我只是在这里汇集了其他人的想法（我不是Java专家，所以这只是伪代码而不是实际代码），但在这个编造的例子中，我会将我的汽车检查方法抽象成一个专门的类，该类只知道汽车并且只在查看车库时关心汽车：
abstract class Vehicle {
public abstract string getDescription() ;
}
class Transmission {
public Transmission(bool isAutomatic) {
this.isAutomatic = isAutomatic;
}
private bool isAutomatic;
public bool getIsAutomatic() { return isAutomatic; }
}
class Car extends Vehicle {
@Override
public string getDescription() {
return "a car";
}
private Transmission transmission;
public Transmission getTransmission() {
return transmission;
}
}
class Boat extends Vehicle {
@Override
public string getDescription() {
return "a boat";
}
}
public enum InspectionBoolean {
FALSE, TRUE, UNSUPPORTED
}
public class CarInspector {
public bool isCar(Vehicle v) {
return (v instanceof Car);
}
public bool isAutomatic(Car car) {
Transmission t = car.getTransmission();
return t.getIsAutomatic();
}
public bool isAutomatic(Vehicle vehicle) {
if (!isCar(vehicle)) throw new UnsupportedVehicleException();
return isAutomatic((Car)vehicle);
}
public InspectionBoolean isAutomatic(Vehicle[] garage, int bay) {
if (!isCar(garage[bay])) return InspectionBoolean.UNSUPPORTED;
return isAutomatic(garage[bay])
? InspectionBoolean.TRUE
: InspectionBoolean.FALSE;
}
}
重点是，当你询问汽车的变速器时，已经决定了你只关心汽车。所以，只需询问CarInspector。由于三州枚举，现在您可以知道它是自动还是非汽车。
当然，您需要针对每辆您关心的车辆使用不同的VehicleInspectors。而且，您已经将实例化VehicleInspector的问题推到了链上。
因此，您可能希望查看接口。
将抽象的getTransmission转移到接口中（例如HasTransmission）。这样，您就可以检查车辆是否具有传动系统，或编写一个TransmissionInspector：
abstract class Vehicle { }
class Transmission {
public Transmission(bool isAutomatic) {
this.isAutomatic = isAutomatic;
}
private bool isAutomatic;
public bool getIsAutomatic() { return isAutomatic; }
}
interface HasTransmission {
Transmission getTransmission();
}
class Car extends Vehicle, HasTransmission {
private Transmission transmission;
@Override
public Transmission getTransmission() {
return transmission;
}
}
class Bus extends Vehicle, HasTransmission {
private Transmission transmission;
@Override
public Transmission getTransmission() {
return transmission;
}
}
class Boat extends Vehicle { }
enum InspectionBoolean {
FALSE, TRUE, UNSUPPORTED
}
class TransmissionInspector {
public bool hasTransmission(Vehicle v) {
return (v instanceof HasTransmission);
}
public bool isAutomatic(HasTransmission h) {
Transmission t = h.getTransmission();
return t.getIsAutomatic();
}
public bool isAutomatic(Vehicle v) {
if (!hasTranmission(v)) throw new UnsupportedVehicleException();
return isAutomatic((HasTransmission)v);
}
public InspectionBoolean isAutomatic(Vehicle[] garage, int bay) {
if (!hasTranmission(garage[bay])) return InspectionBoolean.UNSUPPORTED;
return isAutomatic(garage[bay])
? InspectionBoolean.TRUE
: InspectionBoolean.FALSE;
}
}
现在你说，你只关心传输，不管车辆，所以可以询问TransmissionInspector。无论是公交车还是汽车都可以由TransmissionInspector检查，但只能询问有关传输的问题。
现在，您可能会发现布尔值并不是您关心的所有内容。此时，您可能更喜欢使用通用的支持类型，该类型同时显示支持状态和值：
class Supported<T> {
private bool supported = false;
private T value;
public Supported() { }
public Supported(T value) {
this.isSupported = true;
this.value = value;
}
public bool isSupported() { return supported; }
public T getValue() {
if (!supported) throw new NotSupportedException();
return value;
}
}
现在你的检查器可能被定义为：
class TransmissionInspector {
public Supported<bool> isAutomatic(Vehicle[] garage, int bay) {
if (!hasTranmission(garage[bay])) return new Supported<bool>();
return new Supported<bool>(isAutomatic(garage[bay]));
}
public Supported<int> getGearCount(Vehicle[] garage, int bay) {
if (!hasTranmission(garage[bay])) return new Supported<int>();
return new Supported<int>(getGearCount(garage[bay]));
}
}
如我所说，我不是一个Java程序员，因此上述某些语法可能有误，但是概念应该是正确的。然而，在没有进行测试之前，请不要在任何重要的地方运行上述代码。
boolean carIsAutomatic = (Car)myGarage[0].auto对我来说无法编译。 - Vitalii Fedorenko