java - 学习android ...为什么类中存在类

Question

我来自 php 背景，正在尝试学习 android。我在示例代码中看到了这一点，我正在查看类中存在类定义的位置。那到底是如何运作的呢？有人可以向我解释一下吗？您可以看到 LongOperation 类是在 RestfulServices 类中定义的。

public class RestFulWebservice extends Activity {


    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.rest_ful_webservice);  

        final Button GetServerData = (Button) findViewById(R.id.GetServerData);

        GetServerData.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {

                // WebServer Request URL
                String serverURL = "http://androidexample.com/media/webservice/JsonReturn.php";

                // Use AsyncTask execute Method To Prevent ANR Problem
                new LongOperation().execute(serverURL);
            }
        });    

    }


    // Class with extends AsyncTask class
    private class LongOperation  extends AsyncTask<String, Void, Void> {

        // Required initialization

        private final HttpClient Client = new DefaultHttpClient();
        private String Content;
        private String Error = null;
        private ProgressDialog Dialog = new ProgressDialog(RestFulWebservice.this);
        String data =""; 
        TextView uiUpdate = (TextView) findViewById(R.id.output);
        TextView jsonParsed = (TextView) findViewById(R.id.jsonParsed);
        int sizeData = 0;  
        EditText serverText = (EditText) findViewById(R.id.serverText);


        protected void onPreExecute() {
            // NOTE: You can call UI Element here.

            //Start Progress Dialog (Message)

            Dialog.setMessage("Please wait..");
            Dialog.show();

            try{
                // Set Request parameter
                data +="&" + URLEncoder.encode("data", "UTF-8") + "="+serverText.getText();

            } catch (UnsupportedEncodingException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } 

        }

        // Call after onPreExecute method
        protected Void doInBackground(String... urls) {

            /************ Make Post Call To Web Server ***********/
            BufferedReader reader=null;

                 // Send data 
                try
                { 

                   // Defined URL  where to send data
                   URL url = new URL(urls[0]);

                  // Send POST data request

                  URLConnection conn = url.openConnection(); 
                  conn.setDoOutput(true); 
                  OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream()); 
                  wr.write( data ); 
                  wr.flush(); 

                  // Get the server response 

                  reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
                  StringBuilder sb = new StringBuilder();
                  String line = null;

                    // Read Server Response
                    while((line = reader.readLine()) != null)
                        {
                               // Append server response in string
                               sb.append(line + "\n");
                        }

                    // Append Server Response To Content String 
                   Content = sb.toString();
                }
                catch(Exception ex)
                {
                    Error = ex.getMessage();
                }
                finally
                {
                    try
                    {

                        reader.close();
                    }

                    catch(Exception ex) {}
                }

            /*****************************************************/
            return null;
        }

        protected void onPostExecute(Void unused) {
            // NOTE: You can call UI Element here.

            // Close progress dialog
            Dialog.dismiss();

            if (Error != null) {

                uiUpdate.setText("Output : "+Error);

            } else {

                // Show Response Json On Screen (activity)
                uiUpdate.setText( Content );

             /****************** Start Parse Response JSON Data *************/

                String OutputData = "";
                JSONObject jsonResponse;

                try {

                     /****** Creates a new JSONObject with name/value mappings from the JSON string. ********/
                     jsonResponse = new JSONObject(Content);

                     /***** Returns the value mapped by name if it exists and is a JSONArray. ***/
                     /*******  Returns null otherwise.  *******/
                     JSONArray jsonMainNode = jsonResponse.optJSONArray("Android");

                     /*********** Process each JSON Node ************/

                     int lengthJsonArr = jsonMainNode.length();  

                     for(int i=0; i < lengthJsonArr; i++) 
                     {
                         /****** Get Object for each JSON node.***********/
                         JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);

                         /******* Fetch node values **********/
                         String name       = jsonChildNode.optString("name").toString();
                         String number     = jsonChildNode.optString("number").toString();
                         String date_added = jsonChildNode.optString("date_added").toString();


                         OutputData += " Name           : "+ name +" \n "
                                     + "Number      : "+ number +" \n "
                                     + "Time                : "+ date_added +" \n " 
                                     +"--------------------------------------------------\n";

                         //Log.i("JSON parse", song_name);
                    }
                 /****************** End Parse Response JSON Data *************/     

                     //Show Parsed Output on screen (activity)
                     jsonParsed.setText( OutputData );


                 } catch (JSONException e) {

                     e.printStackTrace();
                 }


             }
        }

    }

}

Answer 1

Java 语法允许这种构造。它被称为内部类。 http://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html

内部类是一种特殊类型的嵌套类。 http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html