Java protected modifier

Question

I have just got weird error which involves protected modifier.

I have following code:

package p1;

public class C1 {
    protected void doIt() {}
}


package p2;

public class C2 extends p1.C1 {
    private C1 c1_instance;
    public void doItAgain() {
        c1_instance.doIt(); // wtf!!!!
    }
}

I get error, stating that doIt() has protected access and can't be accessed! But I am in the sub class and do have and access to doIt() method.

Is not it a bug?

Answer 1

I also had the impression what protected meant "accessible from the same package or from a subclass" but the Java Language Specification is of course more precise, and explains that in a subclass S of C, "If the access is by a qualified name Q.Id, where Q is an ExpressionName, then the access is permitted if and only if the type of the expression Q is S or a subclass of S."

So you can only access a protected method of the superclass via a reference to the subclass you are calling from, like this:

public class C2 extends C1 {
    private C2 c2_other_instance;
    public void doItAgain() {
        c2_other_instance.doIt();
    }
}

If you explain why you want to access one instance of the superclass from a different instance of the subclass then someone might be able to suggest a better design. Otherwise you will have to make the method public or put the classes in the same package.

Answer 2

In Java, you can't call protected methods on a different instance of the base class, even from within a subclass:

public class C2 extends p1.C1 {
    private C1 c1_instance;
    public void doItAgain() {
        doIt();             // fine
        c1_instance.doIt(); // disallowed
    }
}

The only exception is when both the base class and the subclass are in the same package.

To quote the Java OO tutorial:

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.

Answer 3

Protected is equivalent to package level access; you can't access the method if you are in a different package.

You should be able to call doIt() directly without going through the c1_instance object however, as C2 is a subclass.

From http://download.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

"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."

Answer 4

C2 may be a sub class of C1, but this does not mean it can access those methods on a different instance, i.e. c1_instance may not be an instance of C2. You could access it if it was in the same package.

Answer 5

No, protected assures access in the same package and in the descendant classes. You are neither in the same package nor you are accessing it directly from a descendant class. C2 is a descendant but c1_instance isn't.

Answer 6

If your method is declared as protected then you can access this within this class , withing the package and within the subclass.

now here is the question , why is there an error inspite of accessing this protected member from its subclass. the answer is to access the protected method from a subclass your subclass should use the method directly from the direct subclass. here the direct subclass is C2 but the instance c1_instance is not. you can use this method directly (doIt() in place of c1_instance.doIt())