import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.net.URLEncoder;
import java.util.*;

public class OAuthAuthenticator {
  private String domain;
  private String path;
  private String appKey;
  private String secret;
  private String accessToken;
  private String accessTokenSecret;

  public static void main(String[] args) {
    String domain = "your domain";
    // Insert the path without the query string: "/api/account/:accountId/eligibility"
    String path = "your path";
    String appKey = "your appkey";
    String secret = "your secret";
    String accessToken = "your accessToken";
    String accessTokenSecret = "your accessTokenSecret";
    // Insert query string as an array
    String[] parameterList = {"v=2.0"};
    OAuthAuthenticator generator = new OAuthAuthenticator(domain, path, appKey, secret, accessToken, accessTokenSecret);
    String result = generator.generateOauthHeader("POST", parameterList);
    System.out.println(result);
  }

  public OAuthAuthenticator(String domain,
                String path,
                String appKey,
                String secret,
                String accessToken,
                String accessTokenSecret) {
    this.domain = domain;
    this.path = path;
    this.appKey = appKey;
    this.secret = secret;
    this.accessToken = accessToken;
    this.accessTokenSecret = accessTokenSecret;
  }

  public String generateOauthHeader(String method,
                    String[] additionalParameters) {
    long timestamp = new Date().getTime() / 1000;
    String nonce = getNonce();

    ArrayList<String> parameters = new ArrayList<String>();
    parameters.add("oauth_consumer_key=" + appKey);
    parameters.add("oauth_nonce=" + nonce);
    parameters.add("oauth_signature_method=HMAC-SHA1");
    parameters.add("oauth_timestamp=" + timestamp);
    if (accessToken != null) {
      parameters.add("oauth_token=" + accessToken);
    }
    parameters.add("oauth_version=1.0");
    if (additionalParameters != null) {
      for (String additionalParameter : additionalParameters) {
        parameters.add(additionalParameter);
      }
    }
    Collections.sort(parameters);
    StringBuffer parametersList = new StringBuffer();
    for (int i = 0; i < parameters.size(); i++) {
      parametersList.append(((i > 0) ? "&" : "") + parameters.get(i));
    }
    String signatureString =
        method + "&" +
            URLEncoder.encode("https://" + domain + path) + "&" +
            URLEncoder.encode(parametersList.toString());
    String signature = null;
    try {
      SecretKeySpec signingKey = new SecretKeySpec((secret + "&"
          + (accessTokenSecret == null ? "" : accessTokenSecret)
      ).getBytes(), "HmacSHA1");
      Mac mac = Mac.getInstance("HmacSHA1");
      mac.init(signingKey);
      byte[] rawHMAC = mac.doFinal(signatureString.getBytes());
      signature = Base64.getEncoder().encodeToString(rawHMAC);
    } catch (Exception e) {
      System.err.println("Unable to append signature");
      System.exit(0);
    }
    String authorizationLine =
        "OAuth " +
            "oauth_consumer_key=\"" + appKey + "\", " +
            "oauth_nonce=\"" + nonce + "\", " +
            "oauth_timestamp=\"" + timestamp + "\", " +
            "oauth_signature_method=\"HMAC-SHA1\", " +
            "oauth_signature=\"" + URLEncoder.encode(signature) + "\", " +
            "oauth_version=\"1.0\"";
    if (accessToken != null) {
      authorizationLine += ", oauth_token=\"" + accessToken + "\"";
    }
    return authorizationLine;
  }

  public String getNonce() {
    int leftLimit = 48;
    int rightLimit = 122;
    int targetStringLength = 10;
    Random random = new Random();
    String generatedString = random.ints(leftLimit, rightLimit + 1)
        .filter(i -> (i <= 57 || i >= 65) && (i <= 90 || i >= 97))
        .limit(targetStringLength)
        .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
        .toString();

    return generatedString;
  }
}