实施工厂设计模式时如何避免“ instanceof”？

Question

我正在尝试实现我的第一个工厂设计模式，但不确定将工厂制造的对象添加到列表时如何避免使用instanceof。这就是我想要做的：

for (ABluePrint bp : bluePrints) {
    AVehicle v = AVehicleFactory.buildVehicle(bp);
    allVehicles.add(v);

    // Can I accomplish this without using 'instanceof'?
    if (v instanceof ACar) {
        cars.add((ACar) v);
    } else if (v instanceof ABoat) {
        boats.add((ABoat) v);
    } else if (v instanceof APlane) {
        planes.add((APlane) v);
    }
}

根据我在SO上阅读的内容，使用“ instanceof”是一种代码味道。有没有一种更好的方法可以检查工厂创建的不使用“ instanceof”的车辆类型？

我欢迎任何有关实施的反馈/建议，因为我什至不确定我是否会采用正确的方法。

完整示例如下：

import java.util.ArrayList;

class VehicleManager {

    public static void main(String[] args) {

        ArrayList<ABluePrint> bluePrints = new ArrayList<ABluePrint>();
        ArrayList<AVehicle> allVehicles = new ArrayList<AVehicle>();
        ArrayList<ACar> cars = new ArrayList<ACar>();
        ArrayList<ABoat> boats = new ArrayList<ABoat>();
        ArrayList<APlane> planes = new ArrayList<APlane>();

        /*
        *  In my application I have to access the blueprints through an API
        *  b/c they have already been created and stored in a data file.
        *  I'm creating them here just for example.
        */
        ABluePrint bp0 = new ABluePrint(0);
        ABluePrint bp1 = new ABluePrint(1);
        ABluePrint bp2 = new ABluePrint(2);
        bluePrints.add(bp0);
        bluePrints.add(bp1);
        bluePrints.add(bp2);

        for (ABluePrint bp : bluePrints) {
            AVehicle v = AVehicleFactory.buildVehicle(bp);
            allVehicles.add(v);

            // Can I accomplish this without using 'instanceof'?
            if (v instanceof ACar) {
                cars.add((ACar) v);
            } else if (v instanceof ABoat) {
                boats.add((ABoat) v);
            } else if (v instanceof APlane) {
                planes.add((APlane) v);
            }
        }

        System.out.println("All Vehicles:");
        for (AVehicle v : allVehicles) {
            System.out.println("Vehicle: " + v + ", maxSpeed: " + v.maxSpeed);
        }

        System.out.println("Cars:");
        for (ACar c : cars) {
            System.out.println("Car: " + c + ", numCylinders: " + c.numCylinders);
        }

        System.out.println("Boats:");
        for (ABoat b : boats) {
            System.out.println("Boat: " + b + ", numRudders: " + b.numRudders);
        }

        System.out.println("Planes:");
        for (APlane p : planes) {
            System.out.println("Plane: " + p + ", numPropellers: " + p.numPropellers);
        }
    }
}

class AVehicle {

    double maxSpeed;

    AVehicle(double maxSpeed) {
        this.maxSpeed = maxSpeed;
    }
}

class ACar extends AVehicle {

    int numCylinders;

    ACar(double maxSpeed, int numCylinders) {
        super(maxSpeed);
        this.numCylinders = numCylinders;
    }
}

class ABoat extends AVehicle {

    int numRudders;

    ABoat(double maxSpeed, int numRudders) {
        super(maxSpeed);
        this.numRudders = numRudders;
    }
}

class APlane extends AVehicle {

    int numPropellers;

    APlane(double maxSpeed, int numPropellers) {
        super(maxSpeed);
        this.numPropellers = numPropellers;
    }
}

class AVehicleFactory {

    public static AVehicle buildVehicle(ABluePrint blueprint) {

        switch (blueprint.type) {

            case 0:
                return new ACar(100.0, 4);

            case 1:
                return new ABoat(65.0, 1);

            case 2:
                return new APlane(600.0, 2);

            default:
                return new AVehicle(0.0);
        }
    }
}

class ABluePrint {

    int type; // 0 = car; // 1 = boat; // 2 = plane;

    ABluePrint(int type) {
        this.type = type;
    }
}

Answer 1

您可以实现Visitor模式。

详细答案

这个想法是使用多态来执行类型检查。每个子类都覆盖该accept(Visitor)方法，该方法应在超类中声明。当我们遇到如下情况时：

void add(Vehicle vehicle) {
    //what type is vehicle??
}

我们可以将一个对象传递到中声明的方法中Vehicle。如果vehicle类型为Car，并class Car覆盖我们将对象传递给的方法，则该对象现在将在Car类中声明的方法内进行处理。我们利用它的优势：创建一个Visitor对象并将其传递给重写的方法：

abstract class Vehicle {
    public abstract void accept(AddToListVisitor visitor);
}

class Car extends Vehicle {
    public void accept(AddToListVisitor visitor) {
        //gets handled in this class
    }
}

