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Eclipse java issue with the class path

Im creating a message publishing application on eclipse for the minute there is no GUI involved it is just going to be running from the console. But when I compile and try to run the server class I get an error saying
The weird thing is the error on server.java line 24 is just a constructor creating a message class. I asked my tutor about this and he said it was some issue with the classpath not being defined properly in the IDE. I have tried compiling it and running it from command prompt and it works find it is just an Eclipse issue I believe. I have tried looking for an answer myself online but haven't come up with anything. If anyone has come across this before, how did you solve it?

Recompiling and running a decompiled MCP minecraft client throws a noSuchMethod error

So in the past days i've decompiled minecraft 1.12.2 using mcp to make an "own" client. I ran it from eclipse and all was working just fine. But once i exported it, added the needed JSON file and started it from the minecraft launcher, it resulted the following error:
Exception in thread "Client thread" java.lang.NoSuchMethodError: org.apache.logging.log4j.Logger.info(Ljava/lang/String;Ljava/lang/Object;)V
at net.minecraft.client.Minecraft.<init>(Minecraft.java:420)
at net.minecraft.client.main.Main.main(Main.java:115)
In my opinion that'a a bit strange, because when I ran it from eclipse that error did NOT occur.
I was thinking about adding the logger in the specific JSON file for the version but I do not know how to do it. I took the JSON file from a video where I expected my problem to be solved, made some changes, but it didn't. I also put the folder containing the logger-jar (org.apache....) in the client jar, but still no success.
Also i just decompiled 1.12.2 and recompiled it right away to see if i had messed up my code some how.
I hope someone can help.

This means at runtime you have a class that is different than what you had at compile time. You should try unzipping the jar and searching for the class file. It sounds like you need to potentially remove it and replace it with your own version.

Thanks to all who viewed an wanted to help!
Now I found an other way to use it. I wanted to export it to test
it on my server. But instead I used my account credentials to log in directly from eclipse.
:)

Netbeans 8.02: An Instance of the program already running

Suddenly I have started getting this error.
It was working fine as expected until recently for months without any issue.
The problem I am facing is when I am opening file associated with Netbeans using double click, on 1st instance it opens fine but while opening the second file using double click.
I get a window telling:
An instance of the program is already running with your user directory
Either a previous session of the program is running on a different computer
and using the same user directory.
If another session of program is running with the same user
directory,please click to cancel to prevent the corruption of the user
directory. If you are sure that no other instances of the program is running
with your user directory, click OK to continue.
I have google for error. The only solution I found is that there is lock file in your user directory which needs to be deleted.
But in my case what I observed that there is no stale lock file. The lock is present only when Netbeans is running. once I close the netbeans, the lock file is removed on its own.
I have also deleted all the cache, thinking that it might be causing some issues.
But none of it could resolve the issue.
Again If I directly starts Netbeans without double clicking the file, then I am not able to open even a single file using double click and I get the same error box.

Maybe it is late to write but i have had exactly the same problem. So many times i uninstalled and installed old version (like 7.x) , even i uninstalled java jdk :)
I thought to start using JetBrains but i couldnt open the JetBrains. I searched why JetBrains doesnt open for a few hours and i realized that the problem is Kaspersky.
https://intellij-support.jetbrains.com/hc/en-us/community/posts/206489659-Error-Cannot-Lock-System-Folders
I uninstalled Kaspersky and opened the JetBrains.
Bingo! :)
And with that NetBeans (8.2) started to open without any problem.
I have no idea why Kaspersky would be a problem.
(Platform: Win 10 , x64)

Android : The Missing R

I have seen alot of cases of "R cannot be resolved to a variable". Though my case is odd.
It was working perfectly about 15 minutes ago and within that frame, R has now vanished and cleaning the project won't cut it.
I have also ensured that all of my SDK parts are up to date before and after the incident. And the problem still occurs.
Can anyone figure out a solution as to why it would disappear just like that? Even though the SDK was up to date before R vanished?
And another question. Why does this R file cause so much trouble? (I am using Eclipse btw)

This can certainly be a nuisance.
Make sure any R. imports haven't sneaked their way in. Your imports should be bereft of any 'R.'
If you have changed your package name recently, ensure that AndroidManifest.xml is still pointing to the correct package. Using eclipse to refactor does not guarantee the manifest will update.
A side note, if you have errors in your xml files (layouts etc) this will cause a Resource error. Not all errors will be caught by eclipse, so you have to go through those files with a fine comb.
Sometimes simply cleaning, closing and restarting eclipse can help.

Check if you've recently added some music or pictures (any type of resource generally) to your res folder structure. Name of resource must be in lowercase completely, must not start with number...

Also look at the output in the eclipse console. It sometimes show error messages if it can't generate R.java

Runtime Error in JavaHelp

This is an issue that I have been running into for months and have been unable to solve it. We have continued to develop our JavaHelp; however, we are at a point where this needs to be fixed and I am out of ideas.
What this is: A Java Application
Folder structure:
-src
--HelpsetProject.java //Main method to launch JavaHelp
--help-hs.xml
--help-idx.xml
--help-map.xml
--help-toc.xml
--Images/ //images folder
--Topics/ //Topics folder for html files
The issue:
The application builds and compiles fine. It launches and runs fine, displaying the help as intended. However, fairly often when launched, the left side (item list/tree) will have a blank line or will be completely blank:
I originally thought that this issue was caused by missing map-toc tags, mistyped/mismatched xml tags/elements and general typos, as I am not the one actually creating these. However, I went over the files, validated and checked XML, double checked that all items on the list link to a file and clicked on every item in the list to see that it displays. The issue still persists, unless I missed something.
Here is the source code on GitHub:
Here you can download the executable and run it several times to see the error for yourself (It really is a chance for it to work/not work):