如何使用包私有构造函数创建类？

Question

I'm working on a library where I want to return an object representing a real world object. I want to expose this class to developers to manipulate the real world object, but I don't want to allow them to construct these objects themselves.

Example

public class World {
    public static List<RealObject> getAllObjects() {
        // How to create RealObject with physicalID?
    }
}

public class RealObject {
    private int physicalID;

    public RealObject(int physicalID) {
        // Undesirable, user has no knowledge of IDs
    }

    public void setState(int state) {
        // Code using physicalID to change state
    }
}

These objects currently have no constructor and have a private id field that I set with reflection from within my library. This works perfectly, but I can't help but think there must be a better solution. Perhaps a useful constraint in my situation is that it only needs to be possible to construct this object from one other class.

Is there a better solution? And is it still possible to have the class in a separate file in that case for organizational purposes?

Answer 1

If you put default access level on constructor (or any other method), then can be accessed only by classes from same package.

To concretely answer the question in the title:

public class RealObject {
  RealObject(int physicalID) {
    // Package-private constructor
  }
}