Java 7中通用类的变量访问的更改 [英] Changes in access of variables for generic classes in Java 7

问题描述

这里是一些使用Java 6编译但不在Java 7中编译的代码的简单示例。

Here is a simple example of some code that compiles using Java 6, but does not compile in Java 7.

public class Test<T extends Test> {

  private final int _myVar;

  public Test(int myVar) {
    _myVar = myVar;
  }

  public int get(TestContainer<T> container){
    T t = container.get();
    return t._myVar;
  }

  private static class TestContainer<T extends Test> {
    private final T _test;
    private TestContainer(T test) {
      _test = test;
    }
    public T get(){
      return _test;
    }
  }
}

在 get（TestContainer< T> container）方法中编译，出现错误：

In Java 7, it fails to compile in the get(TestContainer<T> container) method, with the error:

错误：_myVar在测试中有私人访问

error: _myVar has private access in Test

我不明白为什么不再编译 - 。变量 t 是 T 类型，必须扩展 Test 。它试图从类中测试 c访问 Test 实例的字段 _myVar / code>。

I don't understand why this no longer compiles - in my mind it should. The variable t is of type T, which must extend Test. It's trying to access the field _myVar of a instance of Test from within the class Test.

确实，如果我改变方法 get（TestContainer< T&到下面，它编译（没有警告）：

Indeed, if I change the method get(TestContainer<T> container) to the following, it compiles (with no warnings):

public int get(TestContainer<T> container){
  Test t = container.get();
  return t._myVar;
}

• 为什么不再编译？


• 这是Java 6中的错误吗？如果是这样，为什么？


• 这是Java 7中的错误吗？



一个google并在Oracle错误数据库中搜索，但没有找到任何东西...

I've had a google and searched in the Oracle bug database, but haven't found anything on this...

推荐答案




§4.9 ...然后交叉类型具有与具有空体，直接超类Ck和在出现交叉类型的相同包中声明的直接超接口T1'，...，Tn'的类类型（§8）相同的成员。

§4.9 ... Then the intersection type has the same members as a class type (§8) with an empty body, direct superclass Ck and direct superinterfaces T1', ..., Tn', declared in the same package in which the intersection type appears.

根据我对JLS部分的理解，你的情况下有一个类型变量< T extends Test> 创建以下交集：

From my understanding of that JLS part, your case with a type variable <T extends Test> creates the following intersection:

package <the same as of Test>;

class I extends Test {}

类型 T 您实际访问的交集类型 I 的成员。由于访问此类成员的子类从不继承私有成员，因此编译错误失败。另一方面，访问package-private（默认）和受保护的成员是允许的交集是

Therefore when you access members of the type T you actually access members of the intersection type I. Since private members are never inherited by subtypes accessing such member fails with compile-error. On the other hand access to package-private (default) and protected members is allowed by the fact the intersection is




...在其中出现交叉路口类型的相同包。

... declared in the same package in which the intersection type appears.

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