java - 尝试在空对象引用上调用虚拟方法 'boolean java.util.ArrayList.add(java.lang.Object)'

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

嘿伙计们，我尝试获取 json 格式的数据，然后在微调器中显示它们。我遇到了一个问题，你能帮我吗？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();
            }

        }
    }

}

城市.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) 

你们能帮帮我吗？？？真的很感激。

Answer 1

替换:

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

与:

citylist = new ArrayList<>();

这样，您就可以正确初始化 citylist 字段，而不是定义新的(未使用的)局部变量，因此以后它不会为 null。