance is a specific realization of that class with actual values for its properties.

Question 3

Explain the purpose of a constructor in Kotlin.

Answer 3

constructor initializes an object when it is created. Kotlin classes can have primary constructors (defined in the class header) and secondary constructors.

Question 4

What is the role of the init block in Kotlin classes?

Answer 4

the init block is used to initialize code that needs to run when an object is created. It serves as the body of the primary constructor.

Question 5

How can you make a Kotlin class subclassable?

Answer 5

sue the open keyword to allow subclassing, as Kotlin classes are final by default.

Question 6

What is a data class, and what features does it provide automatically?

Answer 6

A data class is a special class used to store data. It automatically generates methods like toString(), equals(), hashCode(), and copy().

Question 7

Define a simple class in Kotlin named Car with properties brand, model, and a method startEngine(). Then, create an object instance and call the startEngine() method.

Answer 7

class Car(val brand: String, val model: String) {
fun startEngine() {
println(“The engine of $brand $model has started.”)
}
}

val myCar = Car(“Toyota”, “Corolla”)
myCar.startEngine()

Question 8

Create a class Rectangle with a primary constructor that takes the length and width of the rectangle. Include an init block to print the area when an object is created.

Answer 8

class Rectangle(val length: Int, val width: Int) {
init {
println(“Area of the rectangle is ${length * width}”)
}
}

val rect = Rectangle(10, 5)
// Output: Area of the rectangle is 50

Question 9

Write a Kotlin class Person with properties firstName and lastName. Use a custom getter for a fullName property that combines firstName and lastName.

Answer 9

class Person(val firstName: String, val lastName: String) {
val fullName: String
get() = “$firstName $lastName”
}

val person = Person(“John”, “Doe”)
println(person.fullName) // Output: John Doe

Question 10

Use secondary constructors in a Circle class to define additional ways to create a circle using a radius or a diameter. Print the area in the init block.

Answer 10

class Circle(val radius: Double) {
constructor(diameter: Int) : this(diameter / 2.0) {
println(“Using diameter constructor.”)
}

init {
    println("Area of the circle is ${Math.PI * radius * radius}")
} }

val c = Circle(4)
// Output: Area of the circle is 12.566370614359172

Question 11

Create an interface Shape with a method computeArea(), and implement it in a Square class.

Answer 11

interface Shape {
fun computeArea(): Double
}

class Square(val side: Int) : Shape {
override fun computeArea(): Double = (side * side).toDouble()
}

val square = Square(5)
println(“Area of square: ${square.computeArea()}”) // Output: Area of square: 25.0

Question 12

Constructors in Kotlin give example

Answer 12

class Rectangle(val length: Int, val width: Int) {
init {
println(“Area: ${length * width}”)
}
}

val rect = Rectangle(5, 3) // Output: Area: 15

Question 13

Getters and Setters in Kotlin

Answer 13

Definition: Getters and setters allow you to retrieve and update property values. Custom versions can override default behavior.

class Person(var name: String) {
var age: Int = 0
set(value) {
field = if (value > 0) value else 0
}
}

val person = Person(“Alice”)
person.age = -5 // Negative value ignored
println(person.age) // Output: 0

Question 14

Inheritance in Kotlin

Answer 14

open class Animal {
fun eat() = println(“Eating…”)
}

class Dog : Animal() {
fun bark() = println(“Barking…”)
}

val myDog = Dog()
myDog.eat() // Output: Eating…
myDog.bark() // Output: Barking…

Question 15

Interfaces in Kotlin

Answer 15

An interface defines a contract that classes must follow. A class can implement multiple interfaces.

interface Drivable {
fun drive()
}

class Car : Drivable {
override fun drive() {
println(“Driving a car!”)
}
}

val car = Car()
car.drive() // Output: Driving a car!

Question 16

Extension Functions

Answer 16

Extension functions allow you to add functionality to existing classes without modifying them.

fun String.isLongerThanFive() = this.length > 5

println(“Hello”.isLongerThanFive()) // Output: false
println(“Kotlin”.isLongerThanFive()) // Output: true

Question 17

Data Classes

Answer 17

Data classes are used to store data and automatically generate toString(), equals(), hashCode(), and copy() methods.

data class Player(val name: String, val score: Int)

val player = Player(“Alice”, 100)
println(player) // Output: Player(name=Alice, score=100)

Question 18

Enum Classes

Answer 18

num classes define a set of constants that represent specific values.

enum class Direction {
NORTH, SOUTH, EAST, WEST
}

val dir = Direction.NORTH
println(dir) // Output: NORTH

Question 19

Singletons (Object Keyword)

Answer 19

The object keyword defines a singleton class, meaning only one instance of the class exists.

object Logger {
fun log(message: String) = println(“Log: $message”)
}

Logger.log(“Singleton example”) // Output: Log: Singleton example

Question 20

Companion Objects

Answer 20

A companion object allows sharing static-like members (variables or methods) across all instances of a class.

class MathOperations {
companion object {
fun add(a: Int, b: Int) = a + b
}
}

println(MathOperations.add(5, 3)) // Output: 8

Question 21

Packages

Answer 21

Packages in Kotlin help organize code by grouping related classes and functions.

package com.example.myapp

class MyClass {
fun greet() = println(“Hello from MyClass!”)
}

Question 22

Visibility Modifiers

Answer 22

isibility modifiers control access to classes, methods, and properties (public, private, protected).

class Person {
private var name: String = “Unknown”

fun setName(newName: String) {
    name = newName
}

fun getName() = name }

val person = Person()
person.setName(“Alice”)
println(person.getName()) // Output: Alice