Nullpointerexception on RecyclerView

Question

Alles was ich versuche zu tun ist eine recyclerview innerhalb eines tablayout zu erstellen Ich habe versucht, den Adapter in der Hauptaktivität sowie das Fragment im Einsatz in der tablayout aber egal wie bekomme ich immer noch diesen Fehler

java.lang.RuntimeException: Aktivität kann nicht gestartet werden ComponentInfo {knightsrealms.managment_app/knightsrealms.managment_app.Dashboard}: java.lang.NullPointerException: Versuch, die virtuelle Methode 'void android.support.v7.widget.RecyclerView.setAdapter (android.support.v7.widget.RecyclerView $ Adapter)‘auf einer null-Objekt Referenz

package knightsrealms.managment_app; 

import android.support.design.widget.TabLayout; 
import android.support.v4.content.ContextCompat; 
import android.support.v4.view.ViewPager; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.support.v7.widget.Toolbar; 
import android.view.Menu; 

import java.util.ArrayList; 
import java.util.Calendar; 

public class Dashboard extends AppCompatActivity { 

private TabLayout tabLayout; 
private ViewPager viewPager; 
private ViewPageAdapter viewPagerAdapter; 
ArrayList<knightsrealms.managment_app.Calendar> contacts = new    ArrayList<knightsrealms.managment_app.Calendar>(5); 

@Override protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_dashboard); 
    tabLayout = (TabLayout) findViewById(R.id.tabs); 
    viewPager = (ViewPager) findViewById(R.id.viewpager); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.tool_bar); 
    viewPagerAdapter = new ViewPageAdapter(getSupportFragmentManager()); 
    viewPager.setAdapter(viewPagerAdapter); 
    final LinearLayoutManager layoutManager = new LinearLayoutManager(this); 
    layoutManager.setOrientation(LinearLayoutManager.VERTICAL); 
    setSupportActionBar(toolbar); 



    final TabLayout.Tab messages = tabLayout.newTab(); 
    final TabLayout.Tab dashboard = tabLayout.newTab(); 

    messages.setText("Messages"); 
    dashboard.setText("Dashboard"); 

    tabLayout.addTab(dashboard, 0); 
    tabLayout.addTab(messages, 1); 

    tabLayout.setTabTextColors(ContextCompat.getColorStateList(this, R.color.tab_selector)); 
    tabLayout.setSelectedTabIndicatorColor(ContextCompat.getColor(this, R.color.indicator)); 
    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() { 
     @Override 
     public void onTabSelected(TabLayout.Tab tab) { 
      viewPager.setCurrentItem(tab.getPosition()); 
     } 

     @Override 
     public void onTabUnselected(TabLayout.Tab tab) { 

     } 

     @Override 
     public void onTabReselected(TabLayout.Tab tab) { 

     } 
    }); 

    viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout)); 
      RecyclerView rvContacts = (RecyclerView)findViewById(R.id.rvcal); 


    contacts.add(0, new knightsrealms.managment_app.Calendar("Number 1",true)); 
    contacts.add(1, new knightsrealms.managment_app.Calendar("Number 2",true)); 
    CalendarAdapter adapter = new CalendarAdapter(contacts); 


    rvContacts.setAdapter(adapter); 
    rvContacts.setLayoutManager(new LinearLayoutManager(this)); 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 

    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.dashboard_menu, menu); 
    return true; 
} 

void selectPage(int pageIndex){ 
    tabLayout.setScrollPosition(pageIndex,0f,true); 
    viewPager.setCurrentItem(pageIndex); 
} 
} 

dies ist mein Adapter

package knightsrealms.managment_app; 

import android.content.Context; 
import android.support.v7.widget.RecyclerView; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.Button; 
import android.widget.TextView; 

import java.util.List; 

public class CalendarAdapter extends  RecyclerView.Adapter<CalendarAdapter.ViewHolder> { 

    // Provide a direct reference to each of the views within a data item 
    // Used to cache the views within the item layout for fast access 
    public static class ViewHolder extends RecyclerView.ViewHolder { 
     // Your holder should contain a member variable 
     // for any view that will be set as you render a row 
     public TextView nameTextView; 
     public Button messageButton; 

     // We also create a constructor that accepts the entire item row 
     // and does the view lookups to find each subview 
     public ViewHolder(View itemView) { 
      // Stores the itemView in a public final member variable that can be used 
      // to access the context from any ViewHolder instance. 
      super(itemView); 

      nameTextView = (TextView) itemView.findViewById(R.id.contact_name); 
      messageButton = (Button) itemView.findViewById(R.id.message_button); 
     } 

    } 

private List<Calendar> mContacts; 

    // Pass in the contact array into the constructor 
    public CalendarAdapter(List<Calendar> contacts) { 
    mContacts = contacts; 
    } 

// Usually involves inflating a layout from XML and returning the holder 
@Override 
public CalendarAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
    Context context = parent.getContext(); 
    LayoutInflater inflater = LayoutInflater.from(context); 

    // Inflate the custom layout 
    View contactView = inflater.inflate(R.layout.recyclercell, parent, false); 

    // Return a new holder instance 
    ViewHolder viewHolder = new ViewHolder(contactView); 
    return viewHolder; 
} 

// Involves populating data into the item through holder 
@Override 
public void onBindViewHolder(CalendarAdapter.ViewHolder viewHolder, int position) { 
    // Get the data model based on position 
    Calendar contact = mContacts.get(position); 

