Adding an image watermark in Java with Retrofit

import retrofit2.*;
import retrofit2.converter.gson.GsonConverterFactory;
import com.google.gson.*;
import java.io.IOException;

// You can get an API key at https://pdfshift.io
public interface PdfShiftApi {
    @POST("v3/convert/pdf")
    Call<ResponseBody> convertToPdf(
        @Header("X-API-Key") String apiKey,
        @Body JsonObject body
    );
}

// Usage
Gson gson = new GsonBuilder().create();
Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.pdfshift.io/")
    .addConverterFactory(GsonConverterFactory.create(gson))
    .build();

PdfShiftApi api = retrofit.create(PdfShiftApi.class);

JsonObject payload = new JsonObject();
payload.addProperty("source", "https://www.example.com");
JsonObject watermark = new JsonObject();
watermark.addProperty("image", "https://example.com/watermark.png");
watermark.addProperty("opacity", 0.5);
watermark.addProperty("position", "center");
payload.add("watermark", watermark);

Call<ResponseBody> call = api.convertToPdf("sk_xxxxxxxxxxxx", payload);
Response<ResponseBody> response = call.execute();

// Handle errors:
if (!response.isSuccessful()) {
    throw new IOException("Request failed with status code " + response.code());
}

ResponseBody responseBody = response.body();
if (responseBody != null) {
    java.nio.file.Files.write(
        java.nio.file.Paths.get("result.pdf"), 
        responseBody.bytes()
    );
}

System.out.println("The PDF document was generated and saved to result.pdf");