Java can'；从活动更改为片段后，不会得到相同的结果

我开始使用导航抽屉，为此我开始使用片段。。在我更改了此活动并将代码稍作更改移动到新片段后。。我在片段中的OnOptions ItemSelected方法上得到一个错误。。我应该把网址放在哪里，如果它不是正确的地方放

这是FoodView.java活动

public class FoodView extends AppCompatActivity {


private ListView lvFood;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);


    setContentView(R.layout.activity_food_view);


    lvFood = (ListView) findViewById(R.id.lvFood);
}

    public class JSONTask extends AsyncTask<String, String, List<FoodModel>> {


        @Override
        protected List<FoodModel> doInBackground(String... params) {

            HttpURLConnection httpURLConnection = null;
            BufferedReader reader = null;


            try {
                URL url = new URL(params[0]);
                httpURLConnection = (HttpURLConnection) url.openConnection();
                httpURLConnection.connect();

                InputStream inputStream = httpURLConnection.getInputStream();
                reader = new BufferedReader(new InputStreamReader(inputStream));
                StringBuffer buffer = new StringBuffer();
                String line = "";
                while ((line = reader.readLine()) != null) {
                    buffer.append(line);
                }
                String finalJson = buffer.toString();


                JSONArray parentArray = new JSONArray(finalJson);

                List<FoodModel> foodModelList = new ArrayList<>();


                for (int i = 0; i < parentArray.length(); i++) {
                    JSONObject finalObject = parentArray.getJSONObject(i);
                    FoodModel foodModel = new FoodModel();
                    foodModel.setFood(finalObject.optString("name"));
                    foodModel.setStatus(finalObject.optString("status"));
                    foodModel.setAmount(finalObject.optInt("amount"));
                    foodModel.setDescription(finalObject.optString("description"));
                    foodModel.setDate(finalObject.optInt("exDate"));


                    foodModelList.add(foodModel);
                }

                return foodModelList;

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


            } catch (JSONException e) {
                e.printStackTrace();
            } finally {
                if (httpURLConnection != null) {
                    httpURLConnection.disconnect();
                }
            }
            try {
                if (reader != null) {
                    reader.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }

            return new ArrayList<>();
        }

        @Override
        protected void onPostExecute(List<FoodModel> result) {
            super.onPostExecute(result);

            FoodAdapter adapter = new FoodAdapter(getApplicationContext(), R.layout.row, result);
            lvFood.setAdapter(adapter);


        }}

        public class FoodAdapter extends ArrayAdapter {

            private List<FoodModel> foodModelList;
            private int resource;
            private LayoutInflater inflater;

            public FoodAdapter(Context context, int resource, List<FoodModel> objects) {
                super(context, resource, objects);
                foodModelList = objects;
                this.resource = resource;
                inflater = (LayoutInflater)getSystemService(LAYOUT_INFLATER_SERVICE);


            }


            @Override
            public View getView(int position, View convertView, ViewGroup parent) {
                if (convertView == null) {
                    convertView = inflater.inflate(resource, null);
                }

                ImageView ivIcon;
                TextView foodName, txt_amount, txt_desc, txt_date, txt_status;

                ivIcon = (ImageView) convertView.findViewById(R.id.ivIcon);
                foodName = (TextView) convertView.findViewById(R.id.foodName);
                txt_amount = (TextView) convertView.findViewById(R.id.txt_amount);
                txt_desc = (TextView) convertView.findViewById(R.id.txt_desc);
               txt_status = (TextView) convertView.findViewById(R.id.txt_status);
                txt_date = (TextView) convertView.findViewById(R.id.txt_date);

                foodName.setText(foodModelList.get(position).getFood());
                txt_amount.setText("Amount: " + foodModelList.get(position).getAmount());
                txt_desc.setText(foodModelList.get(position).getDescription());
                txt_status.setText(foodModelList.get(position).getStatus());
                txt_date.setText("EX Date: " + foodModelList.get(position).getDate());


                return convertView;
            }
        }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.navigation_drawer, menu);
    return true;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();

    if (id == R.id.action_refresh) {
        new JSONTask().execute("http://10.0.3.2/MySqlDemo/food.php");
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

---

将这些方法保留在活动中，从片段中删除它们

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.navigation_drawer, menu);
    return true;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();

         if (id == R.id.action_refresh) {
        // add this line in your activity 


      ((FoodViewFragment)selectedFragment).yourServerCallMethodInF‌​ragment();
            return true;
        }

return super.onOptionsItemSelected(item);
}

在活动级别维护当前片段

private Fragment selectedFragment;

并在中获取所选片段的引用

 public boolean onNavigationItemSelected(MenuItem item) {



// Handle navigation view item clicks here.
int id = item.getItemId();

if (id == R.id.nav_home) {
    setTitle("Food View");

   selectedFragment = new FoodViewFragment();
    FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
    fragmentTransaction.replace(R.id.content_frame,selectedFragment, "fragment1");
    fragmentTransaction.commit();


} else if (id == R.id.nav_food) {

} else if (id == R.id.nav_money) {

} else if (id == R.id.nav_settings) {

} else if (id == R.id.nav_about) {

} else if (id == R.id.nav_notifications) {

}


DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}

现在，您的片段将具有此方法

public void yourServerCallMethodInFragment(){
 new JSONTask().execute("http://10.0.3.2/MySqlDemo/food.php");
}

---

如果（id==R.id.action\u refresh）{//在活动selectedFragment.yourServerCallMethodInFragment（）；return true；}中添加这一行，希望对您有所帮助。上面的部分显示了一个错误。。。selectedFragment.yourServerCallMethodInFragment（）；你可以让我知道这是什么类型的错误。我想应该是类型转换问题，试试这个，否则让我知道（（FoodViewFragment）selectedFragment）；

 public boolean onNavigationItemSelected(MenuItem item) {



// Handle navigation view item clicks here.
int id = item.getItemId();

if (id == R.id.nav_home) {
    setTitle("Food View");

   selectedFragment = new FoodViewFragment();
    FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
    fragmentTransaction.replace(R.id.content_frame,selectedFragment, "fragment1");
    fragmentTransaction.commit();


} else if (id == R.id.nav_food) {

} else if (id == R.id.nav_money) {

} else if (id == R.id.nav_settings) {

} else if (id == R.id.nav_about) {

} else if (id == R.id.nav_notifications) {

}


DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}

public void yourServerCallMethodInFragment(){
 new JSONTask().execute("http://10.0.3.2/MySqlDemo/food.php");
}