形象比喻：
你到一个商店买东西，刚好你要的东西没有货，于是你在店员那里留下了你的电话，过了几天店里有货了，店员就打了你的电话，然后你接到电话后就到店里去取了货。在这个例子里，你的电话号码就叫回调函数，你把电话留给店员就叫登记回调函数，，店里后来有货了叫做触发了回调关联的事件，店员给你打电话叫做调用回调函数，你到店里去取货叫做响应回调事件。（来自知乎）
回调的好处：
降低代码的耦合性，使代码更灵活、简洁
步骤一 ：定义回调接口/** * Created by pengkv on 15/10/22. * 网络请求回调 */{void onFinish(String respose);void onError(Exception e);}步骤二：定义回调函数（将接口作为参数）/** * Created by pengkv on 15/10/22. * 网络请求工具类 */{(final String urlStr, final HttpCallBackListener listener) {new Thread(new Runnable() {() {HttpURLConnection connection = null;try {URL url = new URL(urlStr);connection = (HttpURLConnection) url.openConnection();connection.setRequestMethod(“GET”);connection.setConnectTimeout(8000);connection.setReadTimeout(8000);connection.setDoInput(true);connection.setDoOutput(true);InputStream in = connection.getInputStream();BufferedReader br = new BufferedReader(new InputStreamReader(in));StringBuilder sb = new StringBuilder();String line;while ((line = br.readLine()) != null) {sb.append(line);}if (listener != null) {//回调onFinish方法listener.onFinish(sb.toString());}} catch (Exception e) {if (listener != null) {//回调onError方法listener.onError(e);}} finally {if (connection != null) {connection.disconnect();}}}}).start();}}步骤三：使用回调方法一{(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);HttpUtil.requestData(“请求的网址”, new HttpCallBackListener() {(String respose) {//处理请求}(Exception e) {//处理异常}});}}步骤三：使用回调方法二{(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);HttpUtil.requestData(“请求的网址”, this);}(String respose) {//处理请求}(Exception e) {//处理异常}}
当你能爱的时候就不要放弃爱
