Scala Fundamentals & the JVM

Scala on the JVM

Scala compiles to JVM bytecode — the same bytecode as Java. This means: access to every Java library, deployment on any JVM, and zero-cost Java interoperability. Scala can call Java methods directly; Java can call Scala classes. The Scala compiler adds type-safe abstractions that disappear at runtime (no overhead). Scala 3 (Dotty) modernized the language with cleaner syntax and improved type inference.

Values, Variables, and Types

val is immutable (like Java final); var is mutable. Prefer val — immutability prevents entire categories of bugs. Scala infers types: 'val x = 42' is an Int. Explicit annotation: 'val x: Int = 42'. Types: Int, Long, Double, Boolean, String, Char, Unit (void), Nothing (no value), Any (supertype of all), AnyRef (supertype of all reference types). String interpolation: s'Hello ${name}' (s-string) and f'${price}%.2f' (formatted).

Case Classes and Pattern Matching

Case classes are immutable value types with built-in equals, hashCode, toString, and copy. They enable exhaustive pattern matching. 'case class Point(x: Double, y: Double)' creates an immutable data class. Pattern matching: 'shape match { case Circle(r) => ... case Rectangle(w, h) => ... }'. The compiler warns if a match is not exhaustive. Sealed traits restrict which case classes can extend them — enabling total pattern matching.

// Case classes: immutable value types
sealed trait Shape
case class Circle(radius: Double)          extends Shape
case class Rectangle(width: Double, height: Double) extends Shape
case class Triangle(a: Double, b: Double, c: Double) extends Shape

def area(shape: Shape): Double = shape match {
  case Circle(r)        => math.Pi * r * r
  case Rectangle(w, h)  => w * h
  case Triangle(a, b, c) =>
    val s = (a + b + c) / 2
    math.sqrt(s * (s-a) * (s-b) * (s-c))
}

// Collections: immutable by default
val shapes: List[Shape] = List(
  Circle(5.0),
  Rectangle(3.0, 4.0),
  Triangle(3.0, 4.0, 5.0)
)

val areas = shapes.map(area)
val total = areas.sum
println(s'Total area: ${total.formatted("%.2f")}')

// Option: null-safe optional value
def safeDivide(a: Double, b: Double): Option[Double] =
  if (b == 0) None else Some(a / b)

safeDivide(10, 2).foreach(r => println(s'Result: $r'))
safeDivide(10, 0).getOrElse(0.0)  // 0.0