Java 如何避免'；instanceof'；在实施工厂设计模式时？
我正在尝试实现我的第一个Factory设计模式，我不确定在将Factory生成的对象添加到列表时如何避免使用instanceof。这就是我想做的：Java 如何避免'；instanceof'；在实施工厂设计模式时？,java,design-patterns,factory,factory-pattern,instanceof,Java,Design Patterns,Factory,Factory Pattern,Instanceof,我正在尝试实现我的第一个Factory设计模式，我不确定在将Factory生成的对象添加到列表时如何避免使用instanceof。这就是我想做的： for (ABluePrint bp : bluePrints) { AVehicle v = AVehicleFactory.buildVehicle(bp); allVehicles.add(v); // Can I accomplish this without using 'instanceof'? if (
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);
}
}
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;
}
}
import java.util.ArrayList；
高级车辆管理员{
公共静态void main（字符串[]args）{
ArrayList蓝图=新的ArrayList（）；
ArrayList allVehicles=新的ArrayList（）；
ArrayList cars=新的ArrayList（）；
ArrayList=新的ArrayList（）；
ArrayList平面=新的ArrayList（）；
/*
*在我的应用程序中，我必须通过API访问蓝图
*b/c它们已经创建并存储在数据文件中。
*我在这里创建它们只是举个例子。
*/
ABluePrint bp0=新ABluePrint（0）；
ABluePrint bp1=新的ABluePrint（1）；
ABluePrint bp2=新的ABluePrint（2）；
蓝图。添加（bp0）；
蓝图。添加（bp1）；
蓝图。添加（bp2）；
用于（ABluePrint bp：蓝图）{
AVehicle v=AVehicleFactory.buildVehicle（bp）；
所有车辆。添加（v）；
//我可以不用“instanceof”来完成这个任务吗？
if（ACar的v实例）{
增加（（ACar）v）；
}else if（v ABoat实例）{
添加（（ABoat）v）；
}else if（APlane的v实例）{
添加（（平面）v）；
}
}
系统输出打印项次（“所有车辆：”）；
适用于（车辆五：所有车辆）{
系统输出打印项次（“车辆：+v+”，最大速度：+v.maxSpeed）；
}
System.out.println（“Cars:”）；
适用于（ACar c：汽车）{
System.out.println（“车：+c+”，numCylinders:+c.numCylinders）；
}
System.out.println（“船：”）；
对于（ABoat b：船）{
System.out.println（“船：+b+”，numruders:+b.numruders）；
}
System.out.println（“平面：”）；
对于（APlane p：飞机）{
System.out.println（“平面：+p+”，numPropellers:+p.numPropellers）；
}
}
}
类车辆{
双最大速度；
A车辆（双最大速度）{
this.maxSpeed=maxSpeed；
}
}
ACar类扩展车辆{
国际货币基金组织；
ACar（双最大速度，整数气缸）{
超级（最大速度）；
this.numcylinds=numcylinds；
}
}
类ABoat扩展了AVehicle{
整数乳房；
ABoat（双最大速度，整数个乳房）{
超级（最大速度）；
this.numRudders=numRudders；
}
}
APlane类扩展了AVehicle{
国际货币基金组织；
APlane（双最大速度，整数）{
超级（最大速度）；
this.numPropellers=numPropellers；
}
}
阿维类山核桃工厂{
公共静态车辆制造车辆（ABluePrint蓝图）{
开关（blueprint.type）{
案例0：
返回新的ACar（100.0,4）；
案例1：
返回新的ABoat（65.0,1）；
案例2：
返回新APlane（600.0,2）；
违约：
返回新车辆（0.0）；
}
}
}
ABluePrint类{
int type；//0=汽车；//1=船；//2=飞机；
ABluePrint（整型）{
this.type=type；
}
}
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 = null;
ArrayList<ABoat> boats = null;
ArrayList<APlane> planes = null;
/*
* 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'?
