printTopContributor.java

public void printTopContributor(GitHubApi gitHubApi, final String repoName) {
    // 1. Fetch the repository.
    gitHubApi.getRepository(repoName)
        // 2. Fetch a list of all contributors.
        .flatMap(new Function<Repository, SingleSource<? extends List<Contributor>>>() {
            @Override
            public SingleSource<? extends List<Contributor>> apply(Repository repository) throws Exception {
                return repository.getContributors();
            }
        })
        // 3. Convert the list to an Observable emitting contributors on after another.
        .flatMapObservable(new Function<List<Contributor>, ObservableSource<? extends Contributor>>() {
            @Override
            public ObservableSource<? extends Contributor> apply(List<Contributor> contributors) throws Exception {
                return Observable.fromIterable(contributors);
            }
        })
        // 4. Sort emitted values.
        .sorted(new Comparator<Contributor>() {
            @Override
            public int compare(Contributor o1, Contributor o2) {
                return o2.getNumberOfCommits() - o1.getNumberOfCommits();
            }
        })
        // 5. Take the first contributor and convert the result to a Single.
        .firstOrError()
        // 6. Print the top contributor's name.
        .subscribe(new Consumer<Contributor>() {
            @Override
            public void accept(Contributor contributor) throws Exception {
                print(contributor.getLoginName());
            }
        }, new Consumer<Throwable>() {
            @Override
            public void accept(Throwable throwable) throws Exception {
                logError(throwable, "Error while trying to find the top contributor.");
            }
        });
}

printTopContributor.kt

fun printTopContributor(gitHubApi: GitHubApi, repoName: String) {
    gitHubApi.getRepository(repoName)
        .flatMap { repository -> repository.contributors }
        .flatMapObservable { contributors -> Observable.fromIterable(contributors) }
        .sorted { o1, o2 -> o2.numberOfCommits - o1.numberOfCommits }
        .firstOrError()
        .subscribe(
            { contributor -> print(contributor.loginName) },
            { throwable -> logError(throwable, "Error while trying to find the top contributor.") }
        )
}