How implement an inner class?

I there, I have this scenario: ``` public class A{ //attributes and methods } public class B{ //attributes and methods } public class C{ private B b; //other attributes and methods } public class D{ private C c1, c2, c3; private List<A> a; //other attributes and methods } ``` Every class has its own file. However, I'd like to get classes A, B and C as inner classes of class D because I'm not using them throughout the program, just in a small part of it. How should I implement them? I've read about it, but I'm still not sure about what is the best alternative: Option 1, using static classes: ``` public class D{ static class A{ //attributes and methods } static class B{ //attributes and methods } static class C{ private B b; //other attributes and methods } private C c1, c2, c3; private List<A> a; //other attributes and methods } ``` Option 2, using an interface and a class that implements it. ``` public interface D{ class A{ //attributes and methods } class B{ //attributes and methods } class C{ private B b; //other attributes and methods } } public class Dimpl implements D{ private C c1, c2, c3; private List<A> a; //other attributes and methods } ``` I'd like to know which approach is better in order to get the same behavior using the original scenario. Is it ok if I use Option 1 and use the classes like this? ``` public method(){ List<D.A> list_A = new ArrayList<D.A>(); D.B obj_B = new D.B(); D.C obj_C1 = new D.C(obj_B); D.C obj_C2 = new D.C(obj_B); D.C obj_C3 = new D.C(obj_B); D obj_D = new D(obj_C1, obj_C2, obj_C3, list_A); } ``` Basicly, my concern is how the creation of inner classes will affect the outter class. In the original scenario, I first create the instances of classes A, B and C, then the instance of class D. Can I do the same with the options I mentioned?