从不同包的另一个实例的子类调用受保护的方法

I want to invoke a protected method of another instance from within a subclass of the class providing this protected method. See the following example:

public class Nano {

    protected void computeSize() {
    }

}

public class NanoContainer extends Nano {

    protected ArrayList<Nano> children;

}

public class SomeOtherNode extends NanoContainer {

    // {Nano} Overrides

    protected void computeSize() {
        for (Nano child: children) {
            child.computeSize();            // << computeSize() has protected access in nanolay.Nano
        }
    }

}

javac 告诉我 computeSize() 在 Nano 中具有受保护的访问权限.我看不出这是什么原因(我以为我已经在其他代码中这样做了).我想保持这种方法受到保护，我该怎么办?

javac version "1.7.0_09"

nanolay.Node
nanolay.NanoContainer
nanogui.SomeOtherNode