java中類，抽象類，接口，內部類詳解

內部類詳解：https://www.runoob.com/w3cnote/java-inner-class-intro.html
類，抽象類，接口：https://www.cnblogs.com/tubybassoon/p/9548281.html#classO
https://blog.csdn.net/qq_40728375/article/details/79890223