枚举类型w.r.t中的静态块的执行顺序到构造函数

问题描述

这是从有效的Java：

This is from Effective Java :

// Implementing a fromString method on an enum type
  private static final Map<String, Operation> stringToEnum
      = new HashMap<String, Operation>();

  static { // Initialize map from constant name to enum constant
    for (Operation op : values())
      stringToEnum.put(op.toString(), op);
  }

  // Returns Operation for string, or null if string is invalid
  public static Operation fromString(String symbol) {
    return stringToEnum.get(symbol);
  }

我的问题是关于这一行：

My question is regarding the line :

我以为静态块在构造函数运行之前被执行。实际上是在类加载时执行的。

I thought the static block gets executed before the constructor runs. The are actually executed during class load time.

我在这里缺少什么？

推荐答案

作为：为什么有一个保证枚举常量将在静态块运行之前被初始化。答案在中给出，而具体示例在中给出，其中以下解释：

I understand your question as: why is there a guarantee that the enum constants will be initialised before the static block is run. The answer is given in the JLS, and a specific example is given in #8.9.2.1, with the following explanation:

和枚举常量是隐式的最终静态，并在静态初始化程序块之前声明。

and the enums constants are implicitly final static and are declared before the static initializer block.

编辑

行为与普通类没有什么不同。下面的代码打印：

The behaviour is not different from a normal class. The code below prints:

In constructor: PLUS
PLUS == null MINUS == null

In constructor: MINUS
PLUS != null MINUS == null

In static initialiser
PLUS != null MINUS != null

In constructor: after static
PLUS != null MINUS != null

public class Operation {

    private final static Operation PLUS = new Operation("PLUS");
    private final static Operation MINUS = new Operation("MINUS");

    static {
        System.out.println("In static initialiser");
        System.out.print("PLUS = " + PLUS);
        System.out.println("\tMINUS = " + MINUS);
    }

    public Operation(String s) {
        System.out.println("In constructor: " + s);
        System.out.print("PLUS = " + PLUS);
        System.out.println("\tMINUS = " + MINUS);
    }

    public static void main(String[] args) {
        Operation afterStatic = new Operation ("after static");
    }
}

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