/**
 * a few utility methods that are used in MailAttachment and MailMessage
 */
package io.vertx.ext.mail;

import java.util.ArrayList;
import java.util.List;

import io.vertx.core.MultiMap;
import io.vertx.core.http.CaseInsensitiveHeaders;
import io.vertx.core.json.JsonArray;
import io.vertx.core.json.JsonObject;

/**
 * @author <a href="http://oss.lehmann.cx/">Alexander Lehmann</a>
 *
 */
class Utils {

  private Utils() {
  }

  static JsonObject multiMapToJson(final MultiMap headers) {
    JsonObject json = new JsonObject();
    for (String key : headers.names()) {
      json.put(key, headers.getAll(key));
    }
    return json;
  }

  static void putIfNotNull(final JsonObject json, final String key, final Object value) {
    if (value != null) {
      json.put(key, value);
    }
  }

  static MultiMap jsonToMultiMap(final JsonObject jsonHeaders) {
    MultiMap headers = new CaseInsensitiveHeaders();
    for (String key : jsonHeaders.getMap().keySet()) {
      headers.add(key, getKeyAsStringOrList(jsonHeaders, key));
    }
    return headers;
  }

  @SuppressWarnings("unchecked")
  static List<String> getKeyAsStringOrList(JsonObject json, String key) {
    Object value = json.getValue(key);
    if (value == null) {
      return null;
    } else {
      if (value instanceof String) {
        return asList((String) value);
      } else if (value instanceof JsonArray) {
        return (List<String>) ((JsonArray) value).getList();
      } else {
        throw new IllegalArgumentException("invalid attachment type");
      }
    }
  }

  static <T> List<T> asList(T to) {
    List<T> list = new ArrayList<>(1);
    list.add(to);
    return list;
  }


}

/**
 * a few utility methods that are used in MailAttachment and MailMessage
 */
package io.vertx.ext.mail;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import io.vertx.core.MultiMap;
import io.vertx.core.http.CaseInsensitiveHeaders;
import io.vertx.core.json.JsonArray;
import io.vertx.core.json.JsonObject;

/**
 * @author <a href="http://oss.lehmann.cx/">Alexander Lehmann</a>
 *
 */
class Utils {

  private Utils() {
  }

  static JsonObject multiMapToJson(final MultiMap headers) {
    JsonObject json = new JsonObject();
    for (String key : headers.names()) {
      json.put(key, headers.getAll(key));
    }
    return json;
  }

  static void putIfNotNull(final JsonObject json, final String key, final Object value) {
    if (value != null) {
      json.put(key, value);
    }
  }

  static MultiMap jsonToMultiMap(final JsonObject jsonHeaders) {
    MultiMap headers = new CaseInsensitiveHeaders();
    for (String key : jsonHeaders.getMap().keySet()) {
      headers.add(key, getKeyAsStringOrList(jsonHeaders, key));
    }
    return headers;
  }

  @SuppressWarnings("unchecked")
  static List<String> getKeyAsStringOrList(JsonObject json, String key) {
    Object value = json.getValue(key);
    if (value == null) {
      return null;
    } else {
      if (value instanceof String) {
        return asList((String) value);
      } else if (value instanceof JsonArray) {
        return (List<String>) ((JsonArray) value).getList();
      } else {
        throw new IllegalArgumentException("invalid attachment type");
      }
    }
  }

  static <T> List<T> asList(T element) {
    return Collections.singletonList(element);
  }

}

use singletonList for asList method