Data classes for value holders in Kotlin - Step-by-Step Execution

Concept Flow - Data classes for value holders

Define data class with properties

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Create instance of data class

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Access properties directly

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Use built-in functions: toString(), equals(), copy()

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Modify instance with copy() if needed

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Use instance as value holder

Data classes hold values with auto-generated useful functions like toString, equals, and copy for easy use.

Execution Sample

Kotlin

data class User(val name: String, val age: Int)

fun main() {
  val user1 = User("Alice", 30)
  println(user1)
  val user2 = user1.copy(age = 31)
  println(user2)
}

Defines a User data class, creates an instance, prints it, copies it with a changed age, and prints the new instance.

Execution Table

Step	Action	Evaluation	Result
1	Define data class User	User class with properties name:String, age:Int	Class ready for instances
2	Create user1 = User("Alice", 30)	Call constructor	user1 = User(name="Alice", age=30)
3	Print user1	Call user1.toString()	Output: User(name=Alice, age=30)
4	Create user2 = user1.copy(age=31)	Copy user1 with age changed	user2 = User(name="Alice", age=31)
5	Print user2	Call user2.toString()	Output: User(name=Alice, age=31)
6	End of main	Program ends	Execution stops

💡 Program ends after printing user2

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
user1	undefined	User(name="Alice", age=30)	User(name="Alice", age=30)	User(name="Alice", age=30)
user2	undefined	undefined	User(name="Alice", age=31)	User(name="Alice", age=31)

Key Moments - 3 Insights

Why does printing user1 show User(name=Alice, age=30) instead of a memory address?

Because data classes automatically generate a toString() method that shows property names and values, as seen in execution_table step 3.

What does the copy() function do in step 4?

It creates a new instance copying all properties from user1 but allows changing specified properties like age, shown in execution_table step 4.

Are user1 and user2 the same object after copying?

No, copy() creates a new object with the same or changed values; user1 and user2 are different instances as tracked in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is printed when user1 is printed?

A Alice, 30

B User@12345

C User(name=Alice, age=30)

D Error

💡 Hint

Check the Result column in execution_table step 3 for the printed output.

At which step is a new User instance created with age 31?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the Action and Result columns in execution_table step 4.

If we change user2's name to "Bob" in copy(), what changes in variable_tracker after step 4?

A user1 will have name="Bob"

B user2 will have name="Bob" and age=31

C user1 and user2 will be identical

D user2 will have name="Alice" and age=30

💡 Hint

copy() creates a new instance with changed properties, see variable_tracker and step 4.

Concept Snapshot

Data classes hold values with automatic functions.
Syntax: data class Name(val prop: Type)
Auto-generates toString(), equals(), hashCode(), copy().
Use copy() to create modified copies.
Ideal for simple value holders.

Full Transcript

This example shows how Kotlin data classes work as value holders. First, a data class User is defined with two properties: name and age. When we create an instance user1 with name "Alice" and age 30, Kotlin automatically provides a toString() method that prints the object nicely as User(name=Alice, age=30). Then, using the copy() function, we create user2 from user1 but change the age to 31. This creates a new object with the updated age, leaving user1 unchanged. Printing user2 shows the updated values. Data classes simplify handling data by providing useful functions automatically.