    // Set item views based on the data model 
    TextView textView = viewHolder.nameTextView; 
    textView.setText(contact.getName()); 

    Button button = viewHolder.messageButton; 

    if (contact.isOnline()) { 
     button.setText("Message"); 
     button.setEnabled(true); 
    } 
    else { 
     button.setText("Offline"); 
     button.setEnabled(false); 
    } 

} 

// Return the total count of items 
@Override 
public int getItemCount() { 
    return mContacts.size(); 
} 
} 

Dieser Code für das Fragment ist, das innerhalb des tablayout ist

<android.support.v7.widget.RecyclerView 
android:id="@+id/rvcal" 
android:layout_width="match_parent" 
android:layout_height="match_parent" /> 

</RelativeLayout> 

Und das ist die Haupttätigkeit xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
xmlns:app="http://schemas.android.com/apk/res-auto"> 

<include 
android:id="@+id/tool_bar" 
layout="@layout/tool_bar" 
></include> 

<android.support.design.widget.TabLayout 
android:id="@+id/tabs" 
android:layout_width="match_parent" 
android:layout_height="wrap_content" 
android:background="@color/colorPrimary" 
android:layout_below="@+id/tool_bar" 
android:layout_alignParentLeft="true" 
android:layout_alignParentStart="true" 
app:tabIndicatorHeight="3dp"/> 

<android.support.v4.view.ViewPager 
android:id="@+id/viewpager" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:layout_below="@+id/tabs" 
/> 

</RelativeLayout> 

jede Einsicht/Hilfe in das wird sehr geschätzt danke !!

Answer 1

haben Sie bereits in versuchen:

tabLayout.setupWithViewPager(viewPager); 

Nun, da recyclerView in Ihrem Fragment xml ist nicht in Ihrer Tätigkeit xml, so ist die recyclerView return null. Ok, also ist Ihr Code fast fertig, aber man braucht zwei weitere Dinge:

• erste, eine Klasse, die FragmentPagerAdapter für Ihre ViewPager erstreckt, weiß ich nicht, ob Ihre ViewPageAdapter Klasse schon tun, aber Nehmen wir an, Sie sind noch nicht fertig.
• zweitens, ist ein Fragment für Ihren FragmentPagerAdapter.

Oke Lets beginnen:

1. in Ihrem Dashboard Aktivität, erstellen Sie eine statische Klasse Lassen Sie sagen ViewPagerAdapter:

   static class ViewPagerAdapter extends FragmentPagerAdapter { 
   
       private final List<Fragment> fragmentList = new ArrayList<>(); 
       private final List<String> fragmentTitleList = new ArrayList<>(); 
   
       public ViewPagerAdapter(FragmentManager fm) { 
       super(fm); 
       } 
   
       public void addFragment(Fragment fragment, String title){ 
       fragmentList.add(fragment); 
       fragmentTitleList.add(title); 
       } 
   
       @Override 
       public Fragment getItem(int position) { 
       return fragmentList.get(position); 
       } 
   
       @Override 
       public int getCount() { 
       return fragmentList.size(); 
       } 
   
       @Override 
       public CharSequence getPageTitle(int position) { 
       return fragmentTitleList.get(position); 
       } 
   } 
   

2. erstellen FragmentContacts für die FragmentPagerAdapter, in diesem Fragment sollten Sie verwalten Ihre RecyclerView und ihr Adapter ist nicht in der Aktivität.

   public class FragmentContacts extends Fragment { 
       ArrayList<knightsrealms.managment_app.Calendar> contacts = new ArrayList<knightsrealms.managment_app.Calendar>(5); 
   
       @Nullable 
       @Override 
       public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
        View rootView = (View) inflater.inflate(R.layout.your_fragment_xml, container, false); 
   
        contacts.add(0, new knightsrealms.managment_app.Calendar("Number 1",true)); 
        contacts.add(1, new knightsrealms.managment_app.Calendar("Number 2",true)); 
        CalendarAdapter adapter = new CalendarAdapter(contacts); 
   
        RecyclerView rvContacts = (RecyclerView) rootView.findViewById(R.id.rvcal); 
        rvContacts.setLayoutManager(new LinearLayoutManager(getContext(), LinearLayoutManager.VERTICAL, false)); 
        rvContacts.setAdapter(adapter); 
   
        return rootView; 
       } 
   
   } 
   

3. schließlich zurück in Ihrem Dashboard Aktivität in onCreate, setUp Ihre Viewpager:

   @Override protected void onCreate(Bundle savedInstanceState) { 
       super.onCreate(savedInstanceState); 
       setContentView(R.layout.activity_dashboard); 
       Toolbar toolbar = (Toolbar) findViewById(R.id.tool_bar); 
   
       viewPager = (ViewPager) findViewById(R.id.viewpager); 
       viewPagerAdapter = new ViewPageAdapter(getSupportFragmentManager()); 
       viewPagerAdapter.addFragment(new FragmentContacts(),"Messages"); 
       viewPagerAdapter.addFragment(new FragmentContacts(),"Dashboard"); 
       viewPager.setAdapter(viewPagerAdapter); 
   
       tabLayout = (TabLayout) findViewById(R.id.tabs); 
       tabLayout.setupWithViewPager(viewPager); 
       viewPager.setCurrentItem(0); 
   }