Java 当对象是基类的实例时,调用对象的正确方法
我有以下Java示例:Java 当对象是基类的实例时,调用对象的正确方法,java,polymorphism,subclassing,Java,Polymorphism,Subclassing,我有以下Java示例: public abstract class Vehicle { private final String name; private final String make; public Vehicle(final String name, final String make) { this.make = make; this.name = name; } } public final class Car
public abstract class Vehicle {
private final String name;
private final String make;
public Vehicle(final String name, final String make) {
this.make = make;
this.name = name;
}
}
public final class Car extends Vehicle {
public Car(final String name, final String make) {
super(name, make);
}
}
public final class Truck extends Vehicle {
final Integer grossVehicleWeight;
public Truck(final String name, final String make, final Integer gvw) {
super(name, make);
this.grossVehicleWeight = gvw;
}
假设我想用一辆车做一些工作,工作不依赖于车的子类。我在另一个类中有一个方法,如下所示:
public void doStuff(public final Vehicle vehicle) {
//do stuff here
//then insert it into my database:
insertVehicle(vehicle);
}
public void doStuff(public final Vehicle vehicle) {
//do stuff here
//then insert it into my database:
if (vehicle instanceof Car) {
insertVehicle((Car) vehicle);
} else {
insertVehicle((truck) vehicle);
}
}
但是,我希望在insertVehicle中执行不同的操作,因此我为每个子类重写该方法:
public void insertVehicle(Car car) { //do stuff for a car }
public void insertVehicle(Truck truck) { //do stuff for a truck }
在我的doStuff方法中,我可以使用instanceOf来确定车辆的类别(汽车或卡车),然后将车辆强制转换到该类别中,并调用insertVehicle方法,如下所示:
public void doStuff(public final Vehicle vehicle) {
//do stuff here
//then insert it into my database:
insertVehicle(vehicle);
}
public void doStuff(public final Vehicle vehicle) {
//do stuff here
//then insert it into my database:
if (vehicle instanceof Car) {
insertVehicle((Car) vehicle);
} else {
insertVehicle((truck) vehicle);
}
}
然而,我读到使用instanceof并不是最好的方法
我如何才能最好地修改它,从而不必使用instanceof?您可以使用访问者模式:
public interface VehicleVisitor {
public void visit(Car car);
public void visit(Truck truck);
}
public class Car extends Vehicle {
@Override
public void insert(VehicleVisitor vehicleVisitor) {
vehicleVisitor.visit(this);
}
}
public class Truck extends Vehicle {
@Override
public void insert(VehicleVisitor vehicleVisitor) {
vehicleVisitor.visit(this);
}
}
public abstract class Vehicle {
public abstract void insert(VehicleVisitor vehicleVisitor);
}
public class VehicleVisitorImpl implements VehicleVisitor {
@Override
public void visit(Car car) {
System.out.println("insert car");
}
@Override
public void visit(Truck truck) {
System.out.println("insert truck");
}
}
public class Main {
public static void main(String[] args) {
Vehicle vehicle = new Car();
// finally the agnostic call
vehicle.insert(new VehicleVisitorImpl());
}
}
您可以在车辆内部创建抽象函数
public abstract void doStuff()
从要修改的对象实例调用此函数
ford.doStuff(); //ford is car instance
然后你可以用这个来修改
doStuff()
{
this.cost += 10;
}
否则,您可以为车辆添加一个指示车辆类型的变量,并返回该变量。比如:
public void doStuff(public final Vehicle vehicle) {
//do stuff here
//then insert it into my database:
if (vehicle.getType()== 'Car') {
insertVehicle((Car) vehicle);
} else {
insertVehicle((truck) vehicle);
}
}
此变量“vehicleType”将位于vehicle类中,并将在构造函数中初始化:
public final class Car extends Vehicle {
public Car(final String name, final String make, final String vehicleType) {
super(name, make, type);
}
}
一种方法是在
Vehicle
中抽象insertVehicle()
方法。然后在子类Car
和Truck
中实现它们
但是,这会将逻辑移到POJO中。也许最好将db逻辑与POJO分开,即在这种情况下只使用instanceof
public abstract class Vehicle {
private final String name;
private final String make;
public Vehicle(final String name, final String make) {
this.make = make;
this.name = name;
}
public abstract void insertVehicle();
}
public final class Car extends Vehicle {
public Car(final String name, final String make) {
super(name, make);
}
public void insertVehicle() {
}
}
public final class Truck extends Vehicle {
final Integer grossVehicleWeight;
public Truck(final String name, final String make, final Integer gvw) {
super(name, make);
this.grossVehicleWeight = gvw;
}
public void insertVehicle() {
}
}
public void doStuff(Vehicle vehicle) {
//do stuff here
//then insert it into my database:
vehicle.insertVehicle();
}
这取决于你想解决什么样的问题。如果是坚持,请确保你没有重新创造。如果它是特定类型的处理,那么您可以按照@denis的建议解决它。或者,如果要保持POJO样式的实体,可以使用以下命令:
Map如果您不喜欢将doStuff()
放入Car
和Truck
中,您可以为它们中的每一个设置doStuff()
方法,并将通用的Vehicle
逻辑放入另一个方法中
private void doCommonStuff(final Vehicle vehicle) {
//do stuff here
}
public void doStuff(final Car car) {
doCommonStuff(car);
//then insert it into my database:
insertVehicle(car);
}
public void doStuff(final Truck truck) {
doCommonStuff(truck);
//then insert it into my database:
insertVehicle(truck);
}
不过,我们可以用泛型做得更好
public abstract class StuffDoer<T extends Vehicle> {
public void doStuff(final T vehicle) {
// do stuff here
// then insert it into my database:
insertVehicle(vehicle);
}
public abstract void insertVehicle(T vehicle);
}
public class CarStuffDoer extends StuffDoer<Car> {
public void insertVehicle(Car car) {
// whatever
}
}
public class TruckStuffDoer extends StuffDoer<Truck> {
public void insertVehicle(Truck truck) {
// whatever else
}
}
公共抽象类StuffDoer{
公共车辆(最终T型车辆){
//在这里做事
//然后将其插入我的数据库:
插入车辆(车辆);
}
公共车辆(T型车辆);
}
公共类CarStuffDoer扩展了StuffDoer{
公共车辆(汽车){
//随便
}
}
公共类TruckStuffDoer扩展了StuffDoer{
公共车辆(卡车){
//还有别的吗
}
}
这些insertVehicle方法的作用是什么?OO实现这一点的方法是使insertVehicle(或此方法的特定部分)成为Vehicle的实例方法,在每个子类中重写。或者使用访问者模式。发布代码以获得更具体的帮助。