Protected constructor and accessibility

Question

Why can\'t we instantiate a class with a protected constructor if its child is in a different package? If protected variables and methods can be accessed, why doesn\'t the s

Answer 1

JLS 6.6.7 answers your question. A subclass only access a protected members of its parent class, if it involves implementation of its parent. Therefore , you can not instantiate a parent object in a child class, if parent constructor is protected and it is in different package...

6.6.7 Example: protected Fields, Methods, and Constructors Consider this example, where the points package declares:

package points;
public class Point {
    protected int x, y;
    void warp(threePoint.Point3d a) {
        if (a.z > 0)        // compile-time error: cannot access a.z
            a.delta(this);
    }
}

and the threePoint package declares:

package threePoint;
import points.Point;
public class Point3d extends Point {
    protected int z;
    public void delta(Point p) {
        p.x += this.x;      // compile-time error: cannot access p.x
        p.y += this.y;      // compile-time error: cannot access p.y
    }
    public void delta3d(Point3d q) {
        q.x += this.x;
        q.y += this.y;
        q.z += this.z;
    }
}

which defines a class Point3d. A compile-time error occurs in the method delta here: it cannot access the protected members x and y of its parameter p, because while Point3d (the class in which the references to fields x and y occur) is a subclass of Point (the class in which x and y are declared), it is not involved in the implementation of a Point (the type of the parameter p). The method delta3d can access the protected members of its parameter q, because the class Point3d is a subclass of Point and is involved in the implementation of a Point3d. The method delta could try to cast (§5.5, §15.16) its parameter to be a Point3d, but this cast would fail, causing an exception, if the class of p at run time were not Point3d.

A compile-time error also occurs in the method warp: it cannot access the protected member z of its parameter a, because while the class Point (the class in which the reference to field z occurs) is involved in the implementation of a Point3d (the type of the parameter a), it is not a subclass of Point3d (the class in which z is declared).