How to do it...

1. Create interfaces that describe the API:

public interface SpeedModel {
   double getSpeedMph(double timeSec, int weightPounds, 
                                            int horsePower);
 }
 public interface Vehicle {
   void setSpeedModel(SpeedModel speedModel);
   double getSpeedMph(double timeSec);
 }
 public interface Car extends Vehicle {
   int getPassengersCount();
 }
 public interface Truck extends Vehicle {
   int getPayloadPounds();
 }

2. Use factories, which are classes that generate objects that implement certain interfaces. A factory hides from the client code the details of the implementation, so the client deals with an interface only. It is especially helpful when an instance creation requires a complex process and/or significant code duplication. In our case, it makes sense to have a FactoryVehicle class that creates objects of classes that implement the Vehicle, Car, or Truck interface. We will also create the FactorySpeedModel class, which generates objects of a class that implements the SpeedModel interface. Such an API allows us to write the following code:

public static void main(String... arg) {
   double timeSec = 10.0;
   int horsePower = 246;
   int vehicleWeight = 4000;
   Properties drivingConditions = new Properties();
   drivingConditions.put("roadCondition", "Wet");
   drivingConditions.put("tireCondition", "New");
   SpeedModel speedModel  = FactorySpeedModel.
                 generateSpeedModel(drivingConditions);
   Car car = FactoryVehicle.
                buildCar(4, vehicleWeight, horsePower);
   car.setSpeedModel(speedModel);
   System.out.println("Car speed (" + timeSec + " sec) = " 
                      + car.getSpeedMph(timeSec) + " mph");
}

3. Observe that the code behavior is the same as in the previous examples:

However, the design is much more extensible.