I am new to android, working on a project in which I need to get objects from a JSON server and display them in a layout, in the previous activation I authorize and get a token for authorization, pass it using Intent

here is my code:

package com.example.sanzharaubakir.fin; import android.app.Activity; import android.content.Intent; import android.os.AsyncTask; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.LinearLayout; import android.widget.TextView; import android.widget.Toast; import org.json.JSONArray; import org.json.JSONException; import java.io.IOException; import cz.msebera.android.httpclient.HttpEntity; import cz.msebera.android.httpclient.HttpResponse; import cz.msebera.android.httpclient.client.HttpClient; import cz.msebera.android.httpclient.client.methods.HttpGet; import cz.msebera.android.httpclient.impl.client.DefaultHttpClient; import cz.msebera.android.httpclient.util.EntityUtils; /** * Created by sanzharaubakir on 28.07.16. */ public class show extends Activity { LinearLayout l; As a; String[] s; String token; int n = 0; protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.show_res); LinearLayout linLayout = new LinearLayout(this); linLayout.setOrientation(LinearLayout.VERTICAL); LinearLayout.LayoutParams linLayoutParam = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT); setContentView(linLayout, linLayoutParam); s = new String[1000]; Intent intent = getIntent(); token = intent.getStringExtra("token"); a.execute(); for (int i = 0 ; i < n; i++) { TextView v = new TextView(this); v.setText(s[i]); linLayout.addView(v, linLayoutParam); final int finalI = i; } Button b = new Button(this); b.setText("Сканировать"); linLayout.addView(b, linLayoutParam); b.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Intent intent = new Intent(getApplicationContext(), BarcodeScanner.class); intent.putExtra("token",token); startActivity(intent); } }); } public class As extends AsyncTask<Void, Void, Void> { @Override protected void onPreExecute() { super.onPreExecute(); Toast.makeText(getApplicationContext(), "1", Toast.LENGTH_LONG).show(); } protected Void doInBackground(Void... voids) { func(); return null; } } public void func() { HttpClient httpclient = new DefaultHttpClient(); HttpGet get = new HttpGet("http://144.76.29.144:8001/warehouse/api/list/?access-token=<" + token + ">/"); try { HttpResponse response = httpclient.execute(get); HttpEntity httpEntity = response.getEntity(); String line = EntityUtils.toString(httpEntity, "UTF-8"); JSONArray arr = new JSONArray(line); n = arr.length(); for(int i = 0; i < arr.length(); i++){ //s[i] = arr.getJSONObject(i).getString("items"); s[i] = arr.getJSONObject(i).toString(); } } catch (IOException e) { e.printStackTrace(); } catch (JSONException e) { e.printStackTrace(); } } }

and XML file:

 <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" android:background="@color/windowBackground"> </LinearLayout>

when calling a.execute(); the application crashes

 FATAL EXCEPTION: main Process: com.example.sanzharaubakir.fin, PID: 17677 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sanzharaubakir.fin/com.example.sanzharaubakir.fin.show}: java.lang.NullPointerException at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) at android.app.ActivityThread.access$800(ActivityThread.java:135) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:136) at android.app.ActivityThread.main(ActivityThread.java:5017) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.NullPointerException at com.example.sanzharaubakir.fin.show.onCreate(show.java:35) at android.app.Activity.performCreate(Activity.java:5231) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) at android.app.ActivityThread.access$800(ActivityThread.java:135) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:136) at android.app.ActivityThread.main(ActivityThread.java:5017) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) at dalvik.system.NativeStart.main(Native Method)
  • one
    you are null. a = new As(); forgot - Yura Ivanov
  • besides, in the doInBackground method, the result is returned return null that is, all your work in a separate thread is not returned anywhere, whatever is done there will always be null. - Silento

1 answer 1

There are several reasons for returning null in your code.

First: a you have null. a = new As (); forgot - Yura Ivanov

Secondly, if void - it means you don’t need to return null anything from the doInBackground() method, you must either return null either very carefully or not at all.

I also advise you to inherit from AppCompatActivity - this is the same Activity , only with support for older versions.