Enum中的成员字段顺序

Question

Why the order of declaration important for Java enums, I mean why does this give (compile time) errors

public enum ErrorCodes {
    public int id;
    Undefined;
}

but this one is fine:

public enum ErrorCodes {
    Undefined;
    public int id;

}.

Answer 1

It's not a very satisfying answer, but it's just how enums are defined in Java. See section 8.9 Enums in The Java Language Specification.

Answer 2

Because this is the syntax for enums. It could allow different orders however this may have been open to mistakes such as forgetting to place a type on a field and turning it into a enum value.

EDIT: The reason I say they could be in any order is that fields, methods, initialisers and constructors can be in any order. I believe the restriction is valid if it is to reduce mistakes. Even though fields/constructors/methods can be in any order its very common to see them in that order for readability.

Answer 3

Java Enum is a special kind of class. Its simple and mostly useful form does not contain custom fields:

public enum ErrorCodes {
    Undefined, Defined, Foo, Bar
}

Compiler magic creates class that looks approximately like the following:

public class ErrorCodes {
    public final static ErrorCodes Undefined = new ErrorCodes();
    public final static ErrorCodes Defined = new ErrorCodes();
    public final static ErrorCodes Foo = new ErrorCodes();
    public final static ErrorCodes Bar = new ErrorCodes();
}

This compiler magic expects the fields definition right after the enum header.

Sun were so kind to allow us to add such fields that follow the definition of eunum members: public enum ErrorCodes { Undefined, Defined, Foo, Bar; private String myField; }

This is the reason that your custom code always must be defined after the enum fields.