class A {
    private int foo;
    void bar(B b) { b.foo = 42; }
}

class B extends A { }

编译失败，报错:

A.java:3: error: foo has private access in A
    void bar(B b) { b.foo = 42; }
                     ^
1 error

向基类添加强制转换使其工作。

void bar(B b) { ((A) b).foo = 42; }

有人能解释一下为什么第一个片段是非法的吗？被禁止的原因是什么？这是JLS说:

Otherwise, the member or constructor is declared private, and 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.

据我所知，我的代码符合这个措辞。那么这是 Java 编译器的错误，还是我对 JLS 的解释不正确？

(注意:我不是在寻找解决方法，比如让变量 protected。我知道如何解决这个问题。)

最佳答案

错误消息“在 A 中有私有(private)访问权限”是一个很长一段时间的 java bug。

JDK 1.1:

JDK-4096353 : JLS 6.6.1: When subclass references are used to access privates of superclasses

包含完全符合问题一的代码片段

class X{
  private static int i = 10;
  void f()     {
    Y oy = new  Y();
    oy.i = 5;  // Is this an error? Is i accessable through a reference to Y?
  }
}
class Y extends X {}

他们试图修复它并导致

JDK-4122297 : javac's error messages are not appropriate for a private field.

======TP1======
1  class C extends S {
2      void f(){
3          java.lang.System.out.println("foo");
4      }
5  }
6
7  class S {
8      private int java;
9  }
======
% javac C.java
C.java:3: Variable java in class S not accessible from class C.
    java.lang.System.out.println("foo");
    ^
C.java:3: Attempt to reference field lang in a int.
   java.lang.System.out.println("foo");
       ^
2 errors
======

但是按照规范，java 不是在 C 中继承的，这个程序应该可以编译。

它在 1.2 中修复，但在 1.3 中再次出现

JDK-4240480 : name00705.html: JLS6.3 private members should not be inherited from superclasses

JDK-4249653 : new javac assumes that private fields are inherited by a subclass

当泛型出现时

JDK-6246814 : Private member of type variable wrongly accesible

JDK-7022052 : Invalid compiler error on private method and generics

但是，对于 JLS，这个成员根本不存在于继承的类型中。

JLS 8.2. Class Members

Members of a class that are declared private are not inherited by subclasses of that class.

所以 b.foo 是非法的，因为类 B 没有名为 foo 的字段。 没有限制，是B中没有的字段。

Java 具有强类型，我们无法访问 B 中不存在的字段，即使它们存在于父类(super class) A 中。

强制转换 (A) b 是合法的，因为 B 是 A 的子类。

A 有一个名为 foo 的字段，我们可以访问这个私有(private)字段，因为 b(B b) 是 类中的一个函数>A 即使 b != this 由于

JLS 6.6.1. Determining Accessibility

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.

如果我们写

class A {
  private int foo;
  void baz(A b) { b.foo = 42; }
}

class B extends A { }

class T {
  void x() {
    B b = new B();
    b.baz(b);
  }
}

它将编译，因为 Java 推断多态调用的类型参数。

JLS 15.12.2.7. Inferring Type Arguments Based on Actual Arguments:

A supertype constraint T :> X implies that the solution is one of supertypes of X. Given several such constraints on T, we can intersect the sets of supertypes implied by each of the constraints, since the type parameter must be a member of all of them. We can then choose the most specific type that is in the intersection

关于java - 无法通过子类实例从自己的类访问私有(private)变量，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28377005/