PolymorphismExample.java

import java.util.Collection;
import java.util.LinkedList;

public class Main
{
	public static void main(String[] args) {
		Node carNode = new CarNode();

		for (int i = 1; i <= 4; i++) {
			carNode.getChildren().add(new TireNode(i));
		}
		// Fifthwheel node is defined outside the "library"
		carNode.getChildren().add(new FifthWheelNode());
		// As is ExtendedTask
		carNode.doTask(new ExtendedTask());

		/*
			Output:
			   	Car
				Tire: 1
				Tire: 2
				Tire: 3
				Tire: 4
				Tire: 5

			Notice that the last print statement (Tire: 5) doesn't actually print the message defined in ExtendedTask,
			but instead prints the default doTask(TireNode tireNode) method as defined in the Task interface.  I could 
			add the doTask(FifthWheelNode) to the Task interface, but what about a developer using the library, who couldn't
			modify the original source?
		 */
	}

	private interface Task
	{
		default void doTask(Node node) { System.out.println("Node"); }

		default void doTask(CarNode carNode) { System.out.println("Car"); }

		default void doTask(TireNode tireNode) { System.out.println("Tire: " + tireNode.size); }
	}

	private abstract static class Node
	{
		public abstract Collection<Node> getChildren();

		public abstract void doTask(Task task);
	}

	private static class CarNode extends Node
	{
		private Collection<Node> tires;

		public CarNode() { tires = new LinkedList<>(); }

		@Override
		public void doTask(Task task) {
			task.doTask(this);
			getChildren().forEach(child -> child.doTask(task));
		}

		@Override
		public Collection<Node> getChildren() { return tires; }
	}

	private static class TireNode extends Node
	{
		private int size;

		private TireNode(int size) { this.size = size; }

		@Override
		public void doTask(Task task) {
			task.doTask(this);
			getChildren().forEach(child -> child.doTask(task));
		}

		@Override
		public Collection<Node> getChildren() {
			return new LinkedList<>();
		}
	}
	
	// ~~~~~~~ New Type and task added by developer using this library ~~~~~~~

	private static class ExtendedTask implements Task
	{
		public void doTask(FifthWheelNode fifthWheelNode) { System.out.println("Fifth Wheel"); }
	}

	private static class FifthWheelNode extends TireNode
	{
		FifthWheelNode()
		{
			super(5);
		}

		@Override
		public void doTask(Task task) {
			task.doTask(this);
			getChildren().forEach(child -> child.doTask(task));
		}
	}

}