Navigation Drawer setAdapter NullPointerException

Question

I'm trying to implement a custom Navigation Drawer using this tutorial but am getting a NullPointerException on mDrawerList.setAdapter(adapter); when the app tries to start.

Code as follows;

MainActivity.java

public class MainActivity extends Activity {

    TextView ConnectionStatus;

    // For Navigation Drawer. From "http://www.androidhive.info/2013/11/android-sliding-menu-using-navigation-drawer/"
    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;
    // ***

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

        // For checking connection status
        this.registerReceiver(this.mConnReceiver,
                new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION));
        ConnectionStatus = (TextView)findViewById(R.id.txtViewConnectionStatus);
        // ***

        // Setting up the Navigation Drawer
        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+"));
        // Communities, Will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[6], navMenuIcons.getResourceId(6, -1), true, "22"));
        // Pages
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[7], navMenuIcons.getResourceId(7, -1)));
        // What's hot, We  will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[8], navMenuIcons.getResourceId(8, -1), true, "50+"));
        // Recycle the typed array
        navMenuIcons.recycle();
        // 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
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().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) {
                getActionBar().setTitle(mTitle);
                // calling onPrepareOptionsMenu() to show action bar icons
                invalidateOptionsMenu();
            }
            public void onDrawerOpened(View drawerView) {
                getActionBar().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);
        }
    }
}

NavDrawerListAdapter.java

public class NavDrawerListAdapter extends BaseAdapter {

    private Context context;
    private ArrayList<NavDrawerItem> navDrