Here the inner class is not static, so you need to create an instance of outer class and then invoke new,
A a = new Test2().new C();
But in this case, you can make the inner class static,
static class C extends B implements A
then it's ok to use,
A a = new C()
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…