纯洁的微笑写的：https://www.cnblogs.com/ityouknow/p/5614961.html
集合：http://www.cnblogs.com/ityouknow/category/437541.html
CSDN：https://blog.csdn.net/ochangwen/article/category/6227490
类加载机制：双亲委派机制：
https://blog.csdn.net/ochangwen/article/details/51473120