Why can't I call a protected method from an inheriting class in another package in Java?

Question

Say there's the following base class:

package bg.svetlin.ui.controls; public abstract class Control { protected int getHeight() { //.. } //... }

Also, in the same package, there's a class that inherits:

package bg.svetlin.ui.controls; public abstract class LayoutControl extends Control { public abstract void addControl(Control control); //... }

Then, there's a third class in another package:

package bg.svetlin.ui.controls.screen; public abstract class Screen extends LayoutControl { //... }

And, finally, there's the implementation class, again in a different package:

package bg.svetlin.ui.controls.screen.list; public class List extends Screen { private final Vector controls = new Vector(); public void addControl(Control control) { height += control.getHeight(); controls.addElement(control); } }

Even though List inherits from Control, and the getHeight() is protected, there's the following error:

getHeight() has protected access in bg.svetlin.ui.controls.Control

I've checked that my imports are right. I'm using NetBeans.

Any idea what's wrong? I thought protected fields and methods are visible to the children even if the latter are in a different package.

Thanks!

In your abstract class the method is getWidth(), in the subclass you're using getHeight(). Is this just because it's example code? — Ash
– Ash, Commented Mar 3, 2012 at 12:42

jFrenetic · Accepted Answer · 2012-03-03 13:20:25Z

I thought protected fields and methods are visible to the children even if the latter are in a different package.

That's correct. The class itself has an access to the inherited protected members. But, what you're trying to do it to call the getHeight method on some Control reference. You're allowed to call it only on this instance!

For a better understanding, let me quote Kathy Sierra's SCJP Preparation Guide:

But what does it mean for a subclass-outside-the-package to have access to a superclass (parent) member? It means the subclass inherits the member. It does not, however, mean the subclass-outside-the-package can access the member using a reference to an instance of the superclass. In other words, protected = inheritance. The subclass can see the protected member only through inheritance.

How could I make a method available only to a class and it's sub-classes, then? Thanks!
Like I said, it is available to your subclass. But you only allowed to call it on this reference, such as this.getHeight() (this is optional of course). It's a common practice though, to make an accessor method public.
The dimensions and the positions would be held automatically by my library, so I wanted to leave them private to the library. It looks like I'll have to make them public then. Thanks!

Sam · Accepted Answer · 2012-03-03 13:15:17Z

You're right. Any protected member or method accessible from children class, but you want access to protected method of a parameter instance in addControl method. You can access only to protected method of List class (this.getHeight())

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