Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference [duplicate]

Question

MainActivity.java has following code:

package com.softjourn.redmineclient.activities; import android.os.AsyncTask; import android.os.Bundle; import android.support.v7.app.AppCompatActivity; import android.util.Base64; import android.view.Menu; import android.view.MenuItem; import android.widget.ArrayAdapter; import android.widget.ListView; import com.google.gson.JsonArray; import com.google.gson.JsonParser; import com.softjourn.redmineclient.R; import com.softjourn.redmineclient.models.Issue; import com.softjourn.redmineclient.models.IssuesResponse; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.net.HttpURLConnection; import java.net.ProtocolException; import java.net.URL; import butterknife.Bind; import butterknife.ButterKnife;  public class MainActivity extends AppCompatActivity {  private final static String URI="https://redmineng.softjourn.if.ua/issues.json?assigned_to_id=me"; @Bind(R.id.list_issues) ListView mListIssues; Login li;  @Override protected void onCreate(Bundle savedInstanceState) {     super.onCreate(savedInstanceState);     setContentView(R.layout.activity_main);     ButterKnife.bind(this);      li = new Login();     li.execute(URI); }  @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_main, menu);     return true; }  @Override public boolean onOptionsItemSelected(MenuItem item) {     // Handle action bar item clicks here. The action bar will     // automatically handle clicks on the Home/Up button, so long     // as you specify a parent activity in AndroidManifest.xml.     int id = item.getItemId();      //noinspection SimplifiableIfStatement     if (id == R.id.action_settings) {         return true;     }      return super.onOptionsItemSelected(item); }  class Login extends AsyncTask<String, Void, String> {      @Override     protected String doInBackground(String... strings) {         HttpURLConnection c = null;         try {             c = (HttpURLConnection) new URL(strings[0]).openConnection();         } catch (IOException e) {             e.printStackTrace();         }         c.setUseCaches(false);         try {             c.setRequestMethod("GET");         } catch (ProtocolException e) {             e.printStackTrace();         }         c.setRequestProperty("Accept", "application/json");         c.setRequestProperty("Authorization", "basic " + Base64.encodeToString("osavchak:mhgT7322".getBytes(), 0));         try {             c.connect();         } catch (IOException e) {             e.printStackTrace();         }         BufferedReader br = null;         try {             br = new BufferedReader(new InputStreamReader((c.getInputStream())));         } catch (IOException e) {             e.printStackTrace();         }         StringBuilder sb = new StringBuilder();         String output;         try {             while ((output = br.readLine()) != null) {                 sb.append(output);             }         } catch (IOException e) {             e.printStackTrace();         }          return sb.toString();     }      @Override     protected void onPostExecute(String result) {         super.onPostExecute(result);         JsonArray ja = new JsonParser().parse(result).getAsJsonObject().getAsJsonArray("issues");         IssuesResponse ir = new IssuesResponse(ja);          ArrayAdapter<Issue> adapter = new ArrayAdapter<Issue>(MainActivity.this, android.R.layout.simple_list_item_1, ir.getIssues());         mListIssues.setAdapter(adapter);     } } } 

I've created two model classes.

First, IssuesResponse.java:

package com.softjourn   .redmineclient.models;  import com.google.gson.JsonArray; import com.google.gson.JsonElement; import com.google.gson.annotations.SerializedName; import java.util.List;  public class IssuesResponse { @SerializedName("issues") private List<Issue> issues;  public IssuesResponse(JsonArray ja) {     if (issues == null) {      }     for(JsonElement je : ja) {         Issue issue = new Issue(je);         issues.add(issue);     } }  public List<Issue> getIssues() {     return issues; } } 

The second one, Issue.java:

package com.softjourn.redmineclient.models;  import com.google.gson.Gson; import com.google.gson.JsonElement; import com.google.gson.annotations.SerializedName;  public class Issue { @SerializedName("id") private int id;  @SerializedName("description") private String description;  public Issue(JsonElement je) {     Issue issue = new Gson().fromJson(je,Issue.class);     this.id = issue.id;     this.description = issue.description; } } 

When I run my application an error occurs:

java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference at com.softjourn.redmineclient.models.IssuesResponse.(IssuesResponse.java:26) at com.softjourn.redmineclient.activities.MainActivity$Login.onPostExecute(MainActivity.java:125) at com.softjourn.redmineclient.activities.MainActivity$Login.onPostExecute(MainActivity.java:79) at android.os.AsyncTask.finish(AsyncTask.java:636) at android.os.AsyncTask.access$500(AsyncTask.java:177) at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5254) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

I wait for your suggestions how it could be fixed.

Answer 1

You haven't initialised List<Issue> in the IssuesResponse class. Try

private List<Issue> issues = new ArrayList<>();