blocks|key|1650845|text|当在JVM中装入一个类时，静态字段将按照它们在代码中出现的顺序进行初始化。例如|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1650846|public+class+Test4+{
++++++++private+static+final+Test4+test4+=+new+Test4();
++++++++private+static+int+j+=+6;
++++++++Test4()+{
++++++++++++System.out.println(j);
++++++++}
++++++++private+static+void+test()+{
++++++++}
++++++++public+static+void+main(String[]+args)+{
++++++++++++Test4.test();
++++++++}
++++}|code-block|syntax|javascript|1650847|输出将为0。注意，test4初始化是在静态初始化过程中发生的，并且在这段时间内j还没有被初始化，如稍后出现的那样。现在，如果我们改变静态初始化器的顺序，使得j在test4之前。输出将是6，但是在枚举的情况下，我们不能改变静态字段的顺序。enum中的第一件事必须是常量，这些常量实际上是enums的enum+type.Thus的最终实例它总是保证静态字段不会在enum+constants.Since之前被初始化我们不能给静态字段任何合理的值来在enum构造器中使用，在enum构造器中访问它们是没有意义的。|1650848|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

When a class is loaded in the JVM then static fields are initialized in the order in which they appear in code. For e.g.

<pre><code>public class Test4 {
 private static final Test4 test4 = new Test4();
 private static int j = 6;
 Test4() {
 System.out.println(j);
 }
 private static void test() {
 }
 public static void main(String[] args) {
 Test4.test();
 }
 }
</code></pre>

The output will be 0. Note that test4 initialization takes place in static initialization process and during this time j is not yet initialized as it appears later. Now if we switch order of static initializers such that j comes before test4. The output will be 6.But in case of Enums we cannot alter order of static fields. The first thing in enum must be the constants which are actually static final instances of enum type.Thus for enums its always guaranteed that static fields wont be initialized before enum constants.Since we cannot give any sensible values to static fields for use in enum constructor, it would be meaningless to access them in enum constructor.

blocks|key|1650905|text|通过嵌套类解决了这个问题。优点:它更短，CPU消耗也更好。缺点:+JVM内存中又多了一个类。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1650906|enum+Day+{

++++private+static+final+class+Helper+{
++++++++static+Map<String,Day>+ABBR_TO_ENUM+=+new+HashMap<>();
++++}

++++Day(String+abbr)+{
++++++++this.abbr+=+abbr;
++++++++Helper.ABBR_TO_ENUM.put(abbr,+this);

++++}

++++public+static+Day+getByAbbreviation(String+abbr)+{
++++++++return+Helper.ABBR_TO_ENUM.get(abbr);
++++}|code-block|syntax|javascript|1650907|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The problem solved via a nested class. Pros: it's shorter and also better by CPU consumption. Cons: one more class in JVM memory.

<pre><code>enum Day {

 private static final class Helper {
 static Map&lt;String,Day&gt; ABBR_TO_ENUM = new HashMap&lt;&gt;();
 }

 Day(String abbr) {
 this.abbr = abbr;
 Helper.ABBR_TO_ENUM.put(abbr, this);

 }

 public static Day getByAbbreviation(String abbr) {
 return Helper.ABBR_TO_ENUM.get(abbr);
 }
</code></pre>

blocks|key|1436407|text|也许这就是你想要的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1436408|public+enum+Day+{
++++Sunday("Sun"),+
++++Monday("Mon"),+
++++Tuesday("Tue"),+
++++Wednesday("Wed"),+
++++Thursday("Thu"),+
++++Friday("Fri"),+
++++Saturday("Sat");

++++private+static+final+Map<String,+Day>+ELEMENTS;

++++static+{
++++++++Map<String,+Day>+elements+=+new+HashMap<String,+Day>();
++++++++for+(Day+value+:+values())+{
++++++++++++elements.put(value.element(),+value);
++++++++}
++++++++ELEMENTS+=+Collections.unmodifiableMap(elements);
++++}

++++private+final+String+abbr;

++++Day(String+abbr)+{
++++++++this.abbr+=+abbr;
++++}

++++public+String+element()+{
++++++++return+this.abbr;
++++}

++++public+static+Day+elementOf(String+abbr)+{
++++++++return+ELEMENTS.get(abbr);
++++}
}|code-block|syntax|javascript|1436409|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

maybe this is what you want

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

 private static final Map&lt;String, Day&gt; ELEMENTS;

 static {
 Map&lt;String, Day&gt; elements = new HashMap&lt;String, Day&gt;();
 for (Day value : values()) {
 elements.put(value.element(), value);
 }
 ELEMENTS = Collections.unmodifiableMap(elements);
 }

 private final String abbr;

 Day(String abbr) {
 this.abbr = abbr;
 }

 public String element() {
 return this.abbr;
 }

 public static Day elementOf(String abbr) {
 return ELEMENTS.get(abbr);
 }
}
</code></pre>

Why can't enum's constructor access static fields and methods? This is perfectly valid with a class, but is not allowed with an enum.

What I'm trying to do is store my enum instances in a static Map. Consider this example code which allows lookup by abbreivation:

<pre><code>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&lt;String, Day&gt; ABBREV_MAP = new HashMap&lt;String, Day&gt;();

 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);
 }
}
</code></pre>

This will not work as enum doesn't allow static references in its constructor. It however works just find if implemented as a class:

<pre><code>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
}
</code></pre>

Why can't enum's constructor access static fields?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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为什么枚举的构造函数不能访问静态字段和方法？这在类中是完全有效的，但在枚举中是不允许的。我要做的是将我的枚举实例存储在一个静态映射中。考虑下面的示例代码，它允许通过缩写进行查找：public enum Day {    Sunday("Sun"), Monday("Mon"), Tuesday("Tue"), Wedn...

问为什么枚举的构造函数不能访问静态字段？
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