5

以下代码编译并正常工作。我定义了一个 Builder 接口，然后 CarBuilder 类用于处理与 Car 相关的任何事情，而 BusBuilder 类用于处理与 Bus 相关的任何事情。Car 和 Bus 共享一个名为 Vehicle 的抽象类。代码很简单。代码将输出：

do something to CREATE the Car
do something to UPDATE the Car
do something to CREATE the Bus
do something to UPDATE the Bus

这是编译的原始代码：

public abstract class Vehicle { }
public class Car extends Vehicle { }
public class Bus extends Vehicle { }

public interface Builder<V extends Vehicle> {
    public V createVehicle(String spec);
    public void updateVehicle(String spec, Vehicle vehicle);
}

public class CarBuilder implements Builder<Car> {
    public Car createVehicle(String spec) {
        Car car = new Car();
        System.out.println("do something to CREATE the Car");
        return car;
    }
    public void updateVehicle(String spec, Vehicle vehicle) {
        Car car = (Car) vehicle;
        System.out.println("do something to UPDATE the Car");
        return;
    }
}

public class BusBuilder implements Builder<Bus> {
    public Bus createVehicle(String spec) {
        Bus bus = new Bus();
        System.out.println("do something to CREATE the Bus");
        return bus;
    }
    public void updateVehicle(String spec, Vehicle vehicle) {
        Bus bus = (Bus) vehicle;
        System.out.println("do something to UPDATE the Bus");
        return;
    }
}

@Test
public void main() {
    Builder<? extends Vehicle> builder = null;
    Vehicle vehicle = null;

    builder = new CarBuilder();
    vehicle = builder.createVehicle("my original Car spec");
    builder.updateVehicle("my modified Car spec", vehicle);

    builder = new BusBuilder();
    vehicle = builder.createVehicle("my original Bus spec");
    builder.updateVehicle("my modified Bus spec", vehicle);
}

但是，我想让我的代码更加强类型。我想改变接口方法，

从

public void updateVehicle(String spec, Vehicle vehicle);

至

public void updateVehicle(String spec, V vehicle);

换句话说，我尝试在我的接口签名中使用泛型V而不是基类。Vehicle这迫使 的实现者Builder“关闭”特定的产品类型（即Caror Bus，但不是基类Vehicle）。 CarBuilder只应处理一个Car；BusBuilder应该只处理一个Bus.

代码变成如下：

public interface Builder<V extends Vehicle> {
    public V createVehicle(String spec);
    public void updateVehicle(String spec, V vehicle);
}

public class CarBuilder implements Builder<Car> {
    public Car createVehicle(String spec) {
        Car car = new Car();
        System.out.println("do something to CREATE the Car");
        return car;
    }
    public void updateVehicle(String spec, Car car) {
        System.out.println("do something to UPDATE the Car");
        return;
    }
}

public class BusBuilder implements Builder<Bus> {
    public Bus createVehicle(String spec) {
        Bus bus = new Bus();
        System.out.println("do something to CREATE the Bus");
        return bus;
    }
    public void updateVehicle(String spec, Bus bus) {
        System.out.println("do something to UPDATE the Bus");
        return;
    }
}

@Test
public void main() {
    Builder<? extends Vehicle> builder = null;
    Vehicle vehicle = null;

    builder = new CarBuilder();
    vehicle = builder.createVehicle("my original Car spec");
    builder.updateVehicle("my modified Car spec", vehicle);  <== compilation error

    builder = new BusBuilder();
    vehicle = builder.createVehicle("my original Bus spec");
    builder.updateVehicle("my modified Bus spec", vehicle);  <== compilation error
}

两个编译错误 - Java 编译器不允许这样做，因为“ Builder 类型中的方法 updateVehicle(String, capture#3-of ? extends Vehicle) 不适用于参数 (String, Vehicle) ”。

在Java中有没有办法做我想要完成的事情？在 C# 中，我可以简单地使用var关键字键入我的变量，例如var vehicle代替Vehicle vehicle. 我听说 Java 泛型调用是编译时的决定。有什么技术可以克服吗？

4

3 回答 3

4

有一个习惯用法，称为捕获助手，可以在这种情况下提供帮助。

本质上，给定您的声明builder，编译器所知道的就是vehicle扩展的东西Vehicle。此外，它知道Builder需要将某些特定类型Vehicle传递给它的updateVehicle()方法。编译器无法得出对我们来说显而易见的推论——类型是兼容的。为此，我们必须使用辅助方法摆脱通配符。

从您的简单测试工具来看，实际代码的实际应用并不明显。但是，在这种情况下应用它看起来像下面这样。希望这个例子足以说明这个想法并帮助你将它应用到实际代码中。

public void main()
{
  Builder<? extends Vehicle> builder;
  Vehicle vehicle;

  builder = new CarBuilder();
  vehicle = createAndUpdateHelper(builder, "my original Car spec", "my modified Car spec");

  builder = new BusBuilder();
  vehicle = createAndUpdateHelper(builder, "my original Bus spec", "my modified Bus spec");
}

private static <V extends Vehicle> V createAndUpdateHelper(Builder<V> builder, String s1, String s2)
{
  V v = builder.createVehicle(s1);
  builder.updateVehicle(s2, v);
  return v;
}
于 2013-05-20T20:09:00.303 回答
2

您尝试做的事情没有任何意义：您希望您的构建者只接受 Car 或 Bus（这很好），但随后您给他们一个 Vehicle 的实例。你正在做你试图阻止的事情：接受泛型类 Vehicle。

于 2013-05-20T19:51:46.600 回答
0

最简单的方法是过滤与预期或期望输入不匹配的输入。

if(vehicle instanceof Car) return;

通过使用枚举来控制汽车的各个方面（类型、颜色），可以创建一个更彻底（并且不那么脆弱）的实现。然后，您可以使用带有开关的单个 Builder 来处理它们的任何组合。（我为此编写了一些示例代码，如果您需要它，请随时给我发消息，我会发给您一份）

于 2013-05-20T20:01:26.890 回答