code: public CompareResults compare(Object projectIdOrPath, String from, String to, boolean straight) throws GitLabApiException {
          Form formData = new GitLabApiForm()
                  .withParam("from", from, true)
                  .withParam("to", to, true)
                  .withParam("straight", straight);
          Response response = get(Response.Status.OK, formData.asMap(), "projects",
                  getProjectIdOrPath(projectIdOrPath), "repository", "compare");
          return (response.readEntity(CompareResults.class));
      }
docs: Compare branches, tags or commits. This can be accessed without authentication
  if the repository is publicly accessible.
  
  @param projectIdOrPath the project in the form of an Integer(ID), String(path), or Project instance
  @param from the commit SHA or branch name
  @param to the commit SHA or branch name
  @param straight specifies the comparison method, true for direct comparison between from and to (from..to),
  false to compare using merge base (from…to)’.
  @return a CompareResults containing the results of the comparison
  @throws GitLabApiException if any exception occurs