尝试调用null对象引用虚拟方法'布尔java.util.ArrayList.add（java.lang.Object）'参考

Question

嘿嘿我试图获取json格式的数据，并且我在微调器中显示它们。 我陷入了一个问题，你可以帮助我。 这是我的代码。

public class MainActivity extends Activity { 
    Spinner selectcityspinner, selectareaspinner; 
    ArrayList<city> citylist; 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     return super.onCreateOptionsMenu(menu); 
    } 



    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     selectcityspinner = (Spinner)findViewById(R.id.selectcityspin); 
     selectareaspinner = (Spinner) findViewById(R.id.selectareaspin); 
     List<city> citylist = new ArrayList<>(); 
     new cityAsynTask().execute("http://bund.webs.com/citylist.txt"); 

     /*ArrayAdapter<String> adapter = new ArrayAdapter<String> (MainActivity.this,android.R.layout.simple_spinner_item,citylist); 
     adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); 
     selectcityspinner.setAdapter(adapter); 
     selectcityspinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() { 
      @Override 
      public void onItemSelected(AdapterView<?> parent, View view, int position, long id) { 
       Toast.makeText(getBaseContext(),parent.getItemAtPosition(position) + " selected",Toast.LENGTH_LONG).show(); 

       String Country = String.valueOf(parent.getItemAtPosition(position)); 

       if (Country.contentEquals("Ahmedabad")) 
       { 
        List<String> AreaList = new ArrayList<String>(); 
        AreaList.add("Ghatlodia"); 
        AreaList.add("Chandlodia"); 
        AreaList.add("Others"); 
        ArrayAdapter<String> adapter1 = new ArrayAdapter<String> (MainActivity.this,android.R.layout.simple_spinner_item,AreaList); 
        adapter1.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); 
        adapter1.notifyDataSetChanged(); 
        selectareaspinner.setAdapter(adapter1); 
       } 


      } 

      @Override 
      public void onNothingSelected(AdapterView<?> parent) { 

      } 
     });*/ 

    } 
    public class cityAsynTask extends AsyncTask<String,Void,Boolean> 
    { 

     @Override 
     protected Boolean doInBackground(String... params) { 

      try { 
       HttpClient client = new DefaultHttpClient(); 
       HttpPost post = new HttpPost(params[0]); 
       HttpResponse response = client.execute(post); 
       int status = response.getStatusLine().getStatusCode(); 
       if (status == 200) { 
        HttpEntity entity = response.getEntity(); 
        String data = EntityUtils.toString(entity); 
        JSONObject jobjcity = new JSONObject(data); 
        JSONArray jarray = jobjcity.getJSONArray("actors"); 
        for (int i = 0;i < jarray.length();i++){ 
         city city = new city(); 
         JSONObject jrealobject = jarray.getJSONObject(i); 
         city.setCity(jrealobject.getString("name")); 
         citylist.add(city); 

        } 
        return true; 
       } 

      } catch (IOException e) { 
       e.printStackTrace(); 
      } catch (JSONException e) { 
       e.printStackTrace(); 
      } 


      return false; 
     } 

     @Override 
     protected void onPostExecute(Boolean aBoolean) { 
      super.onPostExecute(aBoolean); 
      if(aBoolean == false){ 
       Toast.makeText(MainActivity.this,"data isnot recieved",Toast.LENGTH_SHORT).show(); 
      } 

     } 
    } 

} 

city.java

public class city { 
    private String city; 

    public city(String city) { 
     this.city = city; 
    } 

    public city() { 

    } 

    public String getCity() { 

     return city; 
    } 

    public void setCity(String city) { 
     this.city = city; 
    } 
} 

这里是我的错误

02-28 02:12:44.044 2863-2880/? E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1 
               Process: com.example.bhaumik.myparsesignup, PID: 2863 
               java.lang.RuntimeException: An error occurred while executing doInBackground() 
                at android.os.AsyncTask$3.done(AsyncTask.java:309) 
                at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354) 
                at java.util.concurrent.FutureTask.setException(FutureTask.java:223) 
                at java.util.concurrent.FutureTask.run(FutureTask.java:242) 
                at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 
                at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
                at java.lang.Thread.run(Thread.java:818) 
                Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference 
                at com.example.bhaumik.myparsesignup.MainActivity$cityAsynTask.doInBackground(MainActivity.java:115) 
                at com.example.bhaumik.myparsesignup.MainActivity$cityAsynTask.doInBackground(MainActivity.java:95) 
                at android.os.AsyncTask$2.call(AsyncTask.java:295) 
                at java.util.concurrent.FutureTask.run(FutureTask.java:237) 
                at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)  
                at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)  
                at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)  
                at java.lang.Thread.run(Thread.java:818)  

你们可以帮助我。??? 它真的很感激。

Answer 1

替换：

List<city> citylist = new ArrayList<>(); 

有：

citylist = new ArrayList<>(); 

这种方式，而不是定义一个新的（未使用）局部变量，你正确初始化citylist领域，所以它不会是null后上。