intellij idea - Can't run the .java file -

i might have problem. have homework, , can't run file, whenever open file downloaded looks this:

i can't figure out, why doesn't normal class. plus testing wrote later main method try simple, won't show me option run file. please appreciated.

(just clear not asking on homework, on file, downloaded, can't program in way right now)

the file shown tree.java doesn't have main method, if want run file in editor, add main method. eg:

public class tree {     public static void main(string[] args)     {          system.out.println("calling main");     } } 

refer this link more details.