Java Eclipse not showing errors and can't run main?

When you mean "isn't showing errors," do you mean that the errors pane is empty, or instead that the error pane is not present? Could it be that you have turned off automatic rebuild of your code?

Does your class have a main method with the proper definition:
public static void main( String[] args ) { ... }
... and are you trying to run that specific class as a main program?

is this code in a document talked approximately as Helloworld.java? The compiler does require that the filename journey the call of the outermost type declared interior it, and supply you blunders in any different case.