Interface Segregation Principle
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public interface IVehicleMaintenance { void changeWheel(int number); void changeEngineOil(); void lubeChain(); void checkBattery(); } public class CarWorkshop implements IVehicleMaintenace { @Override public void changeWheel(int number) { // ... } @Override public void changeEngineOil() { // ... } @Override public void lubeChain() { throw new UnsupportedOperationException("No chain to lube in cars"); } @Override public void checkBattery() { // ... } } public class BikeWorkshop implements IVehicleMaintenace { @Override public void changeWheel(int number) { // ... } @Override public void changeEngineOil() { throw new UnsupportedOperationException("No engine oil to change in bikes"); } @Override public void lubeChain() { // ... } @Override public void checkBattery() { // only valid for electric bikes.. } } /* * Same happens for all other types of vehicles. * Even for the changeWheel operation, which looks more common, it does not * apply to boats, and it may behave differently in different vehicles with * different number and types of wheels. * * In fact, throwing an UnsupportedOperationException in the CarMaintenance * and BikeMaintenance classes is a violation of the LSP as the superclass * (interface in this case) is not replaceable by the subclasses. * * Moreover, with this solution we would be breaking the Open/Closed Principle * (OCP), which states that a class should be open for extension but closed * for modification. This is so because, in case of adding a new type of * vehicle, we would have to modify VehicleMaintenace to add the new types * of maintenance operations, and then we would have to modify all subclasses * to add the new operations even if they would not apply to the specific * vehicle represented by a particular subclass. * */ //////////////////////////////////////////////////////////////////// /* * A simple solution is to separate the interfaces. That is, to define * the hierarchy at the level of interfaces because interfaces are * not meant to be replaced and then the OCP principle would not apply * but we still would be respecting the ISP principle. In this solution * the clients (CarWorkshop and BikeWorkshop) depend on the respective * interface they need. */ public interface IVehicleMaintenance { public void void cleanExterior(); // Other operations that are common to all vehicles } public interface ICarMaintenance extends IVehicleMaintenace { public void changeWheel(int number); public void changeEngineOil(); public void checkBattery(); // Other operations that are common to all cars } public interface IBikeMaintenance extends IVehicleMaintenace { public void lubeChain(); // Other operations that are common to all bikes } public class CarWorkshop implements ICarMaintenance { public void cleanExterior() { // ... } public void changeWheel(int number) { // ... } public void changeEngineOil() { // ... } public void checkBattery() { // ... } // Other operations that are common to all cars } public class BikeWorkshop implements IBikeMaintenance { public void cleanExterior() { // ... } public void lubeChain() { // ... } // Other operations that are common to all bikes } //////////////////////////////////////////////////////////////////// /* * A good solution comes from rethinking our abstractions and using * a more generic the interface that relies on the parameters of the * operations to handle the differences. In such case, we would have * ended up with a simpler design that would have respected both the OCP * and the ISP principles. In some cases this is enough and can work */ public interface IMaintainable { public void void maintain(); } public abstract class MaintenanceOperation { // The only thing we want to ensure is that all operations can be executed abstract void execute(); } /* * We can now extend the MaintenanceOperation class to reflect the different * maintenance operations that each vehicle type can accept */ public enum CarMaintenanceTypes { CLEANEXTERIOR, CHANGEWHEEL, CHANGEENGINEOIL, CHECKBATTERY } } public enum BikeMaintenanceTypes { CLEANEXTERIOR, LUBECHAIN } public class CarMaintenanceOperation extends MaintenanceOperation { private CarMaintenanceTypes opType; private CarMaintenanceParameters opParams; public CarMaintenanceOperation (CarMaintenanceTypes opType, CarMaintenanceParameters opParams){ this.opType = opType; this.opParams = opParams; } public void execute(){ // specific implementation } } public class BikeMaintenanceOperation extends MaintenanceOperation { private BikeMaintenanceTypes opType; private BikeMaintenanceParameters opParams; public CarMaintenanceOperation (BikeMaintenanceTypes opType, BikeMaintenanceParameters opParams){ this.opType = opType; this.opParams = opParams; } public void execute(){ // specific implementation } } public class Vehicle implements IMaintainable { // Vehicle data and operations Collection<MaintenanceOperation> pendingOperations = new ArrayList<MaintenanceOperation>(); public void void maintain(){ Iterator itr=pendingOperations.iterator(); while(itr.hasNext()){ itr.next().execute(); } } } /* * The only thing we have to do now it to make sure each vehicle is * only assigned to maintenance operations that are applicable to it */ public class Car extends Vehicle { public void addPendingMaintenanceOperation(CarMaintenanceOperation op); // ... } public class Bike extends Vehicle { public void addPendingMaintenanceOperation(BikeMaintenanceOperation op); // ... } //////////////////////////////////////////////////////////////////// |
