java - java 中的 protected 访问不起作用

Question

考虑类测试

package access;

public class test {
    public String s;
    protected test(String s){
        this.s = s;
        System.out.println("access.test constructor");
    }

    protected void test1(String s){
        this.s = s;
        System.out.println("access.test test1 method");
    }
}

考虑类操作

package data;
public class Operations extends access.test{

    Operations(String s){
        super(s);
    }

    public static void main(String args []) {
        // TODO Auto-generated method stub

        //Operations O = new Operations("Operations!");
        access.test t = new access.test("hello");//1
        t.test1("hi!");                          //2
    }
}

构造函数 test 和方法 test1 在第 1 行和第 2 行不可见。为什么？

Answer 1

在您的data.Operations.main()中，您尝试通过new实例化access.test:

access.test t = new access.test("hello");//1

你不能这么做。这就是它通过错误告诉您的内容。

Section 6.6.1 of the JLS告诉我们:

A member (class, interface, field, or method) of a reference (class, interface, or array) type or a constructor of a class type is accessible only if the type is accessible and the member or constructor is declared to permit access:

• If the member or constructor is declared public, then access is permitted. All members of interfaces are implicitly public.

• Otherwise, if the member or constructor is declared protected, then access is permitted only when one of the following is true:

  • Access to the member or constructor occurs from within the package containing the class in which the protected member or constructor is declared.

  • Access is correct as described in §6.6.2.

我们跳转到6.6.2.2并找到:

A protected constructor can be accessed by a class instance creation expression (that does not declare an anonymous class) only from within the package in which it is defined.

access.test 位于不同的包中，并且您声明了构造函数 protected。只有 access 中的类可以直接调用构造函数(例如使用 new - 这就是“类实例创建表达式”的含义)。

您的data.Operations类扩展了access.test，这很好，因为access.test被声明为public >。您的构造函数是包私有(private)的，因此您可以调用:

Operations o = new Operations("Operations!");

在data.Operations.main()中。 Operation 的构造函数调用 super(s) ，这是允许的，因为它是子类(事实上，它必须这样做，因为父类(super class)中没有空构造函数)。请注意，这与通过 new 直接调用构造函数不同。

如果你有这个:

Operations(String s){
    super(s);
    access.test t = new access.test(s);
}

尝试使用new时会产生相同的错误；你不能那样做。

protected 方法与 protected 构造函数具有不同的访问规则。

您已将 access.test 中的 test1() 声明为 protected。

将方法声明为 protected 意味着 access 包中的类和子类(无论包如何)都可以调用它。因此，以下内容在 data.Operations.main() 中完全有效:

Operations o = new Operations("Operations!");
o.test1("hi!");

如果您的 main() 位于 data 包中的不同类中(或在另一个包中，并且 Operations 有一个 public 构造函数)，你不能这样做。

package data;

public class ThirdClass {

    public static void main(String[] args) {

        // This is perfectly fine since Operations has a package-private constructor
        Operations o = new Operations("Some String");
        // This won't compile
        o.test1("hi!");
    }

}