Tentativo di richiamare il metodo di interfaccia 'booleana java.util.List.add (java.lang.Object)' su un riferimento oggetto null

Question

MainActivity.java ha seguente codice:

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); 
    } 
} 
} 

ho creato due classi modello.

In primo luogo, 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; 
} 
} 

Il secondo, 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; 
} 
} 

Quando eseguo la mia applicazione di un errore si è verificato:

java.lang.NullPointerException: tentativo di richiamare in metodo "boolean java.util.List.add (java.lang.Object)" su un oggetto nullo di riferimento in com.softjourn.redmineclient.models.IssuesResponse. (IssuesResponse.java:26) in com.softjourn.redmineclient .activities.MainActivity $ Login.onPostExecute (MainActivity.java:125) su com.softjourn.redmineclient.activities.MainActivity $ Login.onPostExecute (MainActivity.java:79) su android.os.AsyncTask.finish (AsyncTask.java : 636) su android.os.AsyncTask.access $ 500 (AsyncTask.java:177) su android.os.AsyncTask $ InternalHandler.handleMessage (AsyncTask.java:653) su android.os.Handler.dispatchMessage (Gestore. java: 102) su android.os.Looper.loop (Looper.java:135) su android.app.ActivityThread.main (ActivityT hread.java:5254) su java.lang.reflect.Method.invoke (metodo nativo) su java.lang.reflect.Method.invoke (Method.java:372) su com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run (ZygoteInit.java:903) a com.android.internal.os.ZygoteInit.main (ZygoteInit.java:698)

aspetto i vostri suggerimenti come potrebbe essere risolto.

Answer 1

Non è stato inizializzato l'elenco nella classe IssuesResponse. Prova

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