public interface CollectionsJavaDocWebPageService {
  @GET("javase/7/docs/api/java/util/Collections.html")
  Call<Document> fetch();
}

Retrofit retrofit = new Retrofit.Builder().baseUrl(baseUrl)
  .addConverterFactory(HtmlConverterFactory.create(baseUrl))
  .build();
CollectionsJavaDocWebPageService service
                      = retrofit.create(CollectionsJavaDocWebPageService.class);
Call<Document> call = service.fetch();

import org.jsoup.nodes.Document;

call.enqueue(new Callback<Document>() {
  @Override
  public void onResponse(Call<Document> call, Response<Document> response) {
    Document document = response.body();
    document.setBaseUri(base);
    String html = document.html(); 
    // contains the html of https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
  }

  @Override
  public void onFailure(Call<Document> call, Throwable t) {
    t.printStackTrace();
   }
});