class A {
  private B b;

  public B getB() {
    return b;
  }
}

class B {
  private A a;

  public A getA() {
    return a;
  }
}

Pretty simple.

    The thing to remember is that "has a" really means
"has a reference to a."

    You give your friend a piece of paper with your telephone
number, and she gives you a piece of paper with hers.  Neither
of you has swallowed the other, and there's no crazy infinite
recursion of mutual swallowing: Each of you walks away with
piece of paper, and each has the ability to call the other.