// dont add objects to list here, do it from constructor or in factory
/*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);
}*/
}
cars = ACar.getCars();
boats = ABoat.getBoats();
planes = APlane.getPlanes();
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;
}
void add(){}
}
class ACar extends AVehicle {
static ArrayList<ACar> cars = new ArrayList<ACar>();
int numCylinders;
ACar(double maxSpeed, int numCylinders) {
super(maxSpeed);
this.numCylinders = numCylinders;
}
void add(){
cars.add(this);
}
public static ArrayList<ACar> getCars(){
return cars;
}
}
class ABoat extends AVehicle {
static ArrayList<ABoat> boats = new ArrayList<ABoat>();
int numRudders;
ABoat(double maxSpeed, int numRudders) {
super(maxSpeed);
this.numRudders = numRudders;
}
void add(){
boats.add(this);
}
public static ArrayList<ABoat> getBoats(){
return boats;
}
}
class APlane extends AVehicle {
static ArrayList<APlane> planes = new ArrayList<APlane>();
int numPropellers;
APlane(double maxSpeed, int numPropellers) {
super(maxSpeed);
this.numPropellers = numPropellers;
}
void add(){
planes.add(this);
}
public static ArrayList<APlane> getPlanes(){
return planes;
}
}
class AVehicleFactory {
public static AVehicle buildVehicle(ABluePrint blueprint) {
AVehicle vehicle;
switch (blueprint.type) {
case 0:
vehicle = new ACar(100.0, 4);
break;
case 1:
vehicle = new ABoat(65.0, 1);
break;
case 2:
vehicle = new APlane(600.0, 2);
break;
default:
vehicle = new AVehicle(0.0);
}
vehicle.add();
return vehicle;
}
}
class ABluePrint {
int type; // 0 = car; // 1 = boat; // 2 = plane;
ABluePrint(int type) {
this.type = type;
}
}
import java.util.ArrayList；
高级车辆管理员{
公共静态void main（字符串[]args）{
ArrayList）。您可以实现
详细答案
其思想是使用来执行类型检查。每个子类重写accept（Visitor）
方法，该方法应在超类中声明。当出现以下情况时：
void add(Vehicle vehicle) {
//what type is vehicle??
}
我们可以将一个对象传递到Vehicle
中声明的方法中。如果Vehicle
类型为Car
，并且类Car
覆盖了我们传递对象的方法，那么该对象现在将在Car
类中声明的方法中进行处理。我们利用这个优势：创建访问者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
}
}
此访问者
应准备好访问类型汽车
。任何想要避免使用实例
查找实际类型的类型都必须在访问者
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...
}
}
这里是进行类型检查的地方！
当Car
接收访问者时，它应该使用this
关键字传入。由于我们在类Car
，将调用方法visit（Car）
。在访问者内部，我们可以执行我们想要的操作，现在我们知道了对象的类型
因此，从顶部：
您创建了一个执行所需操作的访问者
。访问者应包含一个访问者
方法，用于您要对其执行操作的每种类型的对象。在这种情况下，我们为车辆创建访问者：
interface VehicleVisitor {
void visit(Car car);
void visit(Plane plane);
void visit(Boat boat);
}
我们要执行的操作是将车辆添加到某些内容中。我们将创建一个AddTransportVisitor
；一个管理添加运输的访问者：
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
}
}
每辆车都应能够接待车辆访客：
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，传递itsel
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);
}
}
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;
}
}
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);
}
}
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();
}
}
Map<Class<? extends AVehicle>, List<? extends AVehicle>> lists = new HashMap<>();
lists.put(ACar.class, new ArrayList<ACar>());
lists.put(ABoat.class, new ArrayList<ABoat>());
lists.put(APlane.class, new ArrayList<APlane>());
for (ABluePrint bp : bluePrints) {
AVehicle v = AVehicleFactory.buildVehicle(bp);
allVehicles.add(v);
lists.get(v.getClass()).add(v);
}