构建器模式

Question

我需要在没有静态嵌套类的情况下实现Builder模式。如果我有遗传，最好的方法是什么？让我们想象一下我有下面的课。

public class Car {
   private String brand;
   private String speed; 
   //getters an setters  
}

public class PassengerCar extends Car{
   private String capacity; 
   //getters an setters  
}

public class Truck extends Car{
   private String length; 
   //getters an setters  
}

是创建一个Builder类来负责设置PassengerCar和Truck的值更好，还是我们需要另外三个类，CarBuilder,PassengerCarBuilder extends CarBuilder和TruckBuilder extends CarBuilder？

Answer 1

正确的方法是每个类有一个构建器。我已经看到了两种不同的构建器实现，让我们称它们为lazy和eager（也许其中一个不是严格的构建器，但它们实际上都构建了实例）。

以下是cars和traks的惰性生成器：

public abstract class AbstractLazyCarBuilder<T extends Car, B extends AbstractLazyCarBuilder<T, B>> {

    private String brand;

    private String speed;

    public B brand(String brand) {
        this.brand = brand;
        return (B) this;
    }

    public B speed(String speed) {
        this.speed = speed;
        return (B) this;
    }

    public T build() {
        T car = this.create();
        this.fill(car);
        return car;
    }

    protected abstract T create();

    protected void fill(T car) {
        car.setBrand(this.brand);
        car.setSpeed(this.speed);
    }
}

public class LazyCarBuilder extends AbstractLazyCarBuilder<Car, LazyCarBuilder> {

    @Override
    protected Car create() {
        return new Car();
    }
}

public class LazyTruckBuilder extends AbstractLazyCarBuilder<Truck, LazyTruckBuilder> {

    private String length;

    public LazyTruckBuilder length(String length) {
        this.length = length;
        return this;
    }

    @Override
    protected Truck create() {
        return new Truck();
    }

    @Override
    protected void fill(Truck truck) {
        super.fill(truck); // very important! fills truck with car's common attributes 
        truck.setLength(this.length);
    }
}

用法：

Truck truck = new LazyTruckBuilder().brand("ford").speed("40").length("30").build();

public abstract class AbstractEagerCarBuilder<T extends Car, B extends AbstractEagerCarBuilder<T, B>> {

    protected final T instance; // needs to be seen by subclasses

    protected AbstractEagerCarBuilder() {
        try {
            // Reflection magic to get type of specific car
            ParameterizedType type = (ParameterizedType) this.getClass().getGenericSuperclass();
            Class<T> clazz = (Class<T>) type.getActualTypeArguments()[0];
            // Create the specific car by reflection
            this.instance = clazz.getConstructor().newInstance();
        } catch (Exception e) {
            throw new RuntimeException("Could not create specific instance", e);
        }
    }

    public B brand(String brand) {
        this.instance.setBrand(brand);
        return (B) this;
    }

    public B speed(String speed) {
        this.instance.setSpeed(speed);
        return (B) this;
    }

    public T build() {
        return this.instance;
    }
}

public class EagerCarBuilder extends AbstractEagerCarBuilder<Car, EagerCarBuilder> {
    // empty: just pass generic parameters
}

public class EagerTruckBuilder extends AbstractEagerCarBuilder<Truck, EagerTruckBuilder> {

    private String length;

    public EagerTruckBuilder length(String length) {
        this.instance.setLength = length;
        return this;
    }
}

Truck truck = new EagerTruckBuilder().brand("gmc").speed("45").length("32").build();