https://github.com/square/okhttp/okhttp: An HTTP+HTTP/2 client for Android and Java applications 
/*
 * Copyright (C) 2014 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3;

import java.io.IOException;

public interface Callback {
  
Called when the request could not be executed due to cancellation, a connectivity problem or
timeout. Because networks can fail during an exchange, it is possible that the remote server
accepted the request before the failure.

/**
   * Called when the request could not be executed due to cancellation, a connectivity problem or
   * timeout. Because networks can fail during an exchange, it is possible that the remote server
   * accepted the request before the failure.
   */
  void onFailure(Call call, IOException e);

  
Called when the HTTP response was successfully returned by the remote server. The callback may proceed to read the response body with Response.body. The response is still live until its response body is closed. The recipient of the callback may consume the response body on another thread. 
Note that transport-layer success (receiving a HTTP response code, headers and body) does not necessarily indicate application-layer success: response may still indicate an unhappy HTTP response code like 404 or 500. 
/**
   * Called when the HTTP response was successfully returned by the remote server. The callback may
   * proceed to read the response body with {@link Response#body}. The response is still live until
   * its response body is {@linkplain ResponseBody closed}. The recipient of the callback may
   * consume the response body on another thread.
   *
   * <p>Note that transport-layer success (receiving a HTTP response code, headers and body) does
   * not necessarily indicate application-layer success: {@code response} may still indicate an
   * unhappy HTTP response code like 404 or 500.
   */
  void onResponse(Call call, Response response) throws IOException;
}