适用于recyclerview的适配器中的空指针异常

Question

I've been trying to implement a recyclerview for my activity but I keep getting NullPointerException. This is my first time making a recyclerview so if I'm doing anything wrong just tell me. I'll show you all my code then tell where where the error is at

my main xml layout (called activity_main.xml)

<android.support.v4.widget.DrawerLayout
                xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:card_view="http://schemas.android.com/apk/res-auto"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:id="@+id/drawer"
                android:fitsSystemWindows="true">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

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

        <android.support.v7.widget.RecyclerView
            android:id="@+id/main_recycler_view"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:paddingRight="8dp"
            android:paddingLeft="8dp"
            android:paddingTop="8dp"/>

    </LinearLayout>

    <android.support.design.widget.NavigationView
        android:id="@+id/navigation_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:headerLayout="@layout/drawer_header"
        app:menu="@menu/drawer"/>

</android.support.v4.widget.DrawerLayout>

My row xml for the recyclerview (called main_recycler_view_row.xml)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <android.support.v7.widget.CardView
        android:id="@+id/main_card"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

            <TextView
                android:id="@+id/period_title"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textSize="24sp"
                android:textColor="@android:color/black"
                android:paddingTop="24dp"
                android:paddingLeft="16dp"
                android:paddingBottom="4dp"/>

            <TextView
                android:id="@+id/period_subtitle"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textSize="14sp"
                android:paddingLeft="16dp"
                android:paddingBottom="16dp"/>

        </LinearLayout>

    </android.support.v7.widget.CardView>

</LinearLayout>

My main activity

package maxbleggi.afstudentplanner;

import android.content.Intent;
import android.content.res.Configuration;
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.app.ActionBar;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;

import com.parse.GetCallback;
import com.parse.Parse;
import com.parse.ParseException;
import com.parse.ParseObject;
import com.parse.ParseQuery;
import com.parse.ParseUser;

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

public class MainActivity extends AppCompatActivity
{
    // "MACROS" for user's class periods
    private final int PERIOD1 = 0;
    private final int PERIOD2 = 1;
    private final int PERIOD3 = 2;
    private final int PERIOD4 = 3;
    private final int PERIOD5 = 4;
    private final int PERIOD6 = 5;
    private final int PERIOD7 = 6;

    // toolbar
    private Toolbar toolbar;

    // nav drawer
    private DrawerLayout navDrawer;

    // user's data
    private String[] userClasses = new String[10];

    // list to fill Classroom class with
    private List<ClassroomClass> classrooms;

    /*
    * on creation of activity
     */
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // initialise and set toolbar as actionbar
        toolbar = (Toolbar) findViewById(R.id.tool_bar);
        setSupportActionBar(toolbar);

        // initialize drawer layout
        navDrawer = (DrawerLayout) findViewById(R.id.drawer);
        NavigationView navView = (NavigationView) findViewById(R.id.navigation_view);

        // initialize recycler view elements
        RecyclerView mainRecyclerView = (RecyclerView)findViewById(R.id.main_recycler_view);

        // initialize layout manager for recycler view
        RecyclerView.LayoutManager mainLayoutManager = new LinearLayoutManager(this);

        // adapter for recycler view
        RecyclerView.Adapter mainAdapter= new MainRecyclerAdapter(classrooms);;

        // initialize nav bars
        initNavBars();

        // initialize nav drawer
        initNavDrawer();

        // initialize data for all classes
        initClassData();

        // add layout manager to recycler view
        mainRecyclerView.setLayoutManager(mainLayoutManager);

        // add adapter to recycler view
        mainRecyclerView.setAdapter(mainAdapter);

    }

    /*
    * initializes the class data for each class
    */
    public void initClassData()
    {
        // check if a user is not cached
        ParseUser currentUser = ParseUser.getCurrentUser();
        if (currentUser == null)
        {
            // prompt user to Register screen
            Intent intent = new Intent(MainActivity.this, RegisterActivity.class);
            intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
            startActivity(intent);
        }

        // query database for user's classes
        final ParseQuery<ParseObject> query = ParseQuery.getQuery("StudentClasses");
        query.whereEqualTo("student_id", ParseUser.getCurrentUser());

        query.getFirstInBackground(new GetCallback<ParseObject>()
        {
            @Override
            public void done(ParseObject parseObject, ParseException e)
            {
                if (parseObject == null)
                {
                    // retrieved the object
                    userClasses[PERIOD1]= parseObject.getString("first_period");
                    userClasses[PERIOD2]= parseObject.getString("second_period");
                    userClasses[PERIOD3]= parseObject.getString("third_period");
                    userClasses[PERIOD4]= parseObject.getString("fourth_period");
                    userClasses[PERIOD5]= parseObject.getString("fifth_period");
                    userClasses[PERIOD6]= parseObject.getString("sixth_period");
                    userClasses[PERIOD7]= parseObject.getString("seventh_period");
                }
                else
                {
                    // failed lookup. Do something here
                }
            }
        });

