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?
我欢迎任何有关实现的反馈/建议,因为我不确定自己是否会以正确的方式进行操作。
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;
}
}
最佳答案
您可以实现Visitor pattern。
详细答案
这个想法是使用polymorphism执行类型检查。每个子类都覆盖accept(Visitor)
方法,该方法应在父类(super class)中声明。当我们遇到如下情况时:
void add(Vehicle vehicle) {
//what type is vehicle??
}
我们可以将对象传递到
Vehicle
声明的方法中。如果vehicle
的类型为Car
,并且class Car
覆盖了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
}
}
该
Visitor
应该准备访问Car
类型。您想要避免使用instanceof
查找实际类型的任何类型都必须在Visitor
中指定。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...
}
}
在这里进行类型检查!
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Car
类中,因此将调用visit(Car)
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
,它执行所需的操作。对于要对其执行操作的每种类型的对象,访问者应由visit
方法组成。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 Action 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);
}
当访客被送往车辆时,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。正确的
visit
方法将被调用,该方法包含根据该方法的参数执行的正确代码。最后一个问题是让
VehicleVisitor
与列表进行交互。这是VehicleManager
出现的地方:它封装了列表,使您可以通过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。
VehicleManager
应该包含访客并拦截VehicleManager#add(Vehicle)
调用: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
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);
}
}
我强烈建议删除每种车辆的 setter/getter 方法并声明重载的
add
方法。这样可以减少不需要时的“访问”的开销,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;
}
public void visit(Car car) {
manager.add(car);
}
public void visit(Plane plane) {
manager.add(plane);
}
public void visit(Boat boat) {
manager.add(boat);
}
}
public class Main {
public static void main(String[] args) {
Vehicle[] vehicles = {
new Plane(),
new Car(),
new Car(),
new Car(),
new Boat(),
new Boat()
};
VehicleManager manager = new VehicleManager();
for(Vehicle vehicle : vehicles) {
manager.add(vehicle);
}
manager.printAllVehicles();
}
}
关于java - 实现工厂设计模式时如何避免 'instanceof'?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/29458676/