Builder pattern with inheritance

Question

I want to represent an web service URL request as an object, and found that there are lots of common parameters that could be \"bubbled up\" in an inheritance hierarchy. A reque

Answer 1

I'm building my answer upon https://stackoverflow.com/a/9138629/946814, but considering this multi-level hierarchy.

What we need is to replicate the same hierarchy with the Builder inner classes. As we want method chaining, we need a getThis() method that returns the leaf object of the hierarchy. In order to pass its type upward the hierarchy, the parent classes have a generic T, and the leaf binds T to itself.

It assures type-safety and avoids any exception throwing due to uninitialized mandatory parameters or typos, plus the nice fluent interface. However, it's a very costy and complex design to represent such a simple structure as an URL. I hope it is useful to someone - I preferred string concatenation at the end.

RequestUrl:

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...
}

GeocodeRequestUrl:

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...
}

ReverseGeocodeRequestUrl:

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...
}