java - 空指针异常变量

Question

我在MainActivity中声明了两个成员变量，并在获得方法内的位置后立即初始化它们。

问题是，如果我使用静态方法或创建对象从另一个 activity 调用它们，则会出现 NullPointerException。

如何解决这个问题？

public class MainActivity extends ActionBarActivity implements GoogleApiClient.ConnectionCallbacks,
        GoogleApiClient.OnConnectionFailedListener, LocationListener {

public static final String TAG = MainActivity.class.getSimpleName();
public static final String DAILY_FORECAST = "DAILY_FORECAST";
public static final String HOURLY_FORECAST = "HOURLY_FORECAST";

private LocationRequest mLocationRequest;
private Forecast mForecast;
private GoogleApiClient mGoogleApiClient;
private Location mLastLocation;
private double mLatitude;
private double mLongitude;
public static String mCity; // declared here
private static String mAdminArea; // declared here

@Bind(R.id.locationLabel) TextView mLocationLabel;
@Bind(R.id.timeLabel) TextView mTimeLabel;
@Bind(R.id.temperatureLabel) TextView mTemperatureLabel;
@Bind(R.id.humidityValue) TextView mHumidityValue;
@Bind(R.id.precipValue) TextView mPrecipValue;
@Bind(R.id.summaryLabel) TextView mSummaryLabel;
@Bind(R.id.iconImageView) ImageView mIconImageView;
@Bind(R.id.refreshImageView) ImageView mRefreshImageView;
@Bind(R.id.progressBar) ProgressBar mProgressBar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ButterKnife.bind(this);

    mProgressBar.setVisibility(View.INVISIBLE);

    toggleRefresh();

    buildGoogleApiClient();
    mGoogleApiClient.connect();

    mRefreshImageView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getForecast(mLatitude, mLongitude);
        }
    });
}

@Override
 protected void onPause(){
    super.onPause();
    if (mGoogleApiClient != null) {
        LocationServices.FusedLocationApi.removeLocationUpdates(mGoogleApiClient, this);
    }
}

private void getForecast(double latitude, double longitude) {
    String apiKey = "48fb6c0ca3567d0b17bf99b400ef5606";
    String forecastUrl = "https://api.forecast.io/forecast/" + apiKey +
            "/" + latitude + "," + longitude;
    if (isNetworkAvailable()) {
        toggleRefresh();

        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder()
                .url(forecastUrl)
                .build();

        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Request request, IOException e) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        toggleRefresh();
                    }
                });
                alertUserAboutError();
            }

            @Override
            public void onResponse(Response response) throws IOException {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        toggleRefresh();
                    }
                });

                try {
                    String jsonData = response.body().string();
                    Log.v(TAG, jsonData);
                    if (response.isSuccessful()) {
                        mForecast = parseForecastDetails(jsonData);
                        runOnUiThread(new Runnable() {
                            @Override
                            public void run() {
                                updateDisplay();
                            }
                        });
                    } else {
                        alertUserAboutError();
                    }
                } catch (IOException e) {
                    Log.e(TAG, "Exception caught: ", e);
                } catch (JSONException e) {
                    Log.e(TAG, "Exception caught: ", e);
                }
            }
        });
    }
    else {
        Toast.makeText(this, getString(R.string.network_unavailable_message),
                Toast.LENGTH_LONG).show();
    }
}

private void toggleRefresh() {
    if (mProgressBar.getVisibility() == View.INVISIBLE) {
        mProgressBar.setVisibility(View.VISIBLE);
        mRefreshImageView.setVisibility(View.INVISIBLE);
    }
    else {
        mProgressBar.setVisibility(View.INVISIBLE);
        mRefreshImageView.setVisibility(View.VISIBLE);
    }
}

private void updateDisplay() {
    Current current = mForecast.getCurrent();

    mTemperatureLabel.setText(current.getTemperature() + "");
    mTimeLabel.setText("At " + current.getFormattedTime() + " it will be");
    mHumidityValue.setText(current.getHumidity() + "");
    mPrecipValue.setText(current.getPrecipChance() + "%");
    mSummaryLabel.setText(current.getSummary());

    Drawable drawable = getResources().getDrawable(current.getIconId());
    mIconImageView.setImageDrawable(drawable);
}

private Forecast parseForecastDetails(String jsonData) throws JSONException {
    Forecast forecast = new Forecast();

    forecast.setCurrent(getCurrentDetails(jsonData));
    forecast.setHourlyForecast(getHourlyForecast(jsonData));
    forecast.setDailyForecast(getDailyForecast(jsonData));

    return forecast;
}

private Day[] getDailyForecast(String jsonData) throws JSONException {JSONObject forecast = new JSONObject(jsonData);
    String timezone = forecast.getString("timezone");
    JSONObject daily = forecast.getJSONObject("daily");
    JSONArray data = daily.getJSONArray("data");

    Day[] days = new Day[data.length()];
    for (int i = 0; i < data.length(); i++) {
        JSONObject jsonDay = data.getJSONObject(i);
        Day day = new Day();

        day.setSummary(jsonDay.getString("summary"));
        day.setIcon(jsonDay.getString("icon"));
        day.setTime(jsonDay.getLong("time"));
        day.setTemperatureMax(jsonDay.getDouble("temperatureMax"));
        day.setTimezone(timezone);

        days[i] = day;
    }

    return days;
}

private Hour[] getHourlyForecast(String jsonData) throws JSONException {
    JSONObject forecast = new JSONObject(jsonData);
    String timezone = forecast.getString("timezone");
    JSONObject hourly = forecast.getJSONObject("hourly");
    JSONArray data = hourly.getJSONArray("data");

