Creating instances without new keyword in Kotlin - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Kotlin object instantiation without 'new'

What is the output of this Kotlin code that creates an instance without using the 'new' keyword?

Kotlin

class Person(val name: String) {
    fun greet() = "Hello, $name!"
}

fun main() {
    val p = Person("Anna")
    println(p.greet())
}

A Hello, Anna!

B Hello, null!

C Compilation error: 'new' keyword required

D Runtime error: Uninitialized property

💡 Hint

In Kotlin, you create instances by calling the class name like a function.

Kotlin does not use the 'new' keyword. You create an instance by calling the class constructor directly. Here, Person("Anna") creates an instance with name 'Anna'.

❓ Predict Output

intermediate

Creating data class instance without 'new' keyword

What will be the output of this Kotlin code snippet?

Kotlin

data class Point(val x: Int, val y: Int)

fun main() {
    val p = Point(5, 10)
    println("x=${p.x}, y=${p.y}")
}

A x=0, y=0

B x=5, y=10

C Compilation error: Missing 'new' keyword

D Runtime error: Uninitialized properties

💡 Hint

Data classes are instantiated like normal classes in Kotlin.

Kotlin creates instances by calling the constructor directly. Here, Point(5, 10) creates an instance with x=5 and y=10.

🔧 Debug

advanced

Why does this Kotlin code fail to compile?

Identify the reason this Kotlin code does not compile.

Kotlin

class Animal(val type: String)

fun main() {
    val a = new Animal("Dog")
    println(a.type)
}

A The class Animal is missing a constructor

B The println statement is incorrect

C The property 'type' is not initialized

D Kotlin does not use the 'new' keyword to create instances

💡 Hint

Check how Kotlin creates instances compared to Java.

Kotlin creates instances by calling the constructor directly without 'new'. Using 'new' causes a compilation error.

🧠 Conceptual

advanced

How does Kotlin create instances without 'new'?

Which statement best explains how Kotlin creates instances without using the 'new' keyword?

A Kotlin requires a factory method to create instances

B Kotlin uses a hidden 'new' keyword that is not visible in code

C Kotlin calls the constructor directly as a function without requiring 'new'

D Kotlin uses reflection to create instances at runtime

💡 Hint

Think about how you write code to create objects in Kotlin.

Kotlin simplifies instance creation by calling the constructor directly, so no 'new' keyword is needed.

❓ Predict Output

expert

Output of Kotlin companion object instance creation

What is the output of this Kotlin code that creates an instance using a companion object without 'new'?

Kotlin

class Logger private constructor(val level: String) {
    companion object {
        fun create(level: String) = Logger(level)
    }
}

fun main() {
    val log = Logger.create("DEBUG")
    println(log.level)
}

A DEBUG

B Compilation error: private constructor inaccessible

C null

D Runtime error: Cannot access companion object

💡 Hint

Companion objects can create instances even with private constructors.

The companion object method 'create' calls the private constructor internally, allowing instance creation without 'new'.

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