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 final keyword applied to a field has one of two effects:

• on a primitive, it prevents the value of the primitive from being changed (an int can't change value)
• on an object, it prevents the "value of the variable", that is, the reference to the object, from being changed. That is to say that, if you have a final HashMap a, you will only be able to set it once, and you won't be able to do this.a=new HashMap(); again, but nothing keeps you from doing this.a.put("a","b"),s since that doesn't modify the reference, only the content of the object.