RetrofitでJSONをPOSTする ref: http://qiita.com/shts/items/775973783966ce7b19cf

file0.rb

post '/user', provides: :json do
  params = JSON.parse request.body.read
  puts params['name']
  puts params['age']
end

file1.txt

$ curl -v -H "Accept: application/json" -H "Content-type: application/json" -X POST -d '{"name":"hogehoge","age":"30"}' http://localhost:9292/user -w "\n%{http_code}\n"

file2.java

public interface ApiService {
    @Headers({
        "Accept: application/json",
        "Content-type: application/json"
    })
    @POST("/user")
    Observable<Void> createUser(@Body HashMap<String, String> body);
}

file3.java

// post
public static Observable<Void> postUser(String name, int age) {
    // create Retrofit
    Retrofit retrofit = new Retrofit.Builder()
        .client(new OkHttpClient.Builder().build())
        .baseUrl("http://localhost:9292")
        .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
        .addConverterFactory(GsonConverterFactory.create())
        .build();

    return retrofit.createUser(createUserBody(name, age));
}

// create params
private static HashMap<String, String> createUserBody(String name, int age) {
    HashMap<String, String> hashMap = new HashMap<>();
    hashMap.put("name", name);
    hashMap.put("age", String.valueOf(age));
    return hashMap;
}

// API
public interface ApiService {
    @Headers({
        "Accept: application/json",
        "Content-type: application/json"
    })
    @POST("/user")
    Observable<Void> createUser(@Body HashMap<String, String> body);
}