Question

How it comes that I instantiate an abstract class?

  abstract class A {
    val a: Int
  }

  val a = new A {val a = 3}

Or is some concrete class implicitly created? And what do those braces after new A mean?

Answer 1

With this, you implicitly extend A. What you did is syntactic sugar, equivalent to this:

class A' extends A {
    val a = 3
}
val a = new A'

These brackets merely let you extend a class on the fly, creating a new, anonymous class, which instantiates the value a and is therefore not abstract any more.