Why must a final variable be initialized before constructor completes?

Question

Why must a final variable be initialized before constructor completes?

public class Ex
{
  final int q;
}

When I compile this code I get er

Answer 1

The official reason is that it is defined by the Java Language Specification 8.3.1.2:

A blank final instance variable must be definitely assigned at the end of every constructor of the class in which it is declared; otherwise a compile-time error occurs.

A blank final is a final variable whose declaration lacks an initializer (i.e. what you describe).