从一个班级内的班级修改变量

Question

这是我基本上想要做的，但不能做到的。我希望urlData和size的值可以从内部类的函数中更新。该应用程序正在崩溃，我试图从CreateTextView()函数访问这些变量的行上提供Null Pointer Exception。

package edu.ahduni.seas.gyapak; 

import android.os.AsyncTask; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 

import java.io.BufferedReader; 
import java.io.InputStream; 
import java.io.InputStreamReader; 
import java.net.HttpURLConnection; 
import java.net.URL; 
import java.util.ArrayList; 

import org.json.JSONException; 
import org.json.JSONArray; 
import org.json.JSONObject; 

import android.util.Log; 
import android.widget.Button; 
import android.widget.LinearLayout; 
import android.widget.TextView; 

public class MainActivity extends AppCompatActivity { 

    public ArrayList<Data> urlData; //Culprit 1 
    public int size; //no. of dirs + files, Culprit 2 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
    } 

    public void parseJSON() throws JSONException { 
     ParseActivity parse = new ParseActivity(); 
     String url = "RDovQWNhZGVtaWMvU2FuamF5IENoYXVkaGFyeQ=="; 
     parse.execute(url); 
     CreateTextView(); 
     Log.e("RETURNING JSON","RETURNING"); 
    } 

    public void CreateTextView() { 
     TextView text = (TextView) findViewById(R.id.text); 
     text.setText(Integer.toString(size)); 
     Button myButton = new Button(this); 
     if(urlData==null) //workaround for crash 
      return; 
     myButton.setText(urlData.get(0).getName()); 

     LinearLayout ll = (LinearLayout)findViewById(R.id.textlayout); 
     LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, 
                    LinearLayout.LayoutParams.WRAP_CONTENT); 
     ll.addView(myButton, lp); 
    } 

    class ParseActivity extends AsyncTask<String, Void, ArrayList<Data>> { 
     InputStream is = null; 
     JSONArray jObj = null; 
     String json = ""; 


     @Override 
     public void onPostExecute(ArrayList<Data> data) { 
      size = data.size(); //here! 
     } 

     @Override 
     protected ArrayList<Data> doInBackground(String... params) { 
      final String BASE_URL = "http://111.93.66.162/json/main.php?f=" + params[0]; 
      HttpURLConnection urlConnection; 

      // Making HTTP request 
      try { 

       Log.e("OPENING: ", "URL"); 
       URL url = new URL(BASE_URL); 

       urlConnection = (HttpURLConnection) url.openConnection(); 
       urlConnection.setRequestMethod("GET"); 
       urlConnection.connect(); 

       is = urlConnection.getInputStream(); 

       StringBuffer buffer = new StringBuffer(); 
       if (is == null) { 
        // Nothing to do. 
        return null; 
       } 

       BufferedReader in = new BufferedReader(new InputStreamReader(is)); 
       String line; 

       while ((line = in.readLine()) != null) { 
        buffer.append(line + "\n"); 
        Log.e("JSON: ", line); 
       } 
       if (buffer.length() == 0) { 
        // Stream was empty. No point in parsing. 
        return null; 
       } 
       is.close(); 
       json = buffer.toString(); 

      } catch (Exception e) { 
       e.printStackTrace(); 
      } 

      try { 
       return getData(json); 
      } catch (JSONException e) { 
       e.printStackTrace(); 
      } 
      return null; 
     } 

     public ArrayList<Data> getData(String jsonString) throws JSONException { 
      // try parse the string to a JSON object 
      final String EXT = "ext"; 
      final String PATH = "path"; 
      final String NAME = "name"; 

      try { 
       jObj = new JSONArray(jsonString); 
      } catch (JSONException e) { 
       e.printStackTrace(); 
      } 

      urlData = new ArrayList<Data>(); 

      for (int i=0;i<jObj.length();i++) { 
       JSONObject obj = jObj.getJSONObject(i); 
       String ext = obj.getString(EXT); 
       String path = obj.getString(PATH); 
       String name = obj.getString(NAME); 

       urlData.add(new Data(ext,path,name)); //and here! 
      } 
      return urlData; 
     } 
    } 
} 

Answer 1

内部类不是静态的，所以它需要实例化封闭类的实例。像这样：

public class abcd { 
    static int x; 

    public static void main(String[] args) { 
     abcd o = new abcd(); // Create instance of abcd called 'o' 
     abcd.efgh o2 = o.new efgh(); // Use instance 'o' to create an instance of efgh 

     System.out.println(x); // 0 
     o2.ok(); 
     System.out.println(x); // 5 
    } 

    class efgh { 
     public void ok() { 
      x = 5; 
     } 
    } 
}