What about public method returns private class instance?

Question

I am trying to test the following situation:

1. There is a main. (city.Main)
2. There is a package. (package castle)
3. Within the package, there is a public class (castle.Guard), and a package-private class (castle.Princess).
4. What if, the public class is returning an instance of the private class?

Here is my code:

Main.java

package city;

import castle.Guard;

public class Main {

    public static void main(String[] args) {
        Princess princess = Guard.getPrincessStatic();
        // Error: Princess cannot be resolved to a type
        
        Guard.getPrincessStatic().sayHi();
        // Error: The type Princess is not visible
        
        Guard guard = new Guard();
        guard.getPrincess().sayHi();
        // Error: The type Princess is not visible
        
        guard.getPrincessMember().sayHi();
        // Error: The type Princess is not visible
    }

}

Guard.java

package castle;

public class Guard {

    public Princess getPrincess() {
        return new Princess();
    }

    public static Princess getPrincessStatic() {
        return new Princess();
    }
    
    private Princess m_princess = new Princess();
    
    public Princess getPrincessMember() {
        return m_princess;
    }
}

Princess.java

package castle;

class Princess {
    public void sayHi() { System.out.println("Hi world"); }
}

Notice all the 4 statements in main() are having errors.

I have done some research too. In fact i want to mimic this answer. But i don't why my codes throw errors.

Thanks for any explanations!

---

Edit:

I intend to make the castle-Princess package-private. I know that, by returning a package-private class out of its package, I should be prepared for errors. But why that answer works, while mine doesn't?

Answer 1

Princess class is default scoped to castle package , so its invisible within city package . To circumvent that :

(We can do one of the following three approaches.)

1) Make Princess class public and use it .

package castle;

// added public modifier
public class Princess {
     public void sayHi() { System.out.println("Hi world"); }
}

2) Or, Define a public interface and let Princess implement it and use the interface reference instead of the class reference . I would prefer this .

castle \ IPrincess.java

// Interface definition
package castle;

public interface IPrincess {
    public void sayHi();
}

castle \ Princess.java

// Implement the interface in Princess class
package castle;

class Princess implements IPrincess {
    public void sayHi() { System.out.println("Hi world"); }
}

castle \ Guard.java

// Modify the Guard class
package castle;

public class Guard {
    public IPrincess getPrincess() {
        return new Princess();
    }
    public static IPrincess getPrincessStatic() {
        return new Princess();
    }

    // for here i use Princess instead of IPrincess. (@midnite)
    private Princess m_princess = new Princess();
    public IPrincess getPrincessMember() {
        return m_princess;
    }
}

city \ Main.java

// Modify the main class
  package city;

  import castle.Guard;
  import castle.IPrincess;

  public class Main {

  public static void main(String[] args) {

     IPrincess princess = Guard.getPrincessStatic();

     Guard.getPrincessStatic().sayHi();

     Guard guard = new Guard();
     guard.getPrincess().sayHi();

     guard.getPrincessMember().sayHi();
   }
}

3) Or, Put all the classes in same package .

Answer 2

You are not going to be able to return a type that is privately scoped within a class. How can any external sources know what that type even IS if it's privately scope inside of another class?

This design won't even compile.

Edit:

The simple solution is to make it public and then you can use it externally outside of the package.