Android Fehler [java.lang.NullPointerException: 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled (boolean)' auf eine Null-Objekt-Referenz]

Question

Ich habe meine Haupt, die eine Menüleiste mit einigen Fragmenten enthalten. Dann eine Host-Seite vor, um das Haupt anzuzeigen. Ich habe einige Fehler über diese Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference

Ich versuchte es getSupportActionBar().setDisplayHomeAsUpEnabled(true); zu ändern, aber nach wie vor nichts, auch mit public class MainActivity extends Activity { oder public class MainActivity extends AppcompactActivity {

Es ist mein Haupt:

package thyroid.com.thyroid; 


import thyroid.com.thyroid.adapter.NavDrawerListAdapter; 
import thyroid.com.thyroid.model.NavDrawerItem; 

import java.util.ArrayList; 

import android.support.v7.app.ActionBarActivity; 
import android.app.Activity; 
import android.app.Fragment; 
import android.app.FragmentManager; 
import android.content.res.Configuration; 
import android.content.res.TypedArray; 
import android.net.Uri; 
import android.os.Bundle; 
import android.support.v4.app.ActionBarDrawerToggle; 
import android.support.v4.widget.DrawerLayout; 
import android.support.v7.app.ActionBar; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.AdapterView; 
import android.widget.ListView; 

import com.google.android.gms.appindexing.Action; 
import com.google.android.gms.appindexing.AppIndex; 
import com.google.android.gms.common.api.GoogleApiClient; 


public class MainActivity extends Activity { 
    private DrawerLayout mDrawerLayout; 
    private ListView mDrawerList; 
    private ActionBarDrawerToggle mDrawerToggle; 

    // nav drawer title 
    private CharSequence mDrawerTitle; 

    // used to store app title 
    private CharSequence mTitle; 

    // slide menu items 
    private String[] navMenuTitles; 
    private TypedArray navMenuIcons; 

    private ArrayList<NavDrawerItem> navDrawerItems; 
    private NavDrawerListAdapter adapter; 
    /** 
    * ATTENTION: This was auto-generated to implement the App Indexing API. 
    * See https://g.co/AppIndexing/AndroidStudio for more information. 
    */ 
    private GoogleApiClient client; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     mTitle = mDrawerTitle = getTitle(); 

     // load slide menu items 
     navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items); 

     // nav drawer icons from resources 
     navMenuIcons = getResources() 
       .obtainTypedArray(R.array.nav_drawer_icons); 

     mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout); 
     mDrawerList = (ListView) findViewById(R.id.list_slidermenu); 

     navDrawerItems = new ArrayList<NavDrawerItem>(); 

     // adding nav drawer items to array 
     // Home 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1))); 
     // Find People 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1))); 
     // Photos 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1))); 
     // Communities, Will add a counter here 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1), true, "22")); 
     // Pages 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1))); 
     // What's hot, We will add a counter here 
     navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1), true, "50+")); 


     // Recycle the typed array 
     navMenuIcons.recycle(); 

     mDrawerList.setOnItemClickListener(new SlideMenuClickListener()); 

     // setting the nav drawer list adapter 
     adapter = new NavDrawerListAdapter(getApplicationContext(), 
       navDrawerItems); 
     mDrawerList.setAdapter(adapter); 

     // enabling action bar app icon and behaving it as toggle button 
     getSupportActionBar().setDisplayHomeAsUpEnabled(true); 
     getSupportActionBar().setHomeButtonEnabled(true); 

     mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout, 
       R.drawable.ic_drawer, //nav menu toggle icon 
       R.string.app_name, // nav drawer open - description for accessibility 
       R.string.app_name // nav drawer close - description for accessibility 
     ) { 
      public void onDrawerClosed(View view) { 
       getSupportActionBar().setTitle(mTitle); 
       // calling onPrepareOptionsMenu() to show action bar icons 
       invalidateOptionsMenu(); 
      } 

      public void onDrawerOpened(View drawerView) { 
       getSupportActionBar().setTitle(mDrawerTitle); 
       // calling onPrepareOptionsMenu() to hide action bar icons 
       invalidateOptionsMenu(); 
      } 
     }; 
     mDrawerLayout.setDrawerListener(mDrawerToggle); 

     if (savedInstanceState == null) { 
      // on first time display view for first nav item 
      displayView(0); 
     } 
     // ATTENTION: This was auto-generated to implement the App Indexing API. 
     // See https://g.co/AppIndexing/AndroidStudio for more information. 
     client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build(); 
    } 

    private ActionBar getSupportActionBar() { 
     return null; 
    } 

    @Override 
    public void onStart() { 
     super.onStart(); 

