应用程序不起作用。它适用于Andorid 5.1，但不适用于android 10.0

Question

我刚刚在听讲座：(这是错误消息

E/AndroidRuntime:致命异常：main进程：com.example.wweather3，PID:8893java.lang.NullPointerException：尝试在null对象引用上调用虚拟方法"boolean java.lang.String.contains(java.lang.CharSequence("网址：com.example.wweather3.MainActivity$GetWeather.onPostExecute(MainActivity.java:166(网址：com.example.wweather3.MainActivity$GetWeather.onPostExecute(MainActivity.java:142(在android.os.AsyncTask.fining上(AsyncTask.java：755(在android.os.AsyncTask.access$900(AsyncTask.java：192(在android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:772(在android.os.Handler.dispatchMessage(Handler.java:107(在android.os.Looper.loop(Looper.java:214(在android.app.ActivityThread.main(ActivityThreads.java：7356(位于java.lang.reflect.Method.ioke(本机方法(网址：com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492(网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:930(

这就是我在上面写的发生错误的部分。

@Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            if(s.contains("Error: Not found city")){
                pd.dismiss();
                return;
            }
            Gson gson = new Gson();
            Type mType = new TypeToken<OpenWeatherMap>(){}.getType();
            openWeatherMap = gson.fromJson(s, mType);
            pd.dismiss();
            txtCity.setText(String.format("%s,%s",openWeatherMap.getName(),openWeatherMap.getSys().getCountry()));
            txtLastUpdate.setText(String.format("Last Updated: %s", Common.getDateNow()));
            txtDescription.setText(String.format("%s",openWeatherMap.getWeather().get(0).getDescription()));
            txtHumidity.setText(String.format("%d%%",openWeatherMap.getMain().getHumidity()));
            txtTime.setText(String.format("%s/%s",Common.unixTimeStampToDateTime(openWeatherMap.getSys().getSunrise()), Common.unixTimeStampToDateTime(openWeatherMap.getSys().getSunset())));
            txtCelsius.setText(String.format("%.2f °C",openWeatherMap.getMain().getTemp()));
            Picasso.with(MainActivity.this)
                    .load(Common.getImage(openWeatherMap.getWeather().get(0).getIcon()))
                    .into(imageView);

        }

如果你需要的帮助，这就是整个代码

package com.example.weather3;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import android.Manifest;
import android.app.ProgressDialog;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ImageView;
import android.widget.TextView;
import com.example.weather3.Common.Common;
import com.example.weather3.Helper.Helper;
import com.example.weather3.Model.OpenWeatherMap;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.squareup.picasso.Picasso;
import java.lang.reflect.Type;
public class MainActivity extends AppCompatActivity implements LocationListener {
    TextView txtCity, txtLastUpdate, txtDescription, txtHumidity, txtTime, txtCelsius;
    ImageView imageView;
    LocationManager locationManager;
    String provider;
    static double lat, lng;
    OpenWeatherMap openWeatherMap = new OpenWeatherMap();
    int MY_PERMISSION = 0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Control
        txtCity = (TextView) findViewById(R.id.txtCity);
        txtLastUpdate = (TextView) findViewById(R.id.txtLastUpdate);
        txtDescription = (TextView) findViewById(R.id.txtDescription);
        txtHumidity = (TextView) findViewById(R.id.txtHumidity);
        txtTime = (TextView) findViewById(R.id.txtTime);
        txtCelsius = (TextView) findViewById(R.id.txtCelsius);
        imageView = (ImageView) findViewById(R.id.imageView);

        //Get Coordinates
        locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
        provider = locationManager.getBestProvider(new Criteria(), false);
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{
                    Manifest.permission.INTERNET,
                    Manifest.permission.ACCESS_COARSE_LOCATION,
                    Manifest.permission.ACCESS_FINE_LOCATION,
                    Manifest.permission.ACCESS_NETWORK_STATE,
                    Manifest.permission.SYSTEM_ALERT_WINDOW,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE
            }, MY_PERMISSION);
        }
        Location location = locationManager.getLastKnownLocation(provider);
        if (location == null) {
            Log.e("TAG", "No Location");
        }
    }

    protected void onPause() {
        super.onPause();
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{
                    Manifest.permission.INTERNET,
                    Manifest.permission.ACCESS_COARSE_LOCATION,
                    Manifest.permission.ACCESS_FINE_LOCATION,
                    Manifest.permission.ACCESS_NETWORK_STATE,
                    Manifest.permission.SYSTEM_ALERT_WINDOW,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE
            }, MY_PERMISSION);
        }
        locationManager.removeUpdates(this);
    }
    protected void onResume() {
        super.onResume();
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(MainActivity.this, new String[]{
                    Manifest.permission.INTERNET,
                    Manifest.permission.ACCESS_COARSE_LOCATION,
                    Manifest.permission.ACCESS_FINE_LOCATION,
                    Manifest.permission.ACCESS_NETWORK_STATE,
                    Manifest.permission.SYSTEM_ALERT_WINDOW,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE
            }, MY_PERMISSION);
        }
        locationManager.requestLocationUpdates(provider, 400, 1, this);
    }

    @Override
    public void onLocationChanged(Location location) {
        lat=location.getLatitude();
        lng=location.getLongitude();
        new GetWeather().execute(Common.apiRequest(String.valueOf(lat), String.valueOf(lng)));
    }
    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
    }
    @Override
    public void onProviderEnabled(String provider) {
    }
    @Override
    public void onProviderDisabled(String provider) {
    }
    private class GetWeather extends AsyncTask<String, Void, String> {
        ProgressDialog pd = new ProgressDialog(MainActivity.this);

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pd.setTitle("Please wait...");
            pd.show();
        }
        @Override
        protected String doInBackground(String... params) {
            String stream = null;
            String urlString = params[0];
            Helper http = new Helper();
            stream = http.getHTTPData(urlString);
            return stream;
        }
        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            if(s.contains("Error: Not found city")){
                pd.dismiss();
                return;
            }
            Gson gson = new Gson();
            Type mType = new TypeToken<OpenWeatherMap>(){}.getType();
            openWeatherMap = gson.fromJson(s, mType);
            pd.dismiss();
            txtCity.setText(String.format("%s,%s",openWeatherMap.getName(),openWeatherMap.getSys().getCountry()));
            txtLastUpdate.setText(String.format("Last Updated: %s", Common.getDateNow()));
            txtDescription.setText(String.format("%s",openWeatherMap.getWeather().get(0).getDescription()));
            txtHumidity.setText(String.format("%d%%",openWeatherMap.getMain().getHumidity()));
            txtTime.setText(String.format("%s/%s",Common.unixTimeStampToDateTime(openWeatherMap.getSys().getSunrise()), Common.unixTimeStampToDateTime(openWeatherMap.getSys().getSunset())));
            txtCelsius.setText(String.format("%.2f °C",openWeatherMap.getMain().getTemp()));
            Picasso.with(MainActivity.this)
                    .load(Common.getImage(openWeatherMap.getWeather().get(0).getIcon()))
                    .into(imageView);

        }
    }
}

Answer 1

错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.contains(java.lang.CharSequence)' on a null object reference at

s是null，此处：

 if(s.contains("Error: Not found city")){

s变量从这里的doInBackground方法传递给onPostExecute：

    @Override
    protected String doInBackground(String... params) {
        String stream = null;
        String urlString = params[0];
        Helper http = new Helper();
        stream = http.getHTTPData(urlString);
        return stream;
    }

这意味着stream为空。

意味着stream = http.getHTTPData(urlString);在某种程度上不起作用/返回null。