java - 当对象是基类的实例时，为对象调用正确的方法

Question

我在 Java 中有以下示例:

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 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;
}

假设我想用车辆做一些工作，而这项工作不依赖于车辆的子类。所以，我在另一个类中有一个这样的方法:

public void doStuff(public final Vehicle vehicle) {
    //do stuff here
    //then insert it into my database:
    insertVehicle(vehicle);
}

但是，我想在我的 insertVehicle 中做不同的事情，所以我为每个子类覆盖了该方法:

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(Car 或 Truck)，然后将车辆转换为该类并调用 insertVehicle 方法，如下所示:

public void doStuff(public final Vehicle vehicle) {
    //do stuff here
    //then insert it into my database:
    if (vehicle instanceof Car) {
        insertVehicle((Car) vehicle);
    } else {
        insertVehicle((truck) vehicle);
    }
}

但是，我了解到使用 instanceof 并不是执行此操作的最佳方法。 1

我怎样才能最好地修改它，以便我不必使用 instanceof？

Answer 1

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());
    }

}