Based on API Specification, here is an example on how to interact with the service in Java:

import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;
import com.google.gson.JsonStreamParser;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonArray;

public class RestSample {

    public static void main(String[] args)  {

        try {
            HttpClient client = HttpClientBuilder.create().build();
            HttpPost post = new HttpPost("http://localhost:9090/cmd");

            JsonObject json = new JsonObject();
            json.addProperty("Stmt", "select * from sales limit 1000");

            StringEntity se = new StringEntity( json.toString());
            post.setEntity(se);
            HttpResponse response = client.execute(post);

            String respString = EntityUtils.toString(response.getEntity());
            JsonStreamParser parser = new JsonStreamParser(respString);
            while (parser.hasNext()) {
                JsonElement element = parser.next();
                JsonObject obj = element.getAsJsonObject();
                JsonArray array = obj.getAsJsonObject("Content").getAsJsonArray("content");
                for (int i=0; i<array.size(); i++) {
                    System.out.println(array.get(i).toString());                    
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        } 
    }
}