Pfthb

I have an Interface (call it Planet) whose implementation class is protected, and I can not change access modifier for the same. However, I need to call the methods of the implementation class inside of my main class. For example, in the implementation class, I have: public void orbit() distance++;
I have tried: private Planet planet = new PlanetImpl, but since the PlanetImpl class is protected and in a different package, it can't find it, making the statement invalid. I have also tried leaving it null, like this
Planet planet; planet.orbit();
but it throws a NullPointerException. Is it possible to call the methods without directly referencing the implementation class?

Assuming that private/protected impl class is inner class. To call the orbit() method which is a instance method(not static), You need to create a public static factory method in the outer class and call the orbit using the instance provided by this method.

basic code example

 public class PlanetImplholder
 public static Planet getInstance()
 return new PlanetImpl();
 

 private static class PlanetImpl implements Planet

 @Override
 public void orbit() 
 System.out.println("111");
 
 

call the method from anywhere like below.

Planet planet = PlanetImplholder.getInstance();

 planet.orbit();

Using anonymous inner class we can do it. Anonymous classes can implement interfaces, and i think at that time only you'll have a chance to see a class implementing an interface without the "implements" keyword.

interface Planet 
public void orbit();

class PlanetMain 
private Planet p = new Planet () 
 public void orbit() 
 System.out.println("interface Planet class executed");
 
 ;

In your case you have to create an object of that PlanetImpl class somewhere to execute the method. The interface has only the definition of that method, not the implementation.

Is it possible to call the methods without directly referencing the implementation class?

NO, THIS IS NOT POSSIBLE. Interface and implementation class are just declaration. To use it, you have to create class or interface instance with new. But you cannot do it, because you want to use logic of PlanetImpl, which is protected.

I think, that you do not want to implement your own class declaration and then create and use new instance of it. I think, that only one way is using reflection. Using it, you can create an instance of protected class PlanetImpl and then use it in normal ways.

Class<Planet> cls = (Class<Planet>)Class.forName("<full PlanetImpl class name, inclusing package>");
Constructor<Planet> constructor = cls.getDeclaredConstructor();
constructor.setAccessible(true);
Planet planet = constructor.newInstance();
planet.orbit();

I can't test it right now, but you can use reflection to access the class properties of an object.

Planet planetImpl = factory.getPlanet(); // getPlanet() returns PlanetImpl
Method orbit = planetImpl.getClass().getDeclaredMethod("orbit");
orbit.setAccessible(true);
orbit.invoke(planetImpl);

This may not work if a SecurityManager prevents access to PlanetImpl.

So what I ended up doing was re-working my code so that I could call PlanetImpl from a different class. So now it's Universe universe = new Universe();
Planet planet = universe.getPlanet(); Thanks so much for all the help! And sorry for not trying this before asking the question.