Class declaration syntax in Kotlin - Mini Project: Build & Apply

Class declaration syntax

📖 Scenario: You are creating a simple program to represent a book in a library system.

🎯 Goal: Build a Kotlin class called Book with properties for the book's title and author.

📋 What You'll Learn

Create a class named Book

Add two properties: title and author of type String

Create an instance of Book with specific values

Print the book's title and author

💡 Why This Matters

🌍 Real World

Classes help organize data about real-world things like books, making programs easier to understand and maintain.

💼 Career

Understanding class declaration is fundamental for Kotlin developers building apps, especially for Android development.

1

Create the Book class with properties

Write a Kotlin class called Book with two properties: title and author, both of type String. Use the primary constructor to declare these properties.

Kotlin

class Book(val title: String, val author: String) {
    // Your code here
}

Hint

Use class Book(val title: String, val author: String) to declare the class and its properties.

2

Create an instance of the Book class

Create a variable called myBook and assign it a new Book object with the title "The Hobbit" and author "J.R.R. Tolkien".

Kotlin

class Book(val title: String, val author: String)

// Your code here

Hint

Use val myBook = Book("The Hobbit", "J.R.R. Tolkien") to create the instance.

3

Access the properties of the Book instance

Create two variables called bookTitle and bookAuthor and assign them the title and author properties of myBook respectively.

Kotlin

class Book(val title: String, val author: String)

val myBook = Book("The Hobbit", "J.R.R. Tolkien")

// Your code here

Hint

Use val bookTitle = myBook.title and val bookAuthor = myBook.author to get the properties.

4

Print the book's title and author

Write a println statement to display the text: "Title: The Hobbit, Author: J.R.R. Tolkien" using the variables bookTitle and bookAuthor.

Kotlin

class Book(val title: String, val author: String)

val myBook = Book("The Hobbit", "J.R.R. Tolkien")

val bookTitle = myBook.title
val bookAuthor = myBook.author

// Your code here

Hint

Use println("Title: $bookTitle, Author: $bookAuthor") to print the output.

Practice

1. What is the correct way to declare a simple class named Car in Kotlin?

easy

A. def Car() { }

B. class Car { }

C. function Car { }

D. class Car()

Solution

1. Step 1: Understand Kotlin class declaration syntax

   Kotlin uses the keyword class followed by the class name and curly braces to declare a class.

2. Step 2: Check each option

   class Car { } uses class Car { }, which is correct. Options B and C use keywords from other languages. class Car() misses curly braces which are required for an empty class body.

3. Final Answer:

   class Car { } -> Option B

4. Quick Check:

   Kotlin class declaration = class ClassName { } [OK]

Hint: Remember: Kotlin classes start with 'class' and use braces [OK]

Common Mistakes:

• Using 'def' or 'function' instead of 'class'
• Omitting curly braces for class body
• Adding parentheses without constructor parameters

2. Which of the following is the correct syntax to declare a class named Person with a primary constructor having a name parameter of type String?

easy

A. class Person(name: String) { }

B. class Person { val name: String }

C. class Person(name String) { }

D. class Person(name: String);

Solution

1. Step 1: Recall Kotlin primary constructor syntax

   In Kotlin, the primary constructor is declared in parentheses after the class name with parameter types specified using a colon.

2. Step 2: Evaluate options

   class Person(name: String) { } correctly declares class Person(name: String) { }. class Person { val name: String } declares a property inside the class but no constructor. class Person(name String) { } misses colon between parameter and type. class Person(name: String) misses curly braces for class body.

3. Final Answer:

   class Person(name: String) { } -> Option A

4. Quick Check:

   Primary constructor syntax = class Name(params) { } [OK]

Hint: Use colon ':' between parameter name and type in constructor [OK]

Common Mistakes:

• Omitting colon between parameter and type
• Forgetting curly braces for class body
• Declaring properties without constructor parameters

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

class Animal(val name: String) {
    fun speak() = "Hello, I am $name"
}

fun main() {
    val dog = Animal("Buddy")
    println(dog.speak())
}

medium

A. Hello, I am name

B. Compilation error

C. Hello, I am Animal

D. Hello, I am Buddy

Solution

1. Step 1: Understand class and function behavior

   The class Animal has a property name and a function speak() that returns a greeting using the name.

2. Step 2: Trace the code execution

   In main, an Animal object named dog is created with name = "Buddy". Calling dog.speak() returns "Hello, I am Buddy" which is printed.

3. Final Answer:

   Hello, I am Buddy -> Option D

4. Quick Check:

   String interpolation with property = Hello, I am Buddy [OK]

Hint: Look for string interpolation with $property inside functions [OK]

Common Mistakes:

• Confusing property name with literal string
• Expecting class name instead of property value
• Assuming syntax error due to string interpolation

4. Identify the error in the following Kotlin class declaration:

class Book(title: String) {
    fun printTitle() {
        println(title)
    }
}

medium

A. Primary constructor parameter should be declared with val or var

B. Function printTitle() should return a String

C. Property title should be nullable

D. Class body must not contain functions

Solution

1. Step 1: Analyze constructor and property declaration

   The primary constructor parameter title is not declared with val or var, so it is not a property of the class.

2. Step 2: Understand Kotlin property initialization

   Since title is only a parameter, it cannot be accessed by name inside the class methods like printTitle(), causing a compilation error. Use val title: String in the constructor to make it a property.

3. Final Answer:

   Primary constructor parameter should be declared with val or var -> Option A

4. Quick Check:

   Constructor params need val/var for properties [OK]

Hint: Add val/var to constructor params to create properties [OK]

Common Mistakes:

• Thinking function must return a value
• Assuming properties must be nullable
• Believing class cannot have functions

5. You want to create a Kotlin class Student that stores name and grade with default grade 0. How do you declare it so that grade is optional when creating an object?

hard

A. class Student(val name: String) { var grade = 0 }

B. class Student(val name: String, val grade: Int) { init { grade = 0 } }

C. class Student(val name: String, val grade: Int = 0) { }

D. class Student(val name: String, grade: Int = 0) { val grade = 0 }

Solution

1. Step 1: Understand default parameter values in constructors

   Kotlin allows default values for constructor parameters, making them optional when creating objects.

2. Step 2: Check options for default grade

   class Student(val name: String, val grade: Int = 0) { } declares grade with default 0 in constructor, so it can be omitted. class Student(val name: String, val grade: Int) { init { grade = 0 } } tries to assign in init block but grade is val and cannot be reassigned. class Student(val name: String) { var grade = 0 } declares grade as property but not in constructor, so not optional parameter. class Student(val name: String, grade: Int = 0) { val grade = 0 } declares grade without val/var in constructor, so it's not a property, and then declares property separately, which is confusing.

3. Final Answer:

   class Student(val name: String, val grade: Int = 0) { } -> Option C

4. Quick Check:

   Default constructor params = val param: Type = value [OK]

Hint: Use default values in constructor parameters for optional fields [OK]

Common Mistakes:

• Trying to assign val properties inside init block
• Not using val/var for constructor parameters
• Declaring properties outside constructor without defaults