Question

I have the following code:

public class Library {

    public void myFunction() {
        // do something
    }
}
public class Book extends Library{

    protected void myFunction() { // Error here
        // do something
    }

}

The above code has error because the Book class is trying to override a function of the supper class with a weaker access modifier. I know this is the rule of Java. But I am curious why is that? What the problem it might cause?

Answer 1

Consider, from an entirely different package:

new Book().myFunction()             // clearly, no access

((Library)new Book()).myFunction()  // now ... access?

The issue really comes down to myFunction being virtual; the method invoked depends on the runtime-type and not the static type of the expression. (I believe C# would actually allow this for a non-virtual method with the new modifier.)