How to defind the relationship between classes in java?

Question

More specific about the question: //There're two classes A and B:

Class A {
public static List<B> b = new ArrayList<B>();
}

Class B {
}

In my schema, I want to an object b from Class B. Then under all circumstances,object b will involve at least two objects "a_x" and "a_y" from Class A. How can I create such a relationship?

Answer 1

First thing,This public static List<B> b = new List<B>(); wont compile.

You might need public static List<B> b = new ArrayList<B>();

You cannot instantiate an Interface.So provide an concreate implementation. Ex:ArrayList

And second thing,You should add them directly where ever your are creating this list .

Answer 2

If you have a relation where each B is related to at least two A instances, then B needs a collection-typed field. For example:

   public class B {
       private List<A> relatedAList = new ArrayList<>();
       ...
   }

This needs to be an instance field, not a static field.

But if you have a 2+ relationship, it doesn't make sense to call the related objects x and y ... because what if there is a z , and a p and so on. Unless there is a fixed upper bound on the number of related A objects for each B , you have to use some kind of collection to represent each B 's related A s.

Answer 3

This is the relationship:

class A
{
    public List<B> b = new ArrayList<B>();
}

class B
{

    A ax;
    A ay;

    B(A ax, A ay)
    {
        this.ax = ax;
        this.ay = ay;
    }
}

This is what you need?

Why do you need such relantioship?