为什么枚举引用变量不能单独声明在方法之外？

Question

I am confused by the rules when it comes to creating reference variables to enum types.

OUTSIDE MAIN METHOD:

[A] When I create a reference variable to enum on its own, I am greeted with a syntax error. However, [B] When I join this reference variable to an enum constant, it is somehow fine.

This situation is further complicated when I experiment with putting [A] and [B] inside the main method - as [X] and [Y].

INSIDE MAIN METHOD:

BOTH [X] and [Y] are fine. I am able to create a reference variable [X] exactly as in [A], but without error.

I therefore think the source of the problem is that [A] is outside main method, but [X] is within method.

I am able to create reference variable to enum inside a method, but doing so outside will produce syntax error. Does anyone know why this is so? Thank you very much.

enum Transport {
    CAR, TRUCK, AIRPLANE, TRAIN, BOAT
}

public class EnumDemo {

    Transport tp; // <A> - syntax error on ';'
    tp = Transport.AIRPLANE;
    Transport tb = Transport.AIRPLANE; // <B>

    public static void main(String[] args) {
        Transport tp; // <X> but this is fine.
        tp = Transport.AIRPLANE;
        Transport tb = Transport.AIRPLANE; // <Y>
    }
}

Answer 1

This is nothing specific to it being an enum: you just can't write statements outside methods or block, you can only declare members.

This would work:

public class EnumDemo {

    Transport tp; // <A> - syntax error on ';'
    {
        tp = Transport.AIRPLANE;
    }
    //...

The {} around tp = is called an instance initializer ; these are relatively uncommon, and somewhat confusing, so you should prefer assigning directly on the field (perhaps defining a method to encapsulate necessary logic), or in a constructor.