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Java error: Could not find or load main class - Netbeans IDE

So I was making a java program in Netbeans IDE 8.2 and when I tried to run it, I got this error:
Error: Could not find or load main class helloworld.HelloWorld
C:\Users\name\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 1 second)
(Assume that helloworld is my project name and HelloWorld is my class name)
Everything was fine before, and I can't think of what I did that would cause this error to happen. This is the first time this error has happened to me so I don't know what to do. The code doesn't seem to have any errors.
Other projects of mine seemed to also have the exact same problem even though I haven't opened or edited them in weeks prior to this error occurring.
I tried many other ways but none of them seems to do the trick. Including deleting the cache folder and setting the main class through the properties tab.
Here's a screenshot of my code with the error, my name is crossed out:
Even if I type my code directly in the main class, which is class6homework.java, the error still occurs but it says class6homework.class6homework instead of the one in the screenshot.
Any ideas on how I could fix this?
UPDATE: I got it to work by moving the folder that contains the project to my desktop, didn't know why it wouldn't work in my original location. Thanks for the help.

Perhaps you don't have the main method included. If that is not the problem, try right clicking your class in the project tree and look for an option that says "Set as main class" or something like that

Possible Fixes:
Fix 1
Go to project properties (right click on the folder of your project in netbeans)
On left tab where it shows the categories, click on the "Run" selection
Then click on Browse to find the Main class you use on your project
Fix 2
Go to C:\Users\name\AppData\Local\Netbeans
delete the Cache folder.
Rebuild and Run
Fix 3
Download most recent version of Netbeans
Fix 4
Download most recent version of JDK and configure Netbeans to use that

must declare a named package eclipse because this compilation unit is associated to the named module

I just downloaded eclipse for Java Yesterday but when I was trying to get make my first program, I kept on getting this error:
must declare a named package eclipse because this compilation unit is associated to the named module x.
How do I fix this?

Just delete module-info.java at your Project Explorer tab.

The "delete module-info.java at your Project Explorer tab" answer is the easiest and most straightforward answer, but
for those who would want a little more understanding or control of what's happening, the following alternate methods may be desirable;
make an ever so slightly more realistic application; com.YourCompany.etc
or just com.HelloWorld (Project name: com.HelloWorld and class name: HelloWorld)
or
when creating the java project; when in the Create Java Project dialog, don't choose Finish but Next, and deselect Create module-info.java file

Reason of the error: Package name left blank while creating a class. This make use of default package. Thus causes this error.
Quick fix:
Create a package eg. helloWorld inside the src folder.
Move helloWorld.java file in that package. Just drag and drop on
the package. Error should disappear.
Explanation:
My Eclipse version: 2020-09 (4.17.0)
My Java version: Java 15, 2020-09-15
Latest version of Eclipse required java11 or above. The module feature is introduced in java9 and onward. It was proposed in 2005 for Java7 but later suspended. Java is object oriented based. And module is the moduler approach which can be seen in language like C. It was harder to implement it, due to which it took long time for the release. Source: Understanding Java 9 Modules
When you create a new project in Eclipse then by default module feature is selected. And in Eclipse-2020-09-R, a pop-up appears which ask for creation of module-info.java file. If you select don't create then module-info.java will not create and your project will free from this issue.
Best practice is while crating project, after giving project name. Click on next button instead of finish. On next page at the bottom it ask for creation of module-info.java file. Select or deselect as per need.
If selected: (by default) click on finish button and give name for module. Now while creating a class don't forget to give package name. Whenever you create a class just give package name. Any name, just don't left it blank.
If deselect: No issue

How to get jar class is coming from

I am currently working on an API for a server software so users can extend my software by programming plugins for it instead of modifying the software themselves, and allow other users without programming knowledge to easily change the software by adding these plugins. So far, everything is working fine. But, I am running into a problem with configuration.
You see, each plugin has a plugin.yml file stored with these 4 attributes:
Main: The main class is stored here
Name: This is where the plugin name is stored
Version: This is where the plugin version is stored
Author: This is where the plugin author is stored
Now, in order for the plugin to print something to the console, they use a function called: this.getServer().getLogger().info("MESSAGE); (They extend another class for plugins, thats why they use "this" instead of another class to log)
But, I do not have any idea on how to get which plugin is which when they are calling the function. I have a ArrayList of PluginSessions which event handlers use to cycle through to run Event Functions.
My solution is to get the jar from which a class is being called so I can then get the plugin.yml from there. But, I have NO idea on how to get that, I have tried using Class.forName(); and some other code. But because the class is non-existent within the jar/project running the code, It will throw a ClassNotFoundException.
Does anyone here know how to get the jar from which a class is coming from without using Class.forName()? Thanks! -Trent

Take a look at Class.getResource.
If you call MyClass.getResource("plugin.yml") (or "/plugin.yml" with leading slash, I forget) you get back a file URL pointing to the plugin.yml file in the same jar as MyClass. (Or null if the path is wrong or the jar doesn't contain a "plugin.yml" file.) You can then open an InputStream to that resource. In a plugin framework you may want to use myPluginInstance.getClass().getResource.

