Add an object in the deph "level" of an object composition with collections

I'm learning java and starting to try to do things in the right way...

I don't know if the Title is right, please advice me if it's not clear, english is not my mother language. I have a too large amount of code to post it. So i made this "simplify" version for evidence my doubt.I think it's more clear. If is not enough clear please tell me.

• The context: I have a object A who have a collections of B object, and B have a collection of C objects, and C... a collection of D. I need to put the D object in this "composition"(?) of objects. The object D have the information about in which C object should be put in. every C object is assigned to a B object according his id.
• The problem:How to know in A to whish B object pass the D object.
• The "solution": make a method to filter the D object according his id and return the B object id.

This is the scheme of my solution:

    public class Test2 {
    public static void test (String args[]){
        A a = new A();
        //... filling the A,B,C data..
        //... done...
        //.. now the cake:
        public String newD = "C=45,D=67,more info...."; // 45 is the C id, 67 is the D id.
        D dude = new D(newD);
        // so i need to put this friend in the C object...
        a.addSomeDude(dude); // looks easy...
    }

}
class A{ // I have 3 B's in my collection = B_1, B_2, B_3. And i can have more...
    private List<B> Blist;
    private String id;
    // blah blah
    public void addSomeDude(D dude){ //... ok, i have some problems... i don't know where to put this dude
        String Bid = SaveTheday.whoGoesWhere(dude.getId()); //oh! you save the day SavetheDay, what a suitable name!
        for(B bDude : Blist)
            if(bDude.getId() ==Bid ){
                bDude.addSomeDude(dude);
            }

    }
    //some things...
}
class B{ // I have some C's in my collections...
    private List<C> Clist;
    private String id;
    //blah blah blah
    public void addSomeDude(D dude){ //... i know where to put this dude, i just get the C id from the dude...
        for(C cGuy : Clist)
            if(dude.getCid() == cGuy.getId()){
                cGuy.addSomeDude(dude);
            }
    }
}
class C{ // I i have some D dudes...
    private List<D> Dlist;
    private String id;
    //yes, more things...
    public void addSomeDude(D dude){
        D objectDude = new D(dude.getCId());
        Dlist.add(objectDude);
    }

}
class D{
    private  String id;
    private String cId;
    //...some things and getters
}
class SaveTheday{
    //...well, maybe i can do the job...
    public static String whoGoesWhere(String id){
        String [] B_1 = {"1","2","3","4","5","6"}; // these are C ids...
        String [] B_2 = {"11","22","33","44","55"};
        String [] B_3 = {"12","45","55","66","77"};
        for( String i : B_1)
            if (i.equals(id))
                return "B_1";
        for( String i : B_2)
            if (i.equals(id))
                return "B_2";
        for( String i : B_3)
            if (i.equals(id))
                return "B_3";
        return null;
    }
}

This eventually works, but I'm starting to thinks it's a very not friendly OO principles solution. Why?

1. What if i need to add a new C id? yes, add it to the comparison method, but, what about the "open to expansion, no to modification" principle?
2. And if i need to add a new B, unknown number of B's?
3. Where the hell put the whoGoesWhere method, i think i did a good job encapsulating the comparison (i can change it without affect the others classes) but, where to put it? is in the right place?
4. i feel out there should be a better solution to this problem, more flexible, expansible and maintainable solution.

So, can you guys tell me what I'm missing? there's some pattern what I'm not seeing? I hope a expressed myself right, if not, please tell me to add more details.