Difference between using var/val or not when creating a Constructor

[JayCesar]

📌 Difference Between Two Kotlin Class Definitions

1️⃣ First Version (Without var or val)

open class Point2D(x: Double, y: Double) {}

Characteristics:

• x and y are constructor parameters, not properties.
• They exist only inside the constructor and are not accessible outside the class.
• If you want to use them inside the class, you must declare properties explicitly:

  open class Point2D(x: Double, y: Double) {
      val xPos = x  // Now xPos is a property
      val yPos = y
  }

Example of Incorrect Usage:

fun main() {
    val point = Point2D(3.0, 4.0)
    println(point.x)  // ❌ Error: Unresolved reference 'x'
}

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2️⃣ Second Version (With var)

open class Point2D(var x: Double, var y: Double) {}

Characteristics:

• x and y are declared as mutable properties of the class.
• They are automatically stored in the object and can be accessed or modified externally.

Example of Correct Usage:

fun main() {
    val point = Point2D(3.0, 4.0)
    println(point.x)  // ✅ Works: Prints 3.0
    point.x = 5.0     // ✅ Can modify since it's `var`
    println(point.x)  // ✅ Prints 5.0
}

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3️⃣ Comparison Table

Feature	open class Point2D(x: Double, y: Double)	open class Point2D(var x: Double, var y: Double)
x and y are...	Constructor parameters only	Properties of the class
Accessible outside?	❌ No	✅ Yes
Mutable?	❌ No	✅ Yes (var allows mutation)
Stored in object?	❌ No	✅ Yes

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4️⃣ When to Use Each Version?

• Use the first version if you only need x and y inside the class (e.g., for calculations).
• Use the second version if you want x and y to be stored as properties and accessible externally.

Would you like a real-world analogy to clarify further? 🚀