Java: classes with instance of each other [duplicate]

Question

I feel like this is a really basic question, but I've never got a situation like this. Right now I have a structure like this:

public class Main{
    ClassA a = new ClassA();
    ClassB b = new ClassB(a);
}

My ClassA is needed in ClassB as well as in the Main.class. Now I noticed that ClassA has to know ClassB as well.

public class Main{
    ClassA a = new ClassA(b);
    ClassB b = new ClassB(a);
}

Obviously won't work. First I thought of using a Observer Pattern but this seems to be an overkill to me, besides it's not what the pattern is usually used for.
Is there a way, that I can make Main.class know ClassA.class & ClassB.class, as well as make ClassA.class know ClassB.class and vice-versa? Since Main.class uses ClassA as well I have to have the same class only initialized once.

Answer 1

You don't need to set the references in the constructor. You can create a setter which you can call after you've created both objects such as b.setA(a);.