为什么会这样，新开的课程不会，求教导~~

public class Hello {
   public static void main(String args[]) {
     System.out.println("大家好");
     System.out.println("Nice to meet you");
     Student stu=new Student();
     stu.speak("We are students");
    }
 }
    class Student{
     public void speak(String s){
        System.out.println(s);
   }
}