公共构造函数，无法创建实例

Question

Have this code:

public final class A {
    public final class B {
    }
    public A(B b) {
    }
}

Problem:

1. You can not create an Instance of A without an instance of B .
2. You can not create an Instance of B without an instance of A .

Without changing the code and without reflection, how to create an Instance of A ?

Answer 1

You can use null :

new A(null);

---

Once you have an A object, you can create a B object too:

A a = new A(null);
B b = a.new B();

Answer 2

To expand on MC Emperor's answer, you can also create an instance of A with non null instance of B :

A a1 = new A(null);
B b1 = a1.new B();
A a2 = new A(b1);

and in a one liner:

A a = new A(new A(null).new B());