You can start with the practical basics and end up with a basic design pattern (such as template method pattern or strategy pattern). The value of teaching though is that the group gets to get their hands dirty in it.
The tricky part is not to let them learn any bad habits; such as breaking encapsulation and using inheritance for everything. This can only be enforced with examples that somehow shows this. Another tricky part is to teach them how to use it practically for day-to-day usage.
Basics
First basic example for inheritance and polymorphism can be, implement a Vehicle that can either be a Car or a Bus. And with all vehicles we can drive() and honk() the horn with them. As you build a list with list with vehicles you can perform the methods on them and see what happens:
public abstract class Vehicle {
private String name;
public Vehicle(String name) {
this.name = name;
}
public void drive() {
System.out.println(name + " is on the move");
}
public void honk() {
System.out.println("The " + name + " honks it's horn");
}
}
public class Car extends Vehicle {
public Car() {
super("car");
}
}
public class Bus extends Vehicle {
public Bus() {
super("bus");
}
@override
public void honk() {
System.out.println("The bus toots it's horn");
}
}
The inheritance part is that every class that extends another inherits it's methods. The polymorphism part is that they both share methods and can be executed without knowing the actual class type. The method to test it all in practice would look something like this:
public static void main(String[] args) {
ArrayList<Vehicle> vehicles = new ArrayList<Vehicle>();
vehicles.add(new Car());
vehicles.add(new Bus());
for(Vehicle v : vehicles) {
v.drive();
v.honk();
}
}
As a further study, they can try to implement registration numbers so that they're visible in the output. Bonus points if they realize they can do so in the abstract class. Also they can try making the vehicles more personalized, such as a SportsCar goes "vroom" when it's driving.
Depending on the user group you're running, you might want to change the actual types and methods with the one that the group can relate to the most. It's easier to learn something that you easily can relate to. If they're not interested in vehicles, maybe something else will help them understand it.
