java重写和弱化访问修饰符

Question

根据JLS 8.4.8.1

An instance method m1, declared in class C, overrides another instance method m2, declared in class A iff all of the following are true:

• C is a subclass of A.

• The signature of m1 is a subsignature (§8.4.2) of the signature of m2.

• Either:

  • m2 is public, protected, or declared with default access in the same package as C, or

  • m1 overrides a method m3 (m3 distinct from m1, m3 distinct from m2), such that m3 overrides m2.

这似乎与以下代码并不矛盾：

public class Main {

    public void f() { }
    public static class A extends Main {
        protected void f() { }
    }

    public static void main(String[] args) {

    }
}

DEMO

但是它不是编译，即使方法f()的重写版本有protected访问修饰符，正如我在提供的规则的第二点中所说的。怎么了

Answer 1

不能使用更严格的访问修饰符重写方法。方法f（）是用访问修饰符public声明的，但您正试图用protected修饰符覆盖它，后者限制性更强。改变

public static class A extends Main{
        protected void f(){ }
    }

到

public static class A extends Main{
        public void f(){ }
    }

Answer 2

这方面的理由在以下JLS中提供8.4.8.3：

The access modifier (§6.6) of an overriding or hiding method must provide at least as much access as the overridden or hidden method, as follows:

• If the overridden or hidden method is public, then the overriding or hiding method must be public; otherwise, a compile-time error occurs.

• If the overridden or hidden method is protected, then the overriding or hiding method must be protected or public; otherwise, a compile-time error occurs.

• If the overridden or hidden method has default (package) access, then the overriding or hiding method must not be private; otherwise, a compile-time error occurs.

请注意，您在问题中引用的部分基本上说明了“基类中方法的访问修饰符不能是私有的”，并且与重写方法的访问修饰符完全无关