The IDEA can't find or unable to load The main class or Module "*" must not contain The source root "*" The root already belongs to The Module "*"

I used IDEA to write code today, but the runtime always prompts error: the main class could not be found or could not be loaded. Baidu and Google have tried all the methods, but they can't solve it. I'm crazy.

After calming down and thinking for a while, click under IDEAout/production directory, found the problem:

There is an arrow on the left side of the previous module after compilation, indicating that there are files (.class files) in it, but the module in question, that is, Day15_exception, has no arrow, Indicating the bytes generated by the compilationThe code file did not go to this directory, so I checked the IDEA path again and found that there was no problem, which made me even more confused.
It suddenly occurred to me that when I first created the module, the name of the module was Day15_exception, and the code was written to run. Later, I found that it was different from my previous naming convention (the first letter was capitalized), and then I changed it again to become Day15_Exception, as followsPictured:

but out/productionThe name in the directory is still Day15_exception, so I changed the name under out/production to Day15_Exception, but it still doesn't work!!!
More crazy!!!
After fiddling for a while, I thought it was still a path problem, so I changed the path of the module Day15_Exception from the path of the inherited project to the manually set path, as shown below:

Click apply, the following error appears:

means that JavaSE this module cannot contain the source root node Day15_Exception\src, this rootThe node already belongs to the Day15_Exception module, the solution is as follows:

Finally, click apply, ok, re-run, and it can run normally.

Summary: Compilation output path in IDEA
IDEA's default compilation output path is the path of the inherited project:

Path to project:

So by default, the bytecode file generated by the java file compilation in the submodule is in the project path\production\module path, where production is automatically generated.For example, the bytecode file generated by compiling the java file in the Day15_Exception module is located at: D:\Java\JavaSE\out\production\Day15_Exception\package name of all levels\MainClass.class.

Of course, instead of using the default path (inheriting the path of the project), you can set the path yourself, as shown below:

Recommended: use the default inherited project path.