A.java

public abstract Class A {
    public B b;

    public A(B b) {
        this.b = b;
    }
}

B.java

public abstract Class B {

}

ConcreteA.java

public class ConcreteA extends A {
    public ConcreteB b;

    public ConcreteA(ConcreteB b) {
        super(b);
    }
}

ConcreteB.java

public class ConcreteB extends B {
}

Main.java

//....

ConcreteB b = new ConcreteB();
ConcreteA a = new ConcreteA(b);

//....