     // ATTENTION: This was auto-generated to implement the App Indexing API. 
     // See https://g.co/AppIndexing/AndroidStudio for more information. 
     client.connect(); 
     Action viewAction = Action.newAction(
       Action.TYPE_VIEW, // TODO: choose an action type. 
       "Main Page", // TODO: Define a title for the content shown. 
       // TODO: If you have web page content that matches this app activity's content, 
       // make sure this auto-generated web page URL is correct. 
       // Otherwise, set the URL to null. 
       Uri.parse("http://host/path"), 
       // TODO: Make sure this auto-generated app URL is correct. 
       Uri.parse("android-app://thyroid.com.thyroid/http/host/path") 
     ); 
     AppIndex.AppIndexApi.start(client, viewAction); 
    } 

    @Override 
    public void onStop() { 
     super.onStop(); 

     // ATTENTION: This was auto-generated to implement the App Indexing API. 
     // See https://g.co/AppIndexing/AndroidStudio for more information. 
     Action viewAction = Action.newAction(
       Action.TYPE_VIEW, // TODO: choose an action type. 
       "Main Page", // TODO: Define a title for the content shown. 
       // TODO: If you have web page content that matches this app activity's content, 
       // make sure this auto-generated web page URL is correct. 
       // Otherwise, set the URL to null. 
       Uri.parse("http://host/path"), 
       // TODO: Make sure this auto-generated app URL is correct. 
       Uri.parse("android-app://thyroid.com.thyroid/http/host/path") 
     ); 
     AppIndex.AppIndexApi.end(client, viewAction); 
     client.disconnect(); 
    } 

    /** 
    * Slide menu item click listener 
    */ 
    private class SlideMenuClickListener implements 
      ListView.OnItemClickListener { 
     @Override 
     public void onItemClick(AdapterView<?> parent, View view, int position, 
           long id) { 
      // display view for selected nav drawer item 
      displayView(position); 
     } 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // toggle nav drawer on selecting action bar app icon/title 
     if (mDrawerToggle.onOptionsItemSelected(item)) { 
      return true; 
     } 
     // Handle action bar actions click 
     switch (item.getItemId()) { 
      case R.id.action_settings: 
       return true; 
      default: 
       return super.onOptionsItemSelected(item); 
     } 
    } 

    /* * 
    * Called when invalidateOptionsMenu() is triggered 
    */ 
    @Override 
    public boolean onPrepareOptionsMenu(Menu menu) { 
     // if nav drawer is opened, hide the action items 
     boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList); 
     menu.findItem(R.id.action_settings).setVisible(!drawerOpen); 
     return super.onPrepareOptionsMenu(menu); 
    } 

    /** 
    * Diplaying fragment view for selected nav drawer list item 
    */ 
    private void displayView(int position) { 
     // update the main content by replacing fragments 
     Fragment fragment = null; 
     switch (position) { 
      case 0: 
       fragment = new LoginFragment(); 
       break; 
      case 1: 
       fragment = new RegisterFragment(); 
       break; 
      case 2: 
       fragment = new LoginFragment(); 
       break; 
      case 3: 
       fragment = new LoginFragment(); 
       break; 
      case 4: 
       fragment = new LoginFragment(); 
       break; 
      case 5: 
       fragment = new LoginFragment(); 
       break; 

      default: 
       break; 
     } 

     if (fragment != null) { 
      FragmentManager fragmentManager = getFragmentManager(); 
      fragmentManager.beginTransaction() 
        .replace(R.id.frame_container, fragment).commit(); 

      // update selected item and title, then close the drawer 
      mDrawerList.setItemChecked(position, true); 
      mDrawerList.setSelection(position); 
      setTitle(navMenuTitles[position]); 
      mDrawerLayout.closeDrawer(mDrawerList); 
     } else { 
      // error in creating fragment 
      Log.e("MainActivity", "Error in creating fragment"); 
     } 
    } 