        // fill all fields for class periods
        classrooms = new ArrayList<>();

        // titles of classes to add to list
        String[] classTitles = {"First Period", "Second Period", "Third Period", "Fourth Period",
                "Fifth Period", "Sixth Period", "Seventh Period"};

        // iterate over every period to add it to the list
        for (int i = 0; i <= PERIOD7; i++)
        {
            // if period registered was "Nothing" don't add it to list
            //if (!userClasses[i].equals("Nothing"))
            //{
                classrooms.add(new ClassroomClass(userClasses[i], classTitles[i], i + 1));
            //}
        }
    }

    public void initNavDrawer()
    {
        navDrawer = (DrawerLayout) findViewById(R.id.drawer);

        final NavigationView navView = (NavigationView) findViewById(R.id.navigation_view);

        navView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener()
        {
            @Override
            public boolean onNavigationItemSelected(MenuItem menuItem)
            {
                menuItem.setChecked(true);
                navDrawer.closeDrawers();
                return true;
            }
        });
    }

    private void initNavBars()
    {
       if (toolbar != null)
       {
            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
            toolbar.setNavigationIcon(R.drawable.ic_menu_white);
            toolbar.setNavigationOnClickListener(new View.OnClickListener()
            {
                @Override
                public void onClick(View v)
                {
                    navDrawer.openDrawer(GravityCompat.START);
                }
            });
       }
    }
}

my class for my data fields

package maxbleggi.afstudentplanner;

/*
 * Define the characteristics that all classroom cards have in common
*/
public class ClassroomClass
{

    String className;
    String classPeriod;
    int classNumber;

    public ClassroomClass(String className, String classPeriod, int classNumber)
    {
        this.className = className;
        this.classPeriod = classPeriod;
        this.classNumber = classNumber;
    }

    public String getClassName()
    {
        return className;
    }

    public String getClassPeriod()
    {
        return classPeriod;
    }

    public int getClassNumber()
    {
        return classNumber;
    }
}

Finally my adapter

package maxbleggi.afstudentplanner;

import android.support.v7.widget.CardView;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.List;

public class MainRecyclerAdapter extends RecyclerView.Adapter<MainRecyclerAdapter.ViewHolder>
{
    private List<ClassroomClass> classrooms;

    public static class ViewHolder extends RecyclerView.ViewHolder
    {
        // hold data items
        public View view;

        ViewHolder(View itemView)
        {
            super(itemView);
            view = itemView;
        }
    }

    public MainRecyclerAdapter(List<ClassroomClass> classrooms)
    {
        this.classrooms = classrooms;
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i)
    {
        // create a new view
        View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.main_recycler_view_row, viewGroup, false);

        // set the view's parameters
        ViewHolder pvh = new ViewHolder(v);
        return pvh;
    }

    @Override
    public void onBindViewHolder(ViewHolder viewHolder, int position)
    {
        // get elements from view
        TextView title = (TextView) viewHolder.view.findViewById(R.id.period_title);
        TextView subtitle = (TextView) viewHolder.view.findViewById(R.id.period_subtitle);

        // retrieve data from data field and put in view
        title.setText(classrooms.get(position).getClassName());
        subtitle.setText(classrooms.get(position).classPeriod);

    }

    @Override
    public int getItemCount()
    {
        return classrooms.size();
    }


    @Override
    public void onAttachedToRecyclerView(RecyclerView recyclerView)
    {
        super.onAttachedToRecyclerView(recyclerView);
    }
}

I get a java.lang.NullPointerException in my MainRecyclerAdapter at the get item count here

    @Override
    public int getItemCount()
    {
        return classrooms.size();
    }

Any help or comments is appreciated thanks and sorry for all the code. I just wanted to make sure I was thorough.

Answer 1

You have to init classrooms data before making constructor, so just invoke

initClassData();

before adapter constructor.

Btw you are supposed to do this code in viewholder constructor

// get elements from view
        TextView title = (TextView) viewHolder.view.findViewById(R.id.period_title);
        TextView subtitle = (TextView) viewHolder.view.findViewById(R.id.period_subtitle);

and later use it like

holder.title.setText

holder.subtitle.setText

Answer 2

You call

initClassData();

after the call of the constructor:

RecyclerView.Adapter mainAdapter= new MainRecyclerAdapter(classrooms);;

this is the reason of the NullPoimter. the object classrooms is still empty so when you invoke the size() on it you get nullPoint.

Just replace the order of those to lines.