Why should data fields be static and final

Question

Deitel's How To Program Java book says:

A final field should also be declared static if it is initialized in its declaration to a value.

Why is that?

public class A
{
   private final int x = 5;
   private static final int y = 5;
}

I think x and y are the same.
What does the static qualifier matter here?
What is the advantage of the static qualifier up there for software engineering observation?

Answer 1

x is an instance variable while y is global.

What does that mean?

Let's look at this example:

public class A {
    public A() {
        System.out.println("create A");
    }
}

public class B {
    public B() {
        System.out.println("create B");
    }
}

public class C {
    private static B b = new B();
    private A a = new A();
}

Then a main:

public static void main(String[] args) {
    C c1 = new C();
    C c2 = new C();
}

Which prints:

> create B
> create A
> create A

c1 and c2 shares the same instance of B while they both create their own instance of A!

So c1.b == c2.b while c1.a != c2.a.

So summary: there is only one and the same place/address for field b for every instance of class C (c1, c2) but for field a there are different places/addresses in the different instances.

The example is a bit oversized with class A and B: Even for simple fields (int, float, ...) is one and the same place/occurrence for a static field in every instance of a class.