I just started learning android recently. Every time I create a new document and use Mainactivity, there will always be a problem of not resolve symbol 'R'. 
Baidu has tried a lot of methods, such as Build->Clean Project (but it doesn't work) ,
Then I checked activity_main.xml again for errors (for example, it is wrong, or the layout file name has uppercase characters, or the image format is wrong, etc.), I checked mine, it is correct, but it still reports an error
After trying all kinds of strange methods on Baidu, the only thing that works for me is: New construction! ! ! ! It is to build a new project again, and then copy all the useful ones before, and no error will be reported. Because of this reason, I can build several projects at a time, which is a bit confusing and a bit helpless. If you have any better ways to use it, please let me know. 
Thank you guys.