Classes, objects and shared references

A student record groups a name and mark. A Kotlin class defines properties and operations, and constructing it creates an instance. On the JVM, assigning another variable to that instance shares a reference rather than copying all its data.

Constructor parameters marked val or var become properties. A plain class does not automatically compare all its properties for structural equality. Use === for identity, and choose a data class or an explicit equals contract when value comparison is intended.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

class Student(val name: String, var score: Int)
fun main() {
  val first = Student("Diya", 70)
  val second = first
  second.score += 5
  println(first.score)
  println(first === second)
}

Expected output

75
true

Follow the execution

First and second identify one instance. Updating through second is therefore visible as 75 through first. Both bindings are val; the score property remains mutable because it was declared var.

Try this in the Kotlin compiler by pasting the complete program.

Your task: How would you create an independent student instance?

Kotlin/JVM · complete program

class Student(val name: String, var score: Int)
fun main() {
  val first = Student("Diya", 70)
  val second = first
  second.score += 5
  println(first.score)
  println(first === second)
}

Worked answer

Construct another Student with the desired values. Assigning first to another name only shares its existing reference.

Shared identity is useful when deliberate, but unexpected aliases can spread mutations. A read-only reference does not freeze an object or synchronize its use across threads.

Construct another Student with the desired values. Assigning first to another name only shares its existing reference. Shared identity is useful when deliberate, but unexpected aliases can spread mutations.

Further reading: official Kotlin documentation for this topic.