    @Override 
    public void setTitle(CharSequence title) { 
     mTitle = title; 
     getSupportActionBar().setTitle(mTitle); 
    } 

    /** 
    * When using the ActionBarDrawerToggle, you must call it during 
    * onPostCreate() and onConfigurationChanged()... 
    */ 

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

    @Override 
    public void onConfigurationChanged(Configuration newConfig) { 
     super.onConfigurationChanged(newConfig); 
     // Pass any configuration change to the drawer toggls 
     mDrawerToggle.onConfigurationChanged(newConfig); 
    } 

} 

Hier ein des Fragments:

package thyroid.com.thyroid; 


import android.app.Fragment; 
import android.app.FragmentTransaction; 
import android.content.SharedPreferences; 
import android.os.Bundle; 
import android.support.design.widget.Snackbar; 
import android.support.v7.widget.AppCompatButton; 
import android.util.Log; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.EditText; 
import android.widget.ProgressBar; 
import android.widget.TextView; 

import retrofit2.Call; 
import retrofit2.Callback; 
import retrofit2.Retrofit; 
import retrofit2.converter.gson.GsonConverterFactory; 
import thyroid.com.thyroid.model.ServerRequest; 
import thyroid.com.thyroid.model.ServerResponse; 
import thyroid.com.thyroid.model.User; 


public class LoginFragment extends Fragment implements View.OnClickListener{ 

    private AppCompatButton btn_login; 
    private EditText et_email,et_password; 
    private TextView tv_register,tv_reset_password; 
    private ProgressBar progress; 
    private SharedPreferences pref; 


    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 

     View view = inflater.inflate(R.layout.fragment_login,container,false); 
     initViews(view); 
     return view; 
    } 

    private void initViews(View view){ 

     pref = getActivity().getPreferences(0); 

     btn_login = (AppCompatButton)view.findViewById(R.id.btn_login); 
     tv_register = (TextView)view.findViewById(R.id.tv_register); 
     tv_reset_password = (TextView)view.findViewById(R.id.tv_reset_password); 
     et_email = (EditText)view.findViewById(R.id.et_email); 
     et_password = (EditText)view.findViewById(R.id.et_password); 

     progress = (ProgressBar)view.findViewById(R.id.progress); 

     btn_login.setOnClickListener(this); 
     tv_register.setOnClickListener(this); 
     tv_reset_password.setOnClickListener(this); 
    } 

    @Override 
    public void onClick(View v) { 

     switch (v.getId()){ 

      case R.id.tv_register: 
       goToRegister(); 
       break; 

      case R.id.btn_login: 
       String email = et_email.getText().toString(); 
       String password = et_password.getText().toString(); 

       if(!email.isEmpty() && !password.isEmpty()) { 

        progress.setVisibility(View.VISIBLE); 
        loginProcess(email,password); 

       } else { 

        Snackbar.make(getView(), "Fields are empty !", Snackbar.LENGTH_LONG).show(); 
       } 
       break; 
      case R.id.tv_reset_password: 
       goToResetPassword(); 
       break; 
     } 
    } 
    private void loginProcess(String email,String password){ 

     Retrofit retrofit = new Retrofit.Builder() 
       .baseUrl(Constants.BASE_URL) 
       .addConverterFactory(GsonConverterFactory.create()) 
       .build(); 

     RequestInterface requestInterface = retrofit.create(RequestInterface.class); 

     User user = new User(); 
     user.setEmail(email); 
     user.setPassword(password); 
     ServerRequest request = new ServerRequest(); 
     request.setOperation(Constants.LOGIN_OPERATION); 
     request.setUser(user); 
     Call<ServerResponse> response = requestInterface.operation(request); 

     response.enqueue(new Callback<ServerResponse>() { 
      @Override 
      public void onResponse(Call<ServerResponse> call, retrofit2.Response<ServerResponse> response) { 

