求解惑Cannot find the class file for java.lang.Object.

[小唐人]

我导入一个项目，出现这个问题：
The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project。
网上解决方法：
step1：点击project----->propertys------>Java Build Path -----> Libraries---->Add Libraries----->JRE System Library------>选第一个workspace default jre-------->finish，
step2：点击project----->clean------>clean all projects -----> OK
但是用完出现如下错误：
Activity cannot be resolved to a variable
AdapterView cannot be resolved to a type
AlertDialog cannot be resolved to a type
Align cannot be resolved to a variable
android cannot be resolved
ArrayAdapter cannot be resolved to a type。。。。重复中
求大神解惑。