Question

1. Why aren’t final variables default initialized? Shouldn’t the default constructor initialize them to default values if you are happy with the constant be the default value.

2. Why must you initialized them in the constructor at all? Why can you can’t you just initialize them before using them like other variables?

ex.

public class Untitled {

public final int zero;

     public static void main(String[] args)
     {
          final int a; // this works
          a = 4; // this works, but using a field doesn't
          new Untitled();
     }
}

Untitled.java:2: variable a might not have been initialized

1. Why must you initialize static final variables when they are declared? Why can’t you just initialize them before using them in any other method?

ex.

public class Untitled
{

      public final static int zero;

      public static void main(String[] args)
      {
           zero = 0;
      }
}

Untitled.java:8: cannot assign a value to final variable zero

I’m asking these question because I’m trying to find a logical/conceptual reason why this won’t work, why it isn’t allowed. Not just because it isn’t.

Answer 1

The idea behind a final variable is that it is set once and only once.

For instance final variables, that means they can only be set during initialization, whether at declaration, in a constructor, or an instance initialization block. For the variable to be set anywhere else, that would have to take place in a non-constructor method, which could be called multiple times – that’s why this is off limits.

Similarly for static final variables, they can only be set at declaration or in a static initialization block. Anywhere else would, again, have to be in a method which could be called more that once:

public static void main(String[] args)
{
     zero = 0;
     main(null);
}

As for your first question, I’m assuming it’s an error not to explicitly set a final variable in order to avoid mistakes by the programmer.