Creating instances without new keyword in Kotlin - Try It Yourself

Creating instances without new keyword

📖 Scenario: Imagine you are building a simple app to manage books in a library. You want to create book objects to store information about each book.

🎯 Goal: You will create instances of a Book class without using the new keyword, which is not used in Kotlin. You will then print the details of the book.

📋 What You'll Learn

Create a data class called Book with properties title and author

Create an instance of Book without using new

Print the book details using string interpolation

💡 Why This Matters

🌍 Real World

Creating instances without the <code>new</code> keyword is common in Kotlin and many modern languages. It makes code cleaner and easier to read.

💼 Career

Understanding how to create and use class instances is fundamental for Kotlin developers working on Android apps, backend services, or any Kotlin-based projects.

1

Create the Book data class

Create a Kotlin data class called Book with two properties: title of type String and author of type String.

Kotlin

# Your code here

Hint

Use the data class keyword to create a simple class with properties.

2

Create a Book instance

Create a variable called myBook and assign it an instance of Book with the title "The Hobbit" and author "J.R.R. Tolkien". Do not use the new keyword.

Kotlin

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

# Your code here

Hint

In Kotlin, you create instances by calling the class name like a function without new.

3

Access properties of the Book instance

Create a variable called bookTitle and assign it the title property of myBook. Also create a variable called bookAuthor and assign it the author property of myBook.

Kotlin

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

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

# Your code here

Hint

Use dot notation to access properties of an instance.

4

Print the book details

Print the text "Title: The Hobbit, Author: J.R.R. Tolkien" using the variables bookTitle and bookAuthor with string interpolation.

Kotlin

data 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 and $variableName inside a string to insert variable values.

Practice

1. In Kotlin, how do you create an instance of a class named Car without using the new keyword?

easy

A. val myCar = Car()

B. val myCar = new Car()

C. val myCar = create Car()

D. val myCar = Car.new()

Solution

1. Step 1: Understand Kotlin instance creation

   Kotlin does not use the new keyword; instead, it calls the class name like a function.

2. Step 2: Apply correct syntax for creating instance

   To create an instance of Car, simply write Car() without new.

3. Final Answer:

   val myCar = Car() -> Option A

4. Quick Check:

   Kotlin instance creation = Car() [OK]

Hint: Remember: Kotlin uses class name with parentheses, no 'new' needed [OK]

Common Mistakes:

• Using 'new' keyword like in Java
• Trying to call a method like 'create Car()'
• Using 'Car.new()' which is invalid

2. Which of the following is the correct syntax to create an instance of class Person in Kotlin?

easy

A. val p = new Person()

B. val p = create Person()

C. val p = Person.new()

D. val p = Person()

Solution

1. Step 1: Recall Kotlin instance creation syntax

   Kotlin creates instances by calling the class name with parentheses, no 'new' keyword.

2. Step 2: Identify the correct syntax

   Only val p = Person() matches Kotlin's syntax for creating an instance.

3. Final Answer:

   val p = Person() -> Option D

4. Quick Check:

   Instance creation = ClassName() [OK]

Hint: Use ClassName() directly to create instances in Kotlin [OK]

Common Mistakes:

• Adding 'new' keyword before class name
• Using 'Person.new()' which is invalid
• Trying to use 'create' keyword

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

class Dog(val name: String) {
    fun bark() = "Woof! I am $name"
}

fun main() {
    val dog = Dog("Buddy")
    println(dog.bark())
}

medium

A. Woof! I am name

B. Woof! I am dog

C. Woof! I am Buddy

D. Compilation error

Solution

1. Step 1: Understand instance creation and property assignment

   The instance dog is created by calling Dog("Buddy"), setting name to "Buddy".

2. Step 2: Evaluate the bark() function output

   The bark() function returns the string with $name replaced by "Buddy".

3. Final Answer:

   Woof! I am Buddy -> Option C

4. Quick Check:

   dog.bark() = "Woof! I am Buddy" [OK]

Hint: Instance properties are set by constructor arguments [OK]

Common Mistakes:

• Expecting 'dog' or 'name' literal instead of property value
• Thinking Kotlin requires 'new' keyword
• Assuming compilation error due to syntax

4. Identify the error in the following Kotlin code snippet:

class Book(val title: String)

fun main() {
    val b = new Book("Kotlin Basics")
    println(b.title)
}

medium

A. Missing parentheses after class name

B. Using 'new' keyword which is invalid in Kotlin

C. Incorrect property access syntax

D. Class Book must be declared as data class

Solution

1. Step 1: Check instance creation syntax

   The code uses new Book("Kotlin Basics"), but Kotlin does not use the new keyword.

2. Step 2: Identify correct Kotlin syntax

   Instance creation should be Book("Kotlin Basics") without new.

3. Final Answer:

   Using 'new' keyword which is invalid in Kotlin -> Option B

4. Quick Check:

   Kotlin instance creation = ClassName() without 'new' [OK]

Hint: Remove 'new' keyword to fix instance creation error [OK]

Common Mistakes:

• Adding 'new' keyword from Java habits
• Thinking parentheses are optional
• Confusing data class requirement

5. Given the Kotlin class below, how can you create a list of Person instances without using the new keyword?

class Person(val name: String, val age: Int)

Choose the correct code snippet.

hard

A. val people = listOf(Person("Alice", 30), Person("Bob", 25))

B. val people = listOf(new Person("Alice", 30), new Person("Bob", 25))

C. val people = listOf(Person.new("Alice", 30), Person.new("Bob", 25))

D. val people = listOf(create Person("Alice", 30), create Person("Bob", 25))

Solution

1. Step 1: Understand Kotlin instance creation in collections

   Instances are created by calling the class name with parentheses, no 'new' keyword.

2. Step 2: Check list creation syntax

   listOf() accepts instances created by Person("Alice", 30) and Person("Bob", 25).

3. Final Answer:

   val people = listOf(Person("Alice", 30), Person("Bob", 25)) -> Option A

4. Quick Check:

   Instances in list = Person() calls without 'new' [OK]

Hint: Use ClassName() inside listOf() to create instances [OK]

Common Mistakes:

• Adding 'new' keyword inside listOf()
• Using invalid 'Person.new()' syntax
• Trying to use 'create' keyword