Why You Should Care About JVM

object Test {

  def main(args: Array[String]): Unit = {
    val a = 1
    val b = 2
    val c = a + b
    println(c)
  }
}

class Test {

  public static void main(String[] args) {
    int a = 1;
    int b = 2;
    int c = a + b;
    scala.Predef$.MODULE$.println(
      scala.runtime.BoxRunTime.boxToInteger(c)
    );
  }
}

 0: iconst_1
 1: istore_2
 2: iconst_2
 3: istore_3
 4: iload_2
 5: iload_3
 6: iadd
 7: istore 4
 9: getstatic #22; scala.Predef$.MODULE$ : scala.Predef$
12: iload 4
14: invokestatic #28 ; scala.runtime.BoxesRunTime
                     ; .boxToInteger(int): java.lang.Integer
17: invokevirtual #32
; scala.Predef$.println(java.lang.Object): void

class Test {

  public static void main(String[] args) {
    int a = 1;
    int b = 2;
    int c = a + b;
    // we finished here
    scala.Predef$.MODULE$.println(
      scala.runtime.BoxRunTime.boxToInteger(c)
    );
  }
}

// what we see

def head[A](nel: NonEmptyList[A]): A = ...

val value: String = head(stringNel)

// pseudocode of what JVM sees

// List can only contain java.lang.Object
def head(list: NonEmptyList): java.lang.Object = ...

// stringNel forgot its type parameter
val value: String = headOption(stringNel)
    .asInstanceOf[String] // so we have to retrieve it

// nothing interesting
val thread = new Thread()
thread.start()

// passing Runnable
val thread = new Thread(() => println("started"))
thread.start()

// extending Thread
val thread = new Thread {
  override def run(): Unit = println("started")
}
thread.start()

// 10 threads that could run submited tasks in parallel
val executorService = Executors.newFixedThreadPool(10)
executorService.submit(() => println("started"))

implicit val ec: ExecutionContext =
    ExecutionContext.fromExecutor(executorService)
// use in e.g. Future