并提供各种排行榜信息。

In fact, employees, experiences and students describe a role, for example, a person who is a "manager" must be an "employee". In the above design, a person cannot have multiple roles at the same time, and if he is an "employee", he can no longer be a "student". This is obviously unreasonable, because many on-the-job graduate students, even employees, are students.All pilots shall obtain the following qualifications and licenses before performing flight tasks:人们静了一瞬间，黄夫子率先出声问道：谁知你是不是周耀辉的儿子？众人听了，都看向地上跪的姐弟俩。雅兰（李素妍饰）从小失去了父母过着不幸的生活，她意外得知父母不是因为事故而是被他人杀害而死。为了报仇，在缜密的计划下和害死自己父母仇家的儿子贤宇（韩尚金饰）结了婚。贤宇知道了雅兰的秘密，却因为交通事故而成了植物人。恢复意识的贤宇透过整形、改头换面成了另外一个人（裴秀彬饰演），展展了对妻子的报复......D > B enters the synchronized block, and since instance is not null at this time, it immediately leaves the synchronized block and returns the result to the program calling the method. E > At this point, thread B intends to use the Singleton instance, only to find that it has not been initialized, so an error occurs. Therefore, there is still the possibility of errors in the program. In fact, the running process of the program is very complicated. From this, we can see that it is more difficult and challenging to write programs especially in multi-threaded environment. We further optimized the program:大人们则轻声细论其根由。