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 same rule also apply for a protected constructor?

pack1:

package pack1;  public class A {     private int a;     protected int b;     public int c;      protected A() {             a = 10;         b = 20;         c = 30;     } } 

pack2:

package pack2;  import pack1.A;  class B extends A {     public void test() {         A obj = new A(); // gives compilation error; why?         //System.out.println("print private not possible :" + a);         System.out.println("print protected possible :" + b);         System.out.println("print public possible :" + c);     } }  class C {     public static void main(String args[]) {         A a = new A(); // gives compilation error; why?         B b = new B();         b.test();     } } 

Answer 1

According to the Java Spec (https://docs.oracle.com/javase/specs/jls/se8/html/jls-6.html#jls-6.6.2.2)

6.6.2.2. Qualified Access to a protected Constructor

Let C be the class in which a protected constructor is declared and let S be the innermost class in whose declaration the use of the protected constructor occurs. Then:

• If the access is by a superclass constructor invocation super(...), or a qualified superclass constructor invocation E.super(...), where E is a Primary expression, then the access is permitted.

• If the access is by an anonymous class instance creation expression new C(...){...}, or a qualified anonymous class instance creation expression E.new C(...){...}, where E is a Primary expression, then the access is permitted.

• If the access is by a simple class instance creation expression new C(...), or a qualified class instance creation expression E.new C(...), where E is a Primary expression, or a method reference expression C :: new, where C is a ClassType, then the access is not permitted. A protected constructor can be accessed by a class instance creation expression (that does not declare an anonymous class) or a method reference expression only from within the package in which it is defined.

In your case, access to the protected constructor of A from B would be legal from a constructor of B through an invocation of super(). However, access using new is not legal.

Answer 2

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).