Creating instances without new keyword in Kotlin - Visual Walkthrough

Concept Flow - Creating instances without new keyword

Class Definition

↓

Call Constructor

↓

Instance Created

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Use Instance

In Kotlin, you create an instance by calling the class name like a function, without using the 'new' keyword.

Execution Sample

Kotlin

class Person(val name: String)

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

This code creates a Person instance named Anna without using 'new' and prints the name.

Execution Table

Step	Action	Code Line	Result
1	Define class Person with property name	class Person(val name: String)	Class Person ready
2	Call Person constructor with "Anna"	val p = Person("Anna")	Instance p created with name = "Anna"
3	Print p.name	println(p.name)	Output: Anna
4	Program ends	}	Execution complete

💡 Reached end of main function, program stops

Variable Tracker

Variable	Start	After Step 2	Final
p	null	Person(name="Anna")	Person(name="Anna")

Key Moments - 2 Insights

Why don't we use 'new' to create an instance in Kotlin?

Kotlin simplifies instance creation by calling the class name directly as a function, as shown in step 2 of the execution_table.

How does Kotlin know to create an instance when we write Person("Anna")?

Person("Anna") calls the primary constructor of the class Person, which creates the instance, as seen in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of variable 'p' after step 2?

A null

B Person instance with name "Anna"

C String "Anna"

D Undefined

💡 Hint

Check variable_tracker row for 'p' after step 2

At which step does the program print the output?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table

If we change the code to val p = Person("Bob"), what changes in the execution_table?

A Step 1 changes

B Step 3 output remains "Anna"

C Step 2 result changes to name = "Bob"

D No changes at all

💡 Hint

Focus on the 'Result' column in step 2 and step 3 of execution_table

Concept Snapshot

Kotlin creates instances by calling the class name like a function.
No 'new' keyword is needed.
Syntax: val obj = ClassName(args)
This calls the constructor and returns the instance.
Use instance properties or methods with dot notation.

Full Transcript

In Kotlin, you create an instance of a class by calling the class name with parentheses and arguments, without using the 'new' keyword. For example, val p = Person("Anna") creates a new Person object with the name Anna. The program then can use this instance, like printing p.name. This is simpler than some other languages that require 'new'. The execution steps show defining the class, creating the instance, printing the name, and ending the program. The variable tracker shows how 'p' changes from null to the new Person instance. This method is clear and concise for beginners.

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