    Hour[] hours = new Hour[data.length()];
    for (int i = 0; i < data.length(); i++) {
        JSONObject jsonHour = data.getJSONObject(i);
        Hour hour = new Hour();

        hour.setSummary(jsonHour.getString("summary"));
        hour.setTemperature(jsonHour.getDouble("temperature"));
        hour.setIcon(jsonHour.getString("icon"));
        hour.setTime(jsonHour.getLong("time"));
        hour.setTimezone(timezone);

        hours[i] = hour;
    }

    return hours;
}

private Current getCurrentDetails(String jsonData) throws JSONException {
    JSONObject forecast = new JSONObject(jsonData);
    String timezone = forecast.getString("timezone");

    Log.i(TAG, "From JSON: " + timezone);

    JSONObject currently = forecast.getJSONObject("currently");

    Current current = new Current();
    current.setHumidity(currently.getDouble("humidity"));
    current.setTime(currently.getInt("time"));
    current.setSummary(currently.getString("summary"));
    current.setTemperature(currently.getInt("temperature"));
    current.setIcon(currently.getString("icon"));
    current.setPrecipChance(currently.getDouble("precipProbability"));
    current.setTimeZone(timezone);

    Log.d(TAG, current.getFormattedTime());

    return current;
}

private boolean isNetworkAvailable() {
    ConnectivityManager manager = (ConnectivityManager)
            getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = manager.getActiveNetworkInfo();
    boolean isAvailable = false;
    if (networkInfo != null && networkInfo.isConnected()) {
        isAvailable = true;
    }
    return isAvailable;
}

private void alertUserAboutError() {
    AlertDialogFragment dialog = new AlertDialogFragment();
    dialog.show(getFragmentManager(), "error_dialog");
}

@OnClick (R.id.dailyButton)
public void startDailyActivity(View view) {
    Intent intent = new Intent(this, DailyForecastActivity.class);
    intent.putExtra(DAILY_FORECAST, mForecast.getDailyForecast());
    startActivity(intent);
}

@OnClick(R.id.hourlyButton)
public void startHourlyActivity(View view) {
    Intent intent = new Intent(this, HourlyForecastActivity.class);
    intent.putExtra(HOURLY_FORECAST, mForecast.getHourlyForecast());
    startActivity(intent);
}

protected synchronized void buildGoogleApiClient() {
    mGoogleApiClient = new GoogleApiClient.Builder(this)
            .addConnectionCallbacks(this)
            .addOnConnectionFailedListener(this)
            .addApi(LocationServices.API)
            .build();
}

@Override
public void onConnected(Bundle bundle) {
    mLocationRequest = new LocationRequest();
    mLocationRequest.setInterval(1000);
    mLocationRequest.setFastestInterval(1000);
    mLocationRequest.setPriority(LocationRequest.PRIORITY_BALANCED_POWER_ACCURACY);

    LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, this);
}

@Override
public void onConnectionSuspended(int i) {
    Toast.makeText(this,getString(R.string.connection_suspended),Toast.LENGTH_LONG).show();
}

@Override
public void onConnectionFailed(ConnectionResult connectionResult) {
    Toast.makeText(this,getString(R.string.connection_failed),Toast.LENGTH_SHORT).show();

    toggleRefresh();

    mLatitude = 37.8267;
    mLongitude = -122.423;

    getForecast(mLatitude, mLongitude);
}

@Override
public void onLocationChanged(Location location) {
    mLastLocation = location;

    mLatitude = location.getLatitude();
    mLongitude = location.getLongitude();

    getLocationName(mLatitude, mLongitude);

    toggleRefresh();
    getForecast(mLatitude, mLongitude);

    //remove location updates if you just need one location:
    if (mGoogleApiClient != null) {
        LocationServices.FusedLocationApi.removeLocationUpdates(mGoogleApiClient, this);
    }
}

private void getLocationName(Double latitude, Double longitude) {
    Geocoder geocoder = new Geocoder(this, Locale.getDefault());
    try {
        List<Address> addresses = geocoder.getFromLocation(latitude,
                longitude, 1);

        if (addresses != null) {
            Address address = addresses.get(0);
            mCity = address.getLocality(); // initialised here
            mAdminArea = address.getAdminArea(); // initialised here


            if (mAdminArea != null) {
                mLocationLabel.setText(mCity.toString() + ", " + mAdminArea);
            }
            else {
                mLocationLabel.setText(mCity.toString());
            }
        }
        else {
            mLocationLabel.setText("No Locality Founded!");
        }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
        mLocationLabel.setText("Cannot get Locality!");
    }
}

public static String getCity() {
    return mCity;
}

public static String getAdminArea() {
    return mAdminArea;
}
}

Answer 1

“问题是，如果我使用静态方法或创建对象从另一个 Activity 调用它们，则会出现 NullPointerException。”

这是一个非常非常错误的决定。如果您在从 MainActivity 开始的 Activity 中需要此变量，请使用 intent.putExtra。在其他情况下，更好的解决方案将使用应用程序类