Classes & Data Classes

A class has a primary constructor with val/var properties. Methods go inside the body:

class Point(val x: Int, val y: Int) {
    fun describe(): String = "Point at ($x, $y)"
}

val p = Point(3, 4)
p.x                  // 3
p.describe()         // "Point at (3, 4)"

val properties are read-only; var properties are mutable.

Add a member function describe() to Point that returns "Point at (X, Y)" — for example, Point(3, 4).describe() returns "Point at (3, 4)". Use string templates.

Starter code

class Point(val x: Int, val y: Int) {
    fun describe(): String {
        return ""
    }
}

A data class automatically generates equals, hashCode, toString, and a copy method:

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

val u = User("Ada", 90)
println(u)                       // User(name=Ada, score=90)

val u2 = u.copy(score = 100)     // new User; u is unchanged

copy is the idiomatic way to build a "modified" version of an immutable value — pass only the fields that change.

Use u.copy(score = u.score + by) inside bumpScore to return a new User with the score increased by by. The original must not be modified.

Starter code

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

fun bumpScore(u: User, by: Int): User {
    return u
}

For things that genuinely change over time, use a var property and a method that updates it:

class Counter {
    var count: Int = 0
    fun bump() { count++ }
}

val c = Counter()
c.bump(); c.bump()
c.count                  // 2

Reach for this when the object models real state (a counter, a connection pool, a session). Otherwise prefer val + copy.

Implement the Counter class with:

• a var count starting at 0
• a method bump() that increments count by 1

Starter code

class Counter {
    var count: Int = 0
    fun bump() {
        // increment count
    }
}