A.java

public abstract class A<T> {
    public T b;

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

B.java

public abstract class B {

}

ConcreteA.java

public class ConcreteA extends A<ConcreteB> {
    public ConcreteA(ConcreteB b) {
        super(b);
    }
}

ConcreteB.java

public class ConcreteB extends B {
}

Main.java

//....

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

//....