我正在阅读ThinkingInJava，我遇到了一个小问题。我正在做第8章的练习12。Createaninterfacewithatleastonemethod,initsownpackage.Createaclassina>separatepackage.Addaprotectedinnerclassthatimplementstheinterface.Inathird>package,inheritfromyourclassand,insideamethod,returnanobjectoftheprotected>innerclass,upcastingtotheinterfac