Null-Zeiger-Ausnahme für Paket-Manager Initialisierung

Question

i versucht haben, auf Klick auf eine Schaltfläche, um die Liste der installierten Anwendungen angezeigt werden, aber der Paket-Manager eine Null-Zeiger-Ausnahme

das ist mein Java-Klasse wirft, die auf Klick aufgerufen wird

import android.app.ListActivity; 
import android.content.Context; 
import android.content.pm.ApplicationInfo; 
import android.content.pm.PackageManager; 
import android.graphics.drawable.Drawable; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.BaseAdapter; 
import android.widget.ImageView; 
import android.widget.ListView; 
import android.widget.TextView; 

import java.util.ArrayList; 
import java.util.List; 


public class AppList extends AppCompatActivity { 

PackageManager packman = getPackageManager(); 
List<ApplicationInfo> apps = packman.getInstalledApplications(0); 
ListView applist; 
List<ApplicationInfo> installedApps = new ArrayList<ApplicationInfo>(); 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_app_list); 

    applist = (ListView)findViewById(R.id.listView); 

    for(ApplicationInfo app : apps) { 
     //checks for flags; if flagged, check if updated system app 
     if((app.flags & ApplicationInfo.FLAG_UPDATED_SYSTEM_APP) != 0) { 
      installedApps.add(app); 
      //it's a system app, not interested 
     } else if ((app.flags & ApplicationInfo.FLAG_SYSTEM) != 0) { 
      //Discard this one 
      //in this case, it should be a user-installed app 
     } else { 
      installedApps.add(app); 
     } 
    } 


    CustomAdapter custom = new CustomAdapter(); 

    applist.setAdapter(custom); 


} 

class CustomAdapter extends BaseAdapter 
{ 

    @Override 
    public int getCount() { 
     return installedApps.size(); 
    } 

    @Override 
    public Object getItem(int position) { 
     return null; 
    } 

    @Override 
    public long getItemId(int position) { 
     return 0; 
    } 

    @Override 
    public View getView(int position, View convertView, ViewGroup parent) { 

     convertView = getLayoutInflater().inflate(R.layout.single_row,null); 
     ImageView app_face = (ImageView)convertView.findViewById(R.id.app_face); 
     TextView app_name = (TextView)convertView.findViewById(R.id.app_name); 

     Drawable face = packman.getApplicationIcon(apps.get(position)); 
     String label = (String)packman.getApplicationLabel(apps.get(position)); 

     app_face.setImageDrawable(face); 
     app_name.setText(label); 

     return convertView; 
    } 
} 
} 

und dies der Fehler habe ich während der App

enter code here 

FATAL EXCEPTION: main 

Process: com.example.limat.applocker, PID: 17438 

java.lang.RuntimeException: Unable to instantiate activity 

Unable to instantiate activity ComponentInfo{com.example.limat.applocker/com.example.limat.applocker.AppList}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference 


at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567) 

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 

at android.app.ActivityThread.-wrap12(ActivityThread.java) 

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 

at android.os.Handler.dispatchMessage(Handler.java:102) 

at android.os.Looper.loop(Looper.java:154) 

at android.app.ActivityThread.main(ActivityThread.java:6119) 

at java.lang.reflect.Method.invoke(Native Method) 

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference 

at com.example.limat.applocker.AppList.<init>(AppList.java:23) 

at java.lang.Class.newInstance(Native Method) 

at android.app.Instrumentation.newActivity(Instrumentation.java:1078) 

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557) 

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)  

at android.app.ActivityThread.-wrap12(ActivityThread.java)  

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)  

at android.os.Handler.dispatchMessage(Handler.java:102)  

at android.os.Looper.loop(Looper.java:154)  

at android.app.ActivityThread.main(ActivityThread.java:6119)  

at java.lang.reflect.Method.invoke(Native Method)  

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)  

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) " 

Answer 1

Setzen Sie die folgenden Zeilen Code in onCreate Methode

PackageManager packman = getPackageManager(); 
List<ApplicationInfo> apps = packman.getInstalledApplications(0); 
ListView applist; 
List<ApplicationInfo> installedApps = new ArrayList<ApplicationInfo>(); 

Sie versuchen, diese Dinge vor onCreate method.Refer Aktivität Lifecycle zu initialisieren: https://developer.android.com/guide/components/activities/activity-lifecycle.html

Answer 2

Initialisiere den Paket-Manager innerhalb Aktivität Lebenszyklus als conte läuft xt verfügbar there.As ist die getPackageManager wird immer auf den Kontext genannt, der frühere Code würde in Null-Kontext geführt haben

Sie können diese Anweisung in onCreate() -Methode aufrufen

packman = getPackageManager();