这Visitor应该准备好访问类型Car。您想要避免使用的任何类型instanceof都必须在中指定Visitor。

class AddToListVisitor {
    public void visit(Car car) {
        //now we know the type! do something...
    }

    public void visit(Plane plane) {
        //now we know the type! do something...
    }
}

这是类型检查的地方！

当Car接收到访问者时，它应该使用this关键字传递自己。由于我们在class中Car，因此visit(Car)将调用该方法。现在我们知道了对象的类型，就可以在访问者内部执行所需的操作。

因此，从顶部开始：

您创建一个Visitor，执行所需的操作。访问者应该visit为要对其执行操作的每种对象类型提供一个方法。在这种情况下，我们正在为车辆创建访客：

interface VehicleVisitor {
    void visit(Car car);
    void visit(Plane plane);
    void visit(Boat boat);
}

我们要执行的动作是将车辆添加到某物上。我们将创建一个AddTransportVisitor；负责管理交通运输的访客：

class AddTransportVisitor implements VehicleVisitor {
    public void visit(Car car) {
        //add to car list
    }

    public void visit(Plane plane) {
        //add to plane list
    }

    public void visit(Boat boat) {
        //add to boat list
    }
}

每辆车都应该能够接纳来访者：

abstract class Vehicle {
    public abstract void accept(VehicleVisitor visitor);
}

当访客被传递到车辆时，车辆应调用其visit方法，并将自身传递给参数：

class Car extends Vehicle {
    public void accept(VehicleVisitor visitor) {
        visitor.visit(this);
    }
}

class Boat extends Vehicle {
    public void accept(VehicleVisitor visitor) {
        visitor.visit(this);
    }
}

class Plane extends Vehicle {
    public void accept(VehicleVisitor visitor) {
        visitor.visit(this);
    }
}

这就是类型检查的地方。visit会调用正确的方法，该方法包含根据方法的参数执行的正确代码。

最后一个问题是VehicleVisitor与列表进行交互。这就是您的VehicleManager来历：它封装了列表，使您可以通过一种VehicleManager#add(Vehicle)方法添加载具。

创建访问者时，我们可以将管理器传递给它（可能通过它的构造函数），这样，既然我们知道对象的类型，就可以执行所需的操作。本VehicleManager应包含的游客和拦截VehicleManager#add(Vehicle)来电：

class VehicleManager {
    private List<Car> carList = new ArrayList<>();
    private List<Boat> boatList = new ArrayList<>();
    private List<Plane> planeList = new ArrayList<>();

    private AddTransportVisitor addVisitor = new AddTransportVisitor(this);

    public void add(Vehicle vehicle) {
        vehicle.accept(addVisitor);
    }

    public List<Car> getCarList() {
        return carList;
    }

    public List<Boat> getBoatList() {
        return boatList;
    }

    public List<Plane> getPlaneList() {
        return planeList;
    }
}

现在，我们可以为这些AddTransportVisitor#visit方法编写实现：

class AddTransportVisitor implements VehicleVisitor {
    private VehicleManager manager;

    public AddTransportVisitor(VehicleManager manager) {
        this.manager = manager;
    }

    public void visit(Car car) {
        manager.getCarList().add(car);
    }

    public void visit(Plane plane) {
        manager.getPlaneList().add(plane);
    }

    public void visit(Boat boat) {
       manager.getBoatList().add(boat);
    }
}

我强烈建议删除add每种车辆的吸气剂方法并声明重载方法。这样可以减少不需要时的“访问”开销，例如manager.add(new Car())：

class VehicleManager {
    private List<Car> carList = new ArrayList<>();
    private List<Boat> boatList = new ArrayList<>();
    private List<Plane> planeList = new ArrayList<>();

    private AddTransportVisitor addVisitor = new AddTransportVisitor(this);

    public void add(Vehicle vehicle) {
        vehicle.accept(addVisitor);
    }

    public void add(Car car) {
        carList.add(car);
    }

    public void add(Boat boat) {
        boatList.add(boat);
    }

    public void add(Plane plane) {
        planeList.add(plane);
    }

    public void printAllVehicles() {
        //loop through vehicles, print
    }
}

class AddTransportVisitor implements VehicleVisitor {
    private VehicleManager manager;

    public AddTransportVisitor(VehicleManager manager) {
        this.manager = manager;
    }