Why should the static field be accessed in a static way?

Solution 1:

Because when you access a static field, you should do so on the class (or in this case the enum). As in

MyUnits.MILLISECONDS;

Not on an instance as in

m.MILLISECONDS;

Edit To address the question of why: In Java, when you declare something as static, you are saying that it is a member of the class, not the object (hence why there is only one). Therefore it doesn't make sense to access it on the object, because that particular data member is associated with the class.

Solution 2:

There's actually a good reason:
The non-static access does not always work, for reasons of ambiguity.

Suppose we have two classes, A and B, the latter being a subclass of A, with static fields with the same name:

public class A {
    public static String VALUE = "Aaa";
}

public class B extends A {
    public static String VALUE = "Bbb";
}

Direct access to the static variable:

A.VALUE (="Aaa")
B.VALUE (="Bbb")

Indirect access using an instance (gives a compiler warning that VALUE should be statically accessed):

new B().VALUE (="Bbb")

So far, so good, the compiler can guess which static variable to use, the one on the superclass is somehow farther away, seems somehow logical.

Now to the point where it gets tricky: Interfaces can also have static variables.

public interface C {
    public static String VALUE = "Ccc";
}

public interface D {
    public static String VALUE = "Ddd";
}

Let's remove the static variable from B, and observe following situations:

• B implements C, D
• B extends A implements C
• B extends A implements C, D
• B extends A implements C where A implements D
• B extends A implements C where C extends D
• ...

The statement new B().VALUE is now ambiguous, as the compiler cannot decide which static variable was meant, and will report it as an error:

error: reference to VALUE is ambiguous
both variable VALUE in C and variable VALUE in D match

And that's exactly the reason why static variables should be accessed in a static way.

Solution 3:

Because ... it (MILLISECONDS) is a static field (hiding in an enumeration, but that's what it is) ... however it is being invoked upon an instance of the given type (but see below as this isn't really true¹).

javac will "accept" that, but it should really be MyUnits.MILLISECONDS (or non-prefixed in the applicable scope).

¹ Actually, javac "rewrites" the code to the preferred form -- if m happened to be null it would not throw an NPE at run-time -- it is never actually invoked upon the instance).

Happy coding.

I'm not really seeing how the question title fits in with the rest :-) More accurate and specialized titles increase the likely hood the question/answers can benefit other programmers.