为什么枚举的构造函数不能访问静态字段？

Question

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。但是，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

}

Answer 1

因为所有静态字段（包括表示枚举值的静态字段）都是按文本顺序初始化的，而枚举值始终位于其他字段之前，所以在初始化静态字段之前会调用构造函数。请注意，在您的类示例中，您没有显示ABBREV_MAP的初始化位置-如果在&nbsp;SUNDAY&nbsp;之后，则在初始化类时会出现异常。是的，这有点痛苦，可能设计得更好。但是，根据我的经验，通常的答案是static {}在所有静态初始化程序的末尾添加一个块，然后在其中进行所有静态初始化，EnumSet.allOf&nbsp;以获取所有值。

Answer 2

引用JLS，“枚举主体声明”部分：没有此规则，由于枚举类型固有的初始化循环性，显然合理的代码将在运行时失败。（在具有“自类型”静态字段的任何类中都存在圆度。）这是将失败的那种代码的示例：enum Color {&nbsp; &nbsp; RED, GREEN, BLUE;&nbsp; &nbsp; static final Map<String,Color> colorMap = new HashMap<String,Color>();&nbsp; &nbsp; Color() {&nbsp; &nbsp; &nbsp; &nbsp;colorMap.put(toString(), this);&nbsp; &nbsp; }}此枚举类型的静态初始化将引发NullPointerException，因为在运行枚举常量的构造函数时，静态变量colorMap未初始化。上面的限制确保了此类代码不会编译。请注意，可以轻松地将示例重构为正常工作：enum Color {&nbsp; &nbsp; RED, GREEN, BLUE;&nbsp; &nbsp; static final Map<String,Color> colorMap = new HashMap<String,Color>();&nbsp; &nbsp; static {&nbsp; &nbsp; &nbsp; &nbsp; for (Color c : Color.values())&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; colorMap.put(c.toString(), c);&nbsp; &nbsp; }}重构版本显然是正确的，因为静态初始化是自上而下进行的。

Answer 3

也许这就是你想要的public enum Day {&nbsp; &nbsp; Sunday("Sun"),&nbsp;&nbsp; &nbsp; Monday("Mon"),&nbsp;&nbsp; &nbsp; Tuesday("Tue"),&nbsp;&nbsp; &nbsp; Wednesday("Wed"),&nbsp;&nbsp; &nbsp; Thursday("Thu"),&nbsp;&nbsp; &nbsp; Friday("Fri"),&nbsp;&nbsp; &nbsp; Saturday("Sat");&nbsp; &nbsp; private static final Map<String, Day> ELEMENTS;&nbsp; &nbsp; static {&nbsp; &nbsp; &nbsp; &nbsp; Map<String, Day> elements = new HashMap<String, Day>();&nbsp; &nbsp; &nbsp; &nbsp; for (Day value : values()) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; elements.put(value.element(), value);&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; ELEMENTS = Collections.unmodifiableMap(elements);&nbsp; &nbsp; }&nbsp; &nbsp; private final String abbr;&nbsp; &nbsp; Day(String abbr) {&nbsp; &nbsp; &nbsp; &nbsp; this.abbr = abbr;&nbsp; &nbsp; }&nbsp; &nbsp; public String element() {&nbsp; &nbsp; &nbsp; &nbsp; return this.abbr;&nbsp; &nbsp; }&nbsp; &nbsp; public static Day elementOf(String abbr) {&nbsp; &nbsp; &nbsp; &nbsp; return ELEMENTS.get(abbr);&nbsp; &nbsp; }}