poly.java

import java.io.*;
import java.util.*;

public class Solution {
  public static class A {

  }
  
  public static class B extends A {
    
  }
  
  public static void mymethod(A a) {
    System.out.println("Hello world!\n");
  }
  
  public static void main(String[] argv) {
    B b = new B();
    
    Solution.mymethod(b);
  }
}