Main.java

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonProperty;
import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.HttpMethod;
import org.springframework.http.ResponseEntity;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.web.client.AsyncRestTemplate;

import java.util.*;
import java.util.concurrent.CompletableFuture;

public class Main {
    public static void main(String[] args) throws Exception {
        Main main = new Main();
        List<String> usernames = Arrays.asList("philwebb", "dsyer", "wilkinsona", "snicoll", "rwinch");

        Optional<CompletableFuture<List<GitHubStar>>> ret = usernames.stream()
                .map(main::findStars)
                .reduce((f1, f2) -> f1.thenCombine(f2,
                        (l1, l2) -> {
                            l1.addAll(l2);
                            return l1;
                        }));
        ret.get().thenAccept(stars -> stars.stream()
                        .distinct()
                        .sorted(Comparator.comparing(GitHubStar::getUpdatedAt).reversed())
                        .forEach(System.out::println)
        ).get();
    }


    AsyncRestTemplate restTemplate = new AsyncRestTemplate();

    public CompletableFuture<List<GitHubStar>> findStars(String username) {
        CompletableFuture<List<GitHubStar>> future = new CompletableFuture<>();

        ListenableFuture<ResponseEntity<List<GitHubStar>>> response = restTemplate.exchange(
                "https://api.github.com/users/{username}/starred", HttpMethod.GET, null,
                new ParameterizedTypeReference<List<GitHubStar>>() {
                }, Collections.singletonMap("username", username));
        response.addCallback(x -> future.complete(x.getBody()), future::completeExceptionally);
        return future;
    }


    @JsonIgnoreProperties(ignoreUnknown = true)
    public static class GitHubStar {
        @JsonProperty("full_name")
        private String name;
        @JsonProperty("html_url")
        private String url;
        private String description;
        @JsonProperty("updated_at")
        private Date updatedAt;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getUrl() {
            return url;
        }

        public void setUrl(String url) {
            this.url = url;
        }

        public String getDescription() {
            return description;
        }

        public void setDescription(String description) {
            this.description = description;
        }

        public Date getUpdatedAt() {
            return updatedAt;
        }

        public void setUpdatedAt(Date updatedAt) {
            this.updatedAt = updatedAt;
        }

        @Override
        public String toString() {
            return "GitHubStar{" +
                    "name='" + name + '\'' +
                    ", url='" + url + '\'' +
                    ", description='" + description + '\'' +
                    ", updatedAt=" + updatedAt +
                    '}';
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            GitHubStar that = (GitHubStar) o;

            if (name != null ? !name.equals(that.name) : that.name != null) return false;
            if (url != null ? !url.equals(that.url) : that.url != null) return false;
            if (description != null ? !description.equals(that.description) : that.description != null) return false;
            return !(updatedAt != null ? !updatedAt.equals(that.updatedAt) : that.updatedAt != null);
        }

        @Override
        public int hashCode() {
            int result = name != null ? name.hashCode() : 0;
            result = 31 * result + (url != null ? url.hashCode() : 0);
            result = 31 * result + (description != null ? description.hashCode() : 0);
            result = 31 * result + (updatedAt != null ? updatedAt.hashCode() : 0);
            return result;
        }
    }
}