Assuming jar for 'PluginSessions' is already added in you classpath by eclipse then you can try the following trick -
Select/highlight PluginSessions by double clicking on it
Now press CTRL+SHIFT+T
A dialog named Open Type is appeared. Here you found from where the PluginSessions class is coming from. If you have more than one jar containing PluginSessions class than you have a list of them.
To benefited from this CTRL+SHIFT+T trick you need to add all of your jar need by the project to be added in your classpath.

Could not find or load main class Swing Java

Error: Could not find or load main class jewelleryerpapplication.GUI.ERPMainMenu
Java Result: 1
Actually i am using Net Beans 7.1 and in that i am continuosly getting the same error but un able to get the solution for that even after installing the latest update for that. The solution which i got from some body that create new project and copy the source and library foldes in that Project from the previous which generate the above error. this solution works for me but i need some simpler solution for this problem.Lot of thanks in advance.
Don't Forget To Give UpVote It If It Helps.

Right click on project node, go to Set configuration, select the main class for your application. Then clean and build.
Even if this doesn't solve your problem, then delete the Netbeans cache by deleting the (index) folder
User\.netbeans\6.9\var\cache\index\

I think that the following is happening:
If this is a third party application, you have not included some jar files on your class path;
Or, (which in my opinion is the most likely) you do not have a method which has this signature: public static void main(String[] args). This method defines the main entry point for your application, not having it might cause that error when you try and run the project. To my knowledge, Netbeans creates a main class with such a method automatically whenever you create a new project. This might be the reason why you are not having this exception when you create a new Project and throw everything in it.
You might be having an issue with Netbeans itself. You can start by either checkin gout this previous SO thread in which a similar (I think) issue is being discussed, or else, as a final resort, you might want to remove Netbeans and all its files, and install an earlier version. You can also try and see if you get the same issue if you use a different IDE such as Eclipse.

the solution is,right click your package in netbeans,go to properties,source now go to the jre option and choose your version,the programme will now run.

Eclipse ClassNotFoundException

Everything works fined, but suddenly eclipse stopped execute and junit tests or even main method, when i run them using run as - > Java application, run as -> junit test
It simply throws error
Caused by: java.lang.ClassNotFoundException: package.ClassName
whene ClassName - is class from where i trying to run method main.
It affect only one of my projects ... Different workspaces works fine, other project in same workspace works fine as well.
I'm sure if i recreate current project, error will gone. But the adjustments of this project in eclipse is really hard, so i want to avoid it.
Any clue?

The ClassName is not in the Class Path, if you start from console you should use -cp parameter , if from eclipse, please add ClassName to the sources of current(start) project.

Thanks to adarshr, I was able to look at the Problems window and determine that the build was failing because it could not find a class I had written.
I had used the MS TFS plugin to create a "shelveset" and it was supposed to have removed my pending changes in the process. However, this integration with the TFS snapin and Eclipse is obviously not well implemented, since the Eclipse project still thought the file existed and was complaining that it could not be compiled.
I went and manually deleted those "files" or "non-existing files" from the Eclipse project (that I thought I had removed with the shelveset action) and the problem was solved.
Also....
Another annoying things is that the Tomcat error I was getting by trying to debug within Eclipse was like this:
SEVERE: Error configuring application listener of class
com.CompanyName.ProjectName.servlet.StartupConfigListener
java.lang.ClassNotFoundException:
com.CompanyName.ProjectName.servlet.StartupConfigListener at
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1643)
In reality, there was no problem at all with StartupConfigListener.java!! The build failed due to the OTHER problems I mentioned above, and therefore I guess this was the first class it attempted to load and failed.... because the entire project hadn't been able to compile perhaps?
(Here's to hoping my next project is using Visual Studio instead of Eclipse!)

Ok, I finally figured it out. The problem was with installed JRE in eclipse setting. I was playing around with this setting and changed installed JRE to JDK, and for some reason it broke the eclipse project.

You can also try going back to the basics. Check your command line and VM args. I've had this situation where a VM arg I was passing in was a path to a file that had a space in the path, and I had forgotten to include the full path in quotes. So e.g., if my arg looked something like
-DFILE=C:\Documents and Settings\myfile
...I'd get a java.lang.NoClassDefFoundError caused by a java.lang.ClassNotFoundException.