为什么枚举类型上的私有字段对于包含类可见？ [英] Why are private fields on an enum type visible to the containing class?

问题描述

public class Parent {

    public enum ChildType {

        FIRST_CHILD("I am the first."),
        SECOND_CHILD("I am the second.");

        private String myChildStatement;

        ChildType(String myChildStatement) {
            this.myChildStatement = myChildStatement;
        }

        public String getMyChildStatement() {
            return this.myChildStatement;
        }
    }

    public static void main(String[] args) {

        // Why does this work?
        System.out.println(Parent.ChildType.FIRST_CHILD.myChildStatement);
    }
}

有关访问控制的其他规则父子类，同一个包中的类等，参考这个枚举？我在哪里可以在规范中找到这些规则？

Are there any additional rules with regard to access control for Parent subclasses, classes within the same package, etc., in reference to this enum? Where might I find those rules in the spec?

推荐答案

它与枚举无关 - 它有一切使用从包含类型到嵌套类型的私有访问。

It has nothing to do with it being an enum - it has everything to do with private access from a containing type to a nested type.

从 Java语言规范，第6.6.1节：

否则，如果成员或构造函数被声明为 private ，那么当且仅当发生在顶级类的主体内时才允许访问（§7.6），其中包含成员或构造函数的声明。

Otherwise, if the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor.

例如，这也是有效的：

public class Test {

    public static void main(String[] args) {
        Nested nested = new Nested();
        System.out.println(nested.x);
    }

    private static class Nested {
        private int x;
    }
}

有趣的是，C＃的工作方式略有不同 - C＃私人成员只能在类型的程序文本中访问，包括任何嵌套的类型。所以上面的Java代码将无法正常工作，但是这将是：

Interestingly, C# works in a slightly different way - in C# a private member is only accessible within the program text of the type, including from any nested types. So the above Java code wouldn't work, but this would:

// C#, not Java!
public class Test
{
    private int x;

    public class Nested
    {
        public Nested(Test t)
        {
            // Access to outer private variable from nested type
            Console.WriteLine(t.x); 
        }
    }
}

...但如果你只需将Console.WriteLine更改为System.out.println，这个将在Java中编译。所以Java比私人会员基本上比C＃更松懈。

... but if you just change Console.WriteLine to System.out.println, this does compile in Java. So Java is basically a bit more lax with private members than C# is.

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