       ServerResponse resp = response.body(); 
       Snackbar.make(getView(), resp.getMessage(), Snackbar.LENGTH_LONG).show(); 

       if(resp.getResult().equals(Constants.SUCCESS)){ 
        SharedPreferences.Editor editor = pref.edit(); 
        editor.putBoolean(Constants.IS_LOGGED_IN,true); 
        editor.putString(Constants.EMAIL,resp.getUser().getEmail()); 
        editor.putString(Constants.NAME,resp.getUser().getName()); 
        editor.putString(Constants.UNIQUE_ID,resp.getUser().getUnique_id()); 
        editor.apply(); 
        goToProfile(); 

       } 
       progress.setVisibility(View.INVISIBLE); 
      } 

      @Override 
      public void onFailure(Call<ServerResponse> call, Throwable t) { 

       progress.setVisibility(View.INVISIBLE); 
       Log.d(Constants.TAG,"failed"); 
       Snackbar.make(getView(), t.getLocalizedMessage(), Snackbar.LENGTH_LONG).show(); 

      } 
     }); 
    } 

    private void goToResetPassword(){ 

     Fragment reset = new Fragment(); 
     FragmentTransaction ft = getFragmentManager().beginTransaction(); 
     ft.replace(R.id.fragment_frame,reset); 
     ft.commit(); 
    } 

    private void goToRegister(){ 

     Fragment register = new RegisterFragment(); 
     FragmentTransaction ft = getFragmentManager().beginTransaction(); 
     ft.replace(R.id.fragment_frame,register); 
     ft.commit(); 
    } 

    private void goToProfile(){ 

     Fragment profile = new ProfileFragment(); 
     FragmentTransaction ft = getFragmentManager().beginTransaction(); 
     ft.replace(R.id.fragment_frame,profile); 
     ft.commit(); 
    } 
} 

Und jetzt die .class, die die "Host-Seite" vor dem Haupt:

package thyroid.com.thyroid; 


import android.content.Intent; 
import android.os.Bundle; 
import android.support.v7.app.AppCompatActivity; 
import android.view.View; 
import android.widget.Button; 

import static thyroid.com.thyroid.R.id.button; 


public class SplashScreen extends AppCompatActivity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     // TODO Auto-generated method stub 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.splashscreen); 

     Button button = (Button) findViewById(R.id.button); 
     assert button != null; 
     button.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       Intent intent = new Intent(v.getContext(),MainActivity.class); 
       startActivityForResult(intent,0); 
      } 
     }); 




     Thread timerThread = new Thread() { 
      public void run() { 
       try { 
        sleep(6000); 
       } catch (InterruptedException e) { 
        e.printStackTrace(); 
       } finally { 
        Intent intent = new Intent(SplashScreen.this, MainActivity.class); 
        startActivity(intent); 
       } 
      } 
     }; 
     timerThread.start(); 
    } 

    @Override 
    protected void onPause() { 
     // TODO Auto-generated method stub 
     super.onPause(); 
     finish(); 
    } 


} 

Das ist mein Fehler:

06-20 11:23:33.603 29939-29939/thyroid.com.thyroid E/AndroidRuntime: FATAL EXCEPTION: main 
                  Process: thyroid.com.thyroid, PID: 29939 
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{thyroid.com.thyroid/thyroid.com.thyroid.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) 
                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
                   at android.app.ActivityThread.access$800(ActivityThread.java:144) 
                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
                   at android.os.Handler.dispatchMessage(Handler.java:102) 
                   at android.os.Looper.loop(Looper.java:135) 
                   at android.app.ActivityThread.main(ActivityThread.java:5221) 
                   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:898) 
                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) 
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                   at thyroid.com.thyroid.MainActivity.onCreate(MainActivity.java:101) 
                   at android.app.Activity.performCreate(Activity.java:5976) 
                   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
                   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
                   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
                   at android.app.ActivityThread.access$800(ActivityThread.java:144) 
                   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
                   at android.os.Handler.dispatchMessage(Handler.java:102) 
                   at android.os.Looper.loop(Looper.java:135) 
                   at android.app.ActivityThread.main(ActivityThread.java:5221) 
                   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:898) 
                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) 

Answer 1

In Ihrem Code Sie haben

private ActionBar getSupportActionBar() { 
    return null; 
} 

Daher getSupportActionBar().setDisplayHomeAsUpEnabled(true) die NPE wirft. Sie müssen diese Methode tatsächlich implementieren ...