package access vs public constructors

1. package access class with public constructor

// hiding/packageaccess/PublicConstructor.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
package hiding.packageaccess;

class PublicConstructor {
  public PublicConstructor() {}
}

2. access public construtor above

// hiding/CreatePackageAccessObject.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// {WillNotCompile}

import hiding.packageaccess.*;

public class CreatePackageAccessObject {
  public static void main(String[] args) {
    new PublicConstructor();// compile error
  }
}

my compiler error message:

CreatePackageAccessObject.java:7: error: package hiding.packageaccess does not exist
import hiding.packageaccess.*;
^
CreatePackageAccessObject.java:11: error: cannot access PublicConstructor
    new PublicConstructor();
        ^
  bad source file: ./packageaccess/PublicConstructor.java
    file does not contain class PublicConstructor
    Please remove or make sure it appears in the correct subdirectory of the sourcepath.
2 errors

Summary, declaring a constructor public inside a packageaccess class doesn't actually make it public, and it should probably be flagged with a compiler error at the point of declaration.

The following table shows the access to members permitted by each modifier.

Access Levels

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

Access levels affect you in two ways.

First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use.

Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.

Let's look at a collection of classes and see how access levels affect visibility. The following figure shows the four classes in this example and how they are related.

 

Classes and Packages of the Example Used to Illustrate Access Levels

The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.

Visibility

Modifier	Alpha	Beta	Alphasub	Gamma
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

Note the difference in column names between table Access Levels and table Visibility.

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/hiding/packageaccess/PublicConstructor.java
3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/hiding/CreatePackageAccessObject.java

4. https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html