Data Class and Class in Kotlin

In Kotlin, both data class and class are used to define classes, but they serve different purposes and come with different features. Here’s a detailed comparison:

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1. Class (class)

A regular class in Kotlin is a blueprint for creating objects, with no additional functionality provided by default. You need to implement properties and methods manually.

Features

• Custom Behavior: You can define any kind of behavior, methods, or properties as needed.
• No Auto-Generated Functions: Functions like equals(), hashCode(), toString(), and copy() are not automatically generated.
• Best for classes with significant logic or behavior.

Example

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

fun main() {
    val person1 = Person("Alice", 25)
    val person2 = Person("Alice", 25)

    // Default behavior
    println(person1.toString()) // Output: Person@1b6d3586
    println(person1 == person2) // Output: false (compares references)
}

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2. Data Class (data class)

A data class in Kotlin is specifically designed to hold data. It provides several useful functions out of the box to reduce boilerplate code.

Features

• Auto-Generated Functions:
  • equals() (compares content)
  • hashCode() (hash based on properties)
  • toString() (human-readable representation)
  • copy() (create a copy with optional property modifications)
• Immutable by Default: val is often used for properties to ensure immutability.
• Best for classes meant to store and manipulate data (e.g., DTOs, POJOs).

Example

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

fun main() {
    val person1 = Person("Alice", 25)
    val person2 = Person("Alice", 25)

    // Auto-generated behavior
    println(person1.toString()) // Output: Person(name=Alice, age=25)
    println(person1 == person2) // Output: true (compares content)

    // Copy with modification
    val person3 = person1.copy(age = 26)
    println(person3) // Output: Person(name=Alice, age=26)
}

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Key Differences

Feature	class	data class
Purpose	General purpose	Storing and manipulating data
Equals Comparison	Reference-based	Content-based
HashCode	Not auto-generated	Auto-generated based on content
ToString	Default implementation	Readable representation
Copy	Not available by default	copy() provided
Use Case	For logic and behaviors	For data models or DTOs

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When to Use Which?

• Use class when:
  • You need more complex behavior, logic, or mutable states.
  • The class is not primarily designed to hold data.
• Use data class when:
  • The primary purpose is to store and manipulate data.
  • You need equality checks, toString(), and copy() functionality without writing boilerplate code.

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