应用程序在 onPostExecute 方法中分配某些内容时崩溃

Question

As a beginner, I am following a lecture on JSON Demo (with exactly the same code and same version) of andriod studio. For some reason the app is crashing in the emulator whenever I add something in onPostExecute method. For instance, it doesn't crash with following code:

@Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
    }

But it crashes when I add very basic to see the result in log like below:

@Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        Log.i("Website Content: ", result);

    }

I am not sure what's wrong here. I tried restarting the Android Studio for the sake RAM memory allocation, my internet is on, it debugs nicely in gradle console but gives this in android monitor:

 06-27 19:16:35.201 18116-18157/com.example.rushi.jsondemo V/RenderScript: 0xae53b000 Launching thread(s), CPUs 2 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: java.io.FileNotFoundException: http://api.openweathermap.org/data/2.5/weather?q=London,uk 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:238) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at com.example.rushi.jsondemo.MainActivity$DownloadTask$override.doInBackground(MainActivity.java:44) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at com.example.rushi.jsondemo.MainActivity$DownloadTask$override.access$dispatch(MainActivity.java) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at com.example.rushi.jsondemo.MainActivity$DownloadTask.doInBackground(MainActivity.java:0) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at com.example.rushi.jsondemo.MainActivity$DownloadTask.doInBackground(MainActivity.java:30) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at android.os.AsyncTask$2.call(AsyncTask.java:295) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at java.util.concurrent.FutureTask.run(FutureTask.java:237) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 06-27 19:16:35.396 18116-19529/com.example.rushi.jsondemo W/System.err: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 06-27 19:16:35.411 18116-19529/com.example.rushi.jsondemo W/System.err: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 06-27 19:16:35.411 18116-19529/com.example.rushi.jsondemo W/System.err: at java.lang.Thread.run(Thread.java:818) 06-27 19:16:35.451 18116-18116/com.example.rushi.jsondemo D/AndroidRuntime: Shutting down VM 06-27 19:16:35.529 18116-18116/com.example.rushi.jsondemo E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.rushi.jsondemo, PID: 18116 java.lang.NullPointerException: println needs a message at android.util.Log.println_native(Native Method) at android.util.Log.i(Log.java:160) at com.example.rushi.jsondemo.MainActivity$DownloadTask$override.onPostExecute(MainActivity.java:74) at com.example.rushi.jsondemo.MainActivity$DownloadTask$override.access$dispatch(MainActivity.java) at com.example.rushi.jsondemo.MainActivity$DownloadTask.onPostExecute(MainActivity.java:0) at com.example.rushi.jsondemo.MainActivity$DownloadTask.onPostExecute(MainActivity.java:30) at android.os.AsyncTask.finish(AsyncTask.java:651) at android.os.AsyncTask.-wrap1(AsyncTask.java) at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 06-27 19:19:47.263 18116-18122/com.example.rushi.jsondemo W/art: Suspending all threads took: 7.513ms 06-27 19:21:35.896 18116-18116/com.example.rushi.jsondemo I/Process: Sending signal. PID: 18116 SIG: 9

Here is my complete code:

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
public class MainActivity extends Activity {

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

        DownloadTask task = new DownloadTask();
        task.execute("http://api.openweathermap.org/data/2.5/weather?q=London,uk");

    }

    public class DownloadTask extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... urls) {

            String result = "";
            URL url;
            HttpURLConnection urlConnection = null;

            try {
                url = new URL(urls[0]);

                urlConnection = (HttpURLConnection) url.openConnection();

                InputStream in = urlConnection.getInputStream();

                InputStreamReader reader = new InputStreamReader(in);

                int data = reader.read();

                while (data != -1) {

                    char current = (char) data;

                    result += current;

                    data = reader.read();

                }

                return result;

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

            return null;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            Log.i("Website Content: ", result);

        }

    }

}

Apologies for long post. Thanks.

Answer 1

app crashes because the API returns nothing that because you haven't specified API KEY as document says https://openweathermap.org/faq#error401 , https://openweathermap.org/appid

Answer 2

Your error basically complains about a null or empty string. This your URL returns with a 401 error, which is explaining why you don't get that your string populated as expected. So to avoid crashes or errors like this, before your log try checking the String result for containing something.