Question: I have a problem with my code, even though it hasn't been modified in any way. It just suddenly threw this message. As you can see on the left, that they are all in the correct package, and have correct names.  

What can I do to fix this problem?

[![enter image description here](https://i.stack.imgur.com/ySp3u.png)](https://i.stack.imgur.com/ySp3u.png)

Answer: 
[Clear](https://stackoverflow.com/questions/8689780/how-to-clear-the-cache-in-netbeans/10722287#10722287) the cache to fix it. In Windows, cache is located at:

```
C:\Users\username\AppData\Local\NetBeans\...

```

On Linux, cache is at:

```
/home/username/.cache/netbeans/...

```

After clearing the cache restart netbeans.
If clearing the cache still doesn't work try adding the jar files **themselves** instead of adding the folder. Looks like a bug in Netbeans. I did it that way and worked like a charm.