问RecyclerView上的空例外

EN

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);
}
}

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();
}
}

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

</RelativeLayout>

<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>