visibility - Understanding java's protected modifier

Question

I have a class called A in package1 and another class called C in package2. Class C extends class A.

A has an instance variable which is declared like this:

protected int protectedInt = 1;

Here is the code for class A

package package1;

public class A {

    public int publicInt = 1;
    private int privateInt = 1;
    int defaultInt = 1;
    protected int protectedInt = 1;

}

And here is the code for class C:

package package2;
import package1.A;

public class C extends A{

    public void go(){
        //remember the import statement
        A a = new A();
        System.out.println(a.publicInt);
        System.out.println(a.protectedInt);

    }
}

Eclipse underlines the last line in C.go() and says "A.protectedInt" is not visible. It seems that this conflicts with the definition of the "protected" keyword, given in the oracle documentation.

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

What's going on here?

question from:https://stackoverflow.com/questions/18573016/understanding-javas-protected-modifier

Answer 1

What's going on here?

You've misunderstood the meaning of protected. You can access the protected members declared in A from within C, but only for instances of C or subclasses of C. See section 6.6.2 of the JLS for details of protected access. In particular:

Let C be the class in which a protected member is declared. Access is permitted only within the body of a subclass S of C.

In addition, if Id denotes an instance field or instance method, then:

• [...]

• If the access is by a field access expression E.Id, where E is a Primary expression, or by a method invocation expression E.Id(. . .), where E is a Primary expression, then the access is permitted if and only if the type of E is S or a subclass of S.

(Emphasis mine.)

So this code would be fine:

C c = new C();
System.out.println(c.publicInt);
System.out.println(c.protectedInt);