Eclipse can't find / load main class

Question

My Eclipse (Indigo) was running just fine. I created a simple class Hello. It is placed in package cont in the folder ch13. However, when I try to run it from Eclipse I get info from Java Virtual Machine Launcher:

Could not find the main class: cont.Hello.   

I tried to run other classes from this package and they run just fine (that is the classes that existed there before). However any new class I create in this package has these problems. Actually any new class I create in Eclipse runs into this problems. I decided to check how it works from the command line. It seems that the problem still exist - I get same error. I checked the path and it is C:\Program Files\Java\jdk1.7.0_02\bin, which is correct (note the other classes are running from Eclipse just fine). I tried to run with java -cp . Hello and there are some Errors produced starting with java.lang.NoClassDefFoundError: Hello (wrong name: cont/Hello). Code itself is simple:

package cont;  public class Hello {     public static void main(String[] args){         System.out.println("Hello");     }  } 

How can I fix it so that my classes still run under Eclipse?

Answer 1

.metadata is corrupted.

Steps:

Warning: Deleting .metadata will delete all your Eclipse configurations, plugins, project setups. Make a backup before you attempt this!

1. Stop eclipse, delete .metadata in workspace and restart eclipse

2. Import Project

3. Run again

Answer 2

• Removing the Run Configuration

  Sometimes I have a similar problems in some pre-release versions of eclipse. For fix the error, I delete the Run Configuration. You can find that in menu Run, Run Configurations...

  Then I launch the app with Alt+Shift+X, then J. If this don't work, Ctrl+F11.

• Deleting the .metadata directory

  In another way, the configuration settings for your current workspace may are corrupted. Those settings are in the .metadata directory in your current workspace ¹. In that case, there is no other choice than delete the directory:

  1. Close eclipse.
  2. Delete the .metadata directory.
  3. Start eclipse.
  4. Import the projects.
  5. Run the project again.

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Notes

1. You will see that directory with File > Switch Workspace > Other...