NPE beim Hinzufügen von Schubladen-Listener

Question

Ich versuche gerade, eine kleine App zu programmieren, jetzt versuche ich eine Navigationsschublade in meiner App zu verwenden. Dies funktioniert nicht mit dem Grund, dass ich eine NPE bei drawer.addDrawerListener(toggle); bekomme. Alles andere scheint in Ordnung zu sein, soweit ich das sagen kann und die App hat bis zum Hinzufügen der Schublade richtig funktioniert. Danke im Voraus.

import android.content.Intent; 
import android.content.res.Configuration; 
import android.net.ConnectivityManager; 
import android.net.NetworkInfo; 
import android.os.Bundle; 
import android.view.View; 
import android.support.design.widget.NavigationView; 
import android.support.v4.view.GravityCompat; 
import android.support.v4.widget.DrawerLayout; 
import android.support.v7.app.ActionBarDrawerToggle; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.widget.TextView; 
import de.nocompany.gotthold.gw2companion.Tools.GwApiAccess; 

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener { 

    private DrawerLayout drawer; 
    private Toolbar toolbar; 
    private ActionBarDrawerToggle toggle; 
    private NavigationView navigationView; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     toolbar = (Toolbar) findViewById(R.id.toolbar); 
     drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
     navigationView = (NavigationView) findViewById(R.id.nav_view); 
     setSupportActionBar(toolbar); 
     setContentView(R.layout.activity_main); 

     toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close) { 

      /** Called when a drawer has settled in a completely closed state. */ 
      public void onDrawerClosed(View view) { 
       super.onDrawerClosed(view); 
       //getActionBar().setTitle(mTitle); 
       invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu() 
      } 

      /** Called when a drawer has settled in a completely open state. */ 
      public void onDrawerOpened(View drawerView) { 
       super.onDrawerOpened(drawerView); 
       //getActionBar().setTitle(mDrawerTitle); 
       invalidateOptionsMenu(); // creates call to onPrepareOptionsMenu() 
      } 
     }; 

     drawer.addDrawerListener(toggle); 
     navigationView.setNavigationItemSelectedListener(this); 

     ConnectivityManager connMgr = (ConnectivityManager) this.getSystemService(CONNECTIVITY_SERVICE); 
     NetworkInfo networkInfo = connMgr.getActiveNetworkInfo(); 
     View navHeader = navigationView.getHeaderView(0); 
     if (networkInfo != null && networkInfo.isConnected()) { 
      TextView connectionStatus = (TextView) navHeader.findViewById(R.id.connection_status); 
      connectionStatus.setText(R.string.common_online); 
     } else { 
      TextView connectionStatus = (TextView) navHeader.findViewById(R.id.connection_status); 
      connectionStatus.setText(R.string.common_offline); 
     } 

     toggle.syncState(); 

     GwApiAccess gwApiAccess = new GwApiAccess(this,"https://api.guildwars2.com/v2/items/30704?lang=de"); 
    } 

    @Override 
    protected void onPostCreate(Bundle savedInstanceState) { 
     super.onPostCreate(savedInstanceState); 
     // Sync the toggle state after onRestoreInstanceState has occurred. 
     toggle.syncState(); 
    } 

    @Override 
    public void onConfigurationChanged(Configuration newConfig) { 
     super.onConfigurationChanged(newConfig); 
     toggle.onConfigurationChanged(newConfig); 
    } 


    @Override 
    public void onBackPressed() { 
     if (drawer.isDrawerOpen(GravityCompat.START)) { 
      drawer.closeDrawer(GravityCompat.START); 
     } else { 
      super.onBackPressed(); 
     } 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onPrepareOptionsMenu(Menu menu) { 
     // If the nav drawer is open, hide action items related to the content view 
     boolean drawerOpen = drawer.isDrawerOpen(GravityCompat.START); 
     return super.onPrepareOptionsMenu(menu); 
    } 


    @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); 
    } 

    @SuppressWarnings("StatementWithEmptyBody") 
    @Override 
    public boolean onNavigationItemSelected(MenuItem item) { 
     // Handle navigation view item clicks here. 
     int id = item.getItemId(); 

     if (id == R.id.nav_profile) { 
      Intent intent = new Intent(this,profile.class); 
      startActivity(intent); 
     } else if (id == R.id.nav_itemDB) { 
      Intent intent = new Intent(this,items.class); 
      startActivity(intent); 
     } else if (id == R.id.nav_guilds) { 
      Intent intent = new Intent(this,guilds.class); 
      startActivity(intent); 
     } else if (id == R.id.nav_maps) { 
      Intent intent = new Intent(this,map.class); 
      startActivity (intent); 
     } else if (id == R.id.settings) { 
      Intent intent = new Intent(this,SettingsActivity.class); 
      startActivity (intent); 
     } 

     drawer.closeDrawer(GravityCompat.START); 
     return true; 
    } 
} 

Answer 1

Erste Änderung dieser Teil

@Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     toolbar = (Toolbar) findViewById(R.id.toolbar); 
     drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
     navigationView = (NavigationView) findViewById(R.id.nav_view); 
     setSupportActionBar(toolbar); 
     setContentView(R.layout.activity_main); 

dieser

@Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState);  
     setContentView(R.layout.activity_main); 
     toolbar = (Toolbar) findViewById(R.id.toolbar); 
     drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
     navigationView = (NavigationView) findViewById(R.id.nav_view); 
     setSupportActionBar(toolbar); 

Weil Sie nicht das Layout zu Actiivty davor gesetzt haben Sie Zugriff auf das innere Element, so dass das Studio kann‘ t finde es und es gibt dir NullpointerExeception.