Demonstrating that child classes can be in a list of Type Parent

ParentChildPolymorphismDemo.java

import java.util.ArrayList;
import java.util.List;

class GrandParent {}
class Parent extends GrandParent {}
class Child extends Parent {}
class Grandchild extends Child {}

class Unrelated {}

class ParentChildPolymorphismDemo
{
    public static void main(String[] args)
    {
        List<Parent> list = new ArrayList<>();

        // Obviously, we cannot add an unrelated class
        // list.add(new Unrelated())

        // Cannot use ancestor: 'Parent' base class no longer matches
        // list.add(new GrandParent());

        // Intance objects of the class itself and any children are allowed
        list.add(new Parent());
        list.add(new Child());
        list.add(new Grandchild());
    }
}