Visual Studio Code – What's different between run code – run java vscode? And error Could not find or load main class? And package?

I am coding java on vscode. I have source folder:

  >...
  >lib
  >>src
    exam1.java
    exam1.class
    >>Month10
      app1.class
      app1.java

With:

exam1.java 
public class exam1 {
    public static void main(String[] args) {
        System.out.println("Hello world");
    } 
}

app1.java
package Month10;

public class app1 {
    public static void main(String[] args) {
        System.out.println("Hello world");
    }
}

I want to ask about why I need to use package Month10 in here. And another, exam1.java run well(both run code and run java), but in app1.java, I only can "run java", can’t "run code" (Ctrl+Alt+N in vs code), it exists error Could not find or load main class app1, I think because the command package? (I saved file before running), can anyone help me, thanks.