The first line of idea plus package packageName cannot find or load the main class

just used idea, to write
package helloWorld;
on the first line like eclipse. The run shows that the main class cannot be found or cannot be loaded. ClassNotFoundExcepttion
what should I do with this error? Eclipse can be run in the same way.

Nov.24,2021

it is estimated that the configuration is incorrect.
it is recommended that you stop messing around. You should create a new project. Right-click on the src folder-> new, according to the settings recommended by idea, and then you can choose to create a new class or create a new package and then create the package name of the class created by class, to help you write it.
you never need to write package and import lines to use idea; they are created automatically by idea.

of course, your problem is not that you can't solve the problem by changing the configuration, but since you are a novice to idea, it is not as fast to change as to build a new project.

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