我想将 Web 服务 URL 请求表示为一个对象，发现有很多公共(public)参数可以在继承层次结构中“冒泡”。一个请求可能有很多参数，一些是必需的，一些是可选的，我相信 Bloch 的构建器模式是一个不错的选择，它使用流畅的界面模拟命名参数。

具体来说，我正在为具有一般网络服务请求的 Google map 网络服务 API 进行设计

http://maps.googleapis.com/maps/api/service/output?{parameters}

service 和output 是强制参数，sensor 是强制参数。还有一个可选参数language。

每个服务都有它的一组强制和可选参数。地理编码服务有两个可选参数，bounds 和region。它还具有相互排斥的强制参数，address 或 location，用于指定服务类型(分别为直接或反向地理编码)。我用新的 child 类(class)来代表这种相互排斥。

我想象类层次结构是这样的:

  .-----.
  | Url |
  '-----'
     ^
     |
.---------.
| Request |
'---------'
     ^
     |----------------------------+--------------...
.---------.                 .------------.
| Geocode |                 | Directions |
'---------'                 '------------'
     ^                            ^
     |------------+               .
 .--------.  .---------.          .
 | Direct |  | Reverse |          .
 '--------'  '---------'

然后，我想做类似下面的事情:

String output = "xml";
boolean sensor = true;
String address = "Av. Paulista, São Paulo, Brasil";
Bounds bounds  = new Bounds(-20, -10, -25, -20); //Geographic rectangle
String region  = "br";
String lang    = "pt-BR";
Coord location = new Coord(-12,-22);

DirectGeocodeRequestUrl direct = 
    new DirectGeocodeRequestUrl.Builder(output, sensor, address)
                               .bounds(bounds)
                               .language(lang)
                               .build();

ReverseGeocodeRequestUrl reverse = 
    new ReverseGeocodeRequestUrl.Builder(output, sensor, location)
                                .language(lang)
                                .region(region)
                                .build();

我如何创建一个 Builder，它使用它所插入的类和父类(super class)中的参数和方法？

最佳答案

我的答案基于 https://stackoverflow.com/a/9138629/946814 , 但考虑到这种多层次结构。

我们需要的是使用 Builder 内部类复制相同的层次结构。由于我们需要方法链接，因此需要一个返回层次结构的叶对象的 getThis() 方法。为了在层次结构中向上传递它的类型，父类有一个泛型 T，叶子将 T 绑定(bind)到它自己。

它确保了类型安全并避免了由于未初始化的强制参数或拼写错误而引发的任何异常，加上流畅的界面。然而，将如此简单的结构表示为 URL 是一项非常昂贵且复杂的设计。我希望它对某人有用 - 我更喜欢最后的字符串连接。

请求地址:

public abstract class RequestUrl{
    public static abstract class Builder<T extends Builder<T>>{
        protected String output;
        protected boolean sensor;
        //Optional parameters can have default values
        protected String lang = "en"; 

        public Builder(String output, boolean sensor){
            this.output = output;
            this.sensor = sensor;
        }

        public T lang(String lang){
            this.lang = lang;
            return getThis();
        }

        public abstract T getThis();
    }

    final private String output;
    final private boolean sensor;
    final private String lang;

    protected <T extends Builder<T>> RequestUrl(Builder<T> builder){
        this.output = builder.output;
        this.sensor = builder.sensor;
        this.lang = builder.lang;
    }

    // other logic...
}

地理编码请求网址:

public abstract class GeocodeRequestUrl extends RequestUrl {
    public static abstract class Builder<T extends Builder<T>>
        extends RequestUrl.Builder<Builder<T>>{

        protected Bounds bounds;
        protected String region = "us";

        public Builder(String output, boolean sensor){
            super( output, sensor );
        }

        public T bounds(Bounds bounds){
            this.bounds = bounds;
            return getThis();
        }

        public T region(String region){
            this.region = region;
            return getThis();
        }

        @Override
        public abstract T getThis();
    }

    final private Bounds bounds;
    final private String region;

    protected <T extends Builder<T>> GeocodeRequestUrl(Builder<T> builder){
        super (builder);
        this.bounds = builder.bounds;
        this.region = builder.region;
    }

    // other logic...
}

DirectGeocodeRequestUrl:

public class DirectGeocodeRequestUrl extends GeocodeRequestUrl {
    public static class Builder<Builder>
        extends GeocodeRequestUrl.Builder<Builder>{

        protected String address;

        public Builder(String output, boolean sensor, String address){
            super( output, sensor );
            this.address = address;
        }

        @Override
        public Builder getThis(){
            return this;
        }

        public DirectGeocodeRequestUrl build(){
            return new DirectGeocodeRequestUrl(this);
        }
    }

    final private String address;

    protected DirectGeocodeRequestUrl(Builder builder){
        super (builder);
        this.address = builder.address;
    }

    // other logic...
}

反向地理编码请求网址:

public class ReverseGeocodeRequestUrl extends GeocodeRequestUrl {
    public static class Builder<Builder>
        extends GeocodeRequestUrl.Builder<Builder>{

        protected Coord location;

        public Builder(String output, boolean sensor, Coord location){
            super( output, sensor );
            this.location = location;
        }

        @Override
        public Builder getThis(){
            return this;
        }

        public ReverseGeocodeRequestUrl build(){
            return new ReverseGeocodeRequestUrl(this);
        }
    }

    final private Coord location;

    protected ReverseGeocodeRequestUrl(Builder builder){
        super (builder);
        this.location = builder.location;
    }

    // other logic...
}

关于java - 具有继承性的构建器模式，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/10941464/