为什么枚举的构造函数不能访问静态字段和方法？这对于一个类是完全有效的，但对于枚举是不允许的。

我正在尝试将枚举实例存储在静态Map中。考虑下面的示例代码，它允许通过abbreivation查找：

public enum Day {
    Sunday("Sun"), Monday("Mon"), Tuesday("Tue"), Wednesday("Wed"), Thursday("Thu"), Friday("Fri"), Saturday("Sat");

    private final String abbreviation;

    private static final Map<String, Day> ABBREV_MAP = new HashMap<String, Day>();

    private Day(String abbreviation) {
        this.abbreviation = abbreviation;
        ABBREV_MAP.put(abbreviation, this);  // Not valid
    }

    public String getAbbreviation() {
        return abbreviation;
    }

    public static Day getByAbbreviation(String abbreviation) {
        return ABBREV_MAP.get(abbreviation);
    }
}

这将不起作用，因为枚举不允许在其构造函数中使用静态引用。然而，它只是在实现为类的情况下才起作用：

public static final Day SUNDAY = new Day("Sunday", "Sun");
private Day(String name, String abbreviation) {
    this.name = name;
    this.abbreviation = abbreviation;
    ABBREV_MAP.put(abbreviation, this);  // Valid
}