6.6.7 Example: protected Fields, Methods, and Constructors

Consider this example, where the point 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 (§5.4, §15.15) 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 Point (the type of the parameter a), it is not a subclass of Point (the class in which z is declared).