Java 如何使用FragmentManager在ViewPager中引用片段？

所以我一直在为android开发一个基本的天气应用程序，在实现了一个带有标签的viewpager之后，每当我试图在主要活动中使用changeCity方法时，该应用程序就会抛出一个NullPointerException；这需要引用片段，因为我已经改变了片段实例化的方式，所以我不知道该怎么做

我的主要活动：

import android.app.AlertDialog;
import android.support.v4.app.FragmentManager;
import android.content.DialogInterface;
import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.InputType;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.RelativeLayout;

public class WeatherActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_weather);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tab_layout);
        tabLayout.addTab(tabLayout.newTab().setText("Current Weather"));
        tabLayout.addTab(tabLayout.newTab().setText("Forecast"));
        tabLayout.setTabGravity(TabLayout.GRAVITY_FILL);

        final ViewPager viewPager = (ViewPager) findViewById(R.id.pager);
        final PagerAdapter adapter = new PagerAdapter
                (getSupportFragmentManager(), tabLayout.getTabCount());
        viewPager.setAdapter(adapter);
        viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
        tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {
            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {
            }

        });

        /*if(savedInstanceState == null){
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new WeatherFragment())
                    .commit();
        }*/
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_weather, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        if (id == R.id.change_city) {
            showInputDialog();
        }
        return false;
    }


    private void showInputDialog(){
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Change city");
        final EditText input = new EditText(this);
        input.setInputType(InputType.TYPE_CLASS_TEXT);
        builder.setView(input);
        builder.setPositiveButton("Go", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                changeCity(input.getText().toString());
            }
        });
        builder.show();
    }

    public void changeCity(String city){
        FragmentManager fm = getSupportFragmentManager();
        WeatherFragment wf = (WeatherFragment)fm.findFragmentById(R.id.fragment_weather); 

        //EXCEPTION IS THROWN HERE

        wf.changeCity(city);
        new CityPreference(this).setCity(city);
    }

    public void changeBackground(String timeOfDay) {
        String day = "DAY";
        String dusk = "DUSK";
        String night = "NIGHT";
        RelativeLayout layout = (RelativeLayout) findViewById(R.id.container);
        if (timeOfDay.equals(day)) {
            layout.setBackgroundColor(getResources().getColor(R.color.background_day));
            //setTheme(R.style.CustomAppTheme_NoActionBarTitle_Day);
        } else if(timeOfDay.equals(dusk)){
            layout.setBackgroundColor(getResources().getColor(R.color.background_dusk));
            //setTheme(R.style.CustomAppTheme_NoActionBarTitle_Dusk);
        } else if(timeOfDay.equals(night)){
            layout.setBackgroundColor(getResources().getColor(R.color.background_night));
            //setTheme(R.style.CustomAppTheme_NoActionBarTitle_Night);
        }
    }
}

我的PagerAdapter类，其中片段被实例化：

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;

/**
 * Created by User on 31/08/2015.
 */
public class PagerAdapter extends FragmentStatePagerAdapter {
    int mNumOfTabs;

    public PagerAdapter(FragmentManager fm, int NumOfTabs){
        super(fm);
        this.mNumOfTabs = NumOfTabs;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                WeatherFragment tab1 = new WeatherFragment();
                return tab1;
            case 1:
                ForecastFragment tab2 = new ForecastFragment();
                return tab2;
            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        return mNumOfTabs;
    }
}

WeatherFragment类：

import android.graphics.Typeface;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageButton;
import android.widget.TextView;
import android.widget.Toast;
import android.os.Handler;

import org.json.JSONObject;

import java.text.DateFormat;
import java.util.Date;
import java.util.Locale;


/**
 * Weather app fragment, handles the display of all elements within the FrameLayout of the main
 * activity.
 */
public class WeatherFragment extends Fragment {

    public static final String TAG = "SimpleWeather Fragment";

    Typeface weatherFont;

    TextView cityField;
    TextView updatedField;
    TextView localTimeField;
    TextView detailsField;
    TextView currentTemperatureField;
    TextView weatherIcon;

    ImageButton refreshButton;

    Handler handler;

    public WeatherFragment(){
        handler = new Handler();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_weather, container, false);
        cityField = (TextView)rootView.findViewById(R.id.city_field);
        updatedField = (TextView)rootView.findViewById(R.id.updated_field);
        localTimeField = (TextView)rootView.findViewById(R.id.local_time_field);
        detailsField = (TextView)rootView.findViewById(R.id.details_field);
        currentTemperatureField = (TextView)rootView.findViewById(R.id.current_temperature_field);
        weatherIcon = (TextView)rootView.findViewById(R.id.weather_icon);

        refreshButton = (ImageButton)rootView.findViewById(R.id.refresh_button);
        refreshButton.setOnClickListener(new View.OnClickListener(){
            public void onClick(View v) {
                updateWeatherData(new CityPreference(getActivity()).getCity());
            }
        });

        weatherIcon.setTypeface(weatherFont);
        return rootView;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        weatherFont = Typeface.createFromAsset(getActivity().getAssets(), "fonts/weather.ttf");
        updateWeatherData(new CityPreference(getActivity()).getCity());


