This indicates that you're trying to run a class called "main.java".  
That is quite unlikely.  It's more likely that you want to run a class
called "main", and that it is defined in a FILE called "main.java".  I
bet you typed the wrong thing into the "Main class:" field when setting
up a run configuration.

You could avoid this by choosing "Run As -> Java Application" from the
Run menu while the correct class is selected rather than creating a Run
configuration by hand... or by using the search button to find your main
class when you create the run configuration.