JSON Builder – Fun with Generics

On the train back to Berlin I spiked a little fluent Json Builder in Java. Here is one of my acceptance tests:

 JsonBuilder builder = new JsonBuilder<NULL>()
        .addObject("name")
            .addProperty("first", "Holden")
            .addProperty("last", "Caulfield")
        .end()
        .addArray("contact")
            .addPrimitive("00447903217666")
            .addObject()
                .addProperty("street", "5 Mayton St")
            .end()
            .addPrimitive("004915151183666")
        .end()
        .addProperty("date", "2011-12-12");
 
JsonObject clientFile =builder.build();

This yields:

{
    "name":
        {
         "first":"Holden",
         "last":"Caulfield"
        },
    "contact":["00447903217666",{"street":"5 Mayton St"},"004915151183666"],
    "date":"2011-12-12"
}

I am wondering whether people find the nesting of builders with end() useful.
The interesting thing is the type parameter, as there is a JsonBuilder and a JsonArrayBuilder, that can be nested arbitrarily but the end() call always returns the the enclosing type.

The type is recursive ;-).
In JsonBuilder we have:

class JsonBuilder <P> {
    public JsonArrayBuilder<JsonBuilder<P>> addArray(String key); 
    public JsonBuilder<JsonBuilder<P>> addObject(String name);
    public P end();
}

And in JsonArrayBuilder things are similar:

class JsonArrayBuilder<P>{   
    public JsonBuilder<JsonArrayBuilder<P>> addObject();
    public JsonArrayBuilder<JsonArrayBuilder<P>> addArray();
    public P end();
}

I was quite surprised, that this thing works. To start with I defined a NULL type for the instance at the root level. Obviously this could be hidden in a static factory method or a subclass.

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