    }

    private void updateWeatherData(final String city){
        new Thread(){
            public void run(){
                final JSONObject json = RemoteFetch.getJSON(getActivity(), city);
                if (json == null){
                    handler.post(new Runnable(){
                        public void run(){
                            Toast.makeText(getActivity(),
                                    getActivity().getString(R.string.place_not_found),
                                    Toast.LENGTH_LONG).show();
                        }
                    });
                } else {
                    handler.post(new Runnable(){
                        @Override
                        public void run() {
                            renderWeather(json);
                        }
                    });
                }
            }
        }.start();
    }

    private void renderWeather(JSONObject json){
        try {
            cityField.setText(json.getString("name").toUpperCase(Locale.UK) +
                    "," +
                    json.getJSONObject("sys").getString("country"));

            JSONObject details = json.getJSONArray("weather").getJSONObject(0);
            JSONObject main = json.getJSONObject("main");
            detailsField.setText(
                    details.getString("description").toUpperCase(Locale.UK) +
                            "\n" + "Humidity: " + main.getString("humidity") + "%" +
                            "\n" + "Pressure: " + main.getString("pressure") + "hPa");

            currentTemperatureField.setText(
                    String.format("%.2f", main.getDouble("temp")) + " ℃");

            DateFormat df = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM);
            String updatedOn = df.format(new Date(json.getLong("dt") * 1000));
            updatedField.setText("Last update: " + updatedOn);

            /* TODO local time field */

            setWeatherIcon(details.getInt("id"),
                    json.getJSONObject("sys").getLong("sunrise") * 1000,
                    json.getJSONObject("sys").getLong("sunset") * 1000);

            String t = new TimeOfDay().getTimeOfDay(
                    json.getJSONObject("sys").getLong("sunrise") * 1000,
                    json.getJSONObject("sys").getLong("sunset") * 1000);

            ((WeatherActivity)getActivity()).changeBackground(t);

        }catch (Exception e) {
            Log.e("SimpleWeather", "One or more fields not found in JSON data");
        }
    }

    private void setWeatherIcon(int id, long sunrise, long sunset){
        int shortId = id / 100;
        long currentTime = new Date().getTime();
        String icon = "";

        if(id==800 || id==801 || id==802 || id==803 || shortId==3){
            if (currentTime>=sunrise && currentTime<sunset) {
                if(shortId==3){
                    icon = getActivity().getString(R.string.weather_day_showers);
                } else {
                    switch (id) {
                        case 800 : icon = getActivity().getString(R.string.weather_sunny);
                            break;
                        case 801 : icon = getActivity().getString(R.string.weather_day_few_clouds);
                            break;
                        case 802: icon = getActivity().getString(R.string.weather_day_overcast);
                            break;
                        case 803: icon = getActivity().getString(R.string.weather_day_overcast);
                            break;
                    }
                }

            } else {
                if (shortId==3){
                    icon = getActivity().getString(R.string.weather_night_showers);
                } else {
                    switch (id) {
                        case 800: icon = getActivity().getString(R.string.weather_clear_night);
                            break;
                        case 801: icon = getActivity().getString(R.string.weather_night_few_clouds);
                            break;
                        case 802: icon = getActivity().getString(R.string.weather_night_overcast);
                            break;
                        case 803: icon = getActivity().getString(R.string.weather_night_overcast);
                            break;
                    }
                }
            }
        } else {
            switch (shortId) {
                case 2 : icon = getActivity().getString(R.string.weather_thunder);
                    break;
                case 7 : icon = getActivity().getString(R.string.weather_foggy);
                    break;
                case 8 : icon = getActivity().getString(R.string.weather_cloudy);
                    break;
                case 6 : icon = getActivity().getString(R.string.weather_snowy);
                    break;
                case 5 : icon = getActivity().getString(R.string.weather_rainy);
                    break;
            }
        }weatherIcon.setText(icon);
    }

    public void changeCity(String city){
        updateWeatherData(city);
    }


}

---

片段由ViewPager自动标记

private static String makeFragmentName(int viewPagerId, int index) {
 return "android:switcher:" + viewPagerId + ":" + index;
}

---

请参见

我添加了这段代码，片段名称看起来都是相同的，仍然得到一个空指针异常。从日志中，名称都是

android:switcher:2131492966:0

，

android:switcher:2131492966:1

。我将索引作为开关箱编号？？我认为我可能错误地获得了索引编号；有没有一个明显的方法来告诉你应该是什么？对于你在ViewPager中的第一个片段，应该是：“android:switcher:R.id.pager:0”是的，我就是这么做的。我不明白为什么它还在扔exceptionOk弄明白了。我的PagerAdapter类扩展了FragmentStatePagerAdapter，改为扩展FragmentPagerAdapter，您的解决方案成功了。谢谢

private static String makeFragmentName(int viewPagerId, int index) {
 return "android:switcher:" + viewPagerId + ":" + index;
}