Simple Class Hierarchy

Main.java

public class Main {
  public static void main(String[] args) {
    A a = new C();
    a.m();
  }
}

class A {
  void m() {
  }
}

class B extends A {
  void m() {
    System.out.println("I am vulnerable");
  }
}

class C extends A {
  void m() {
    System.out.println("I am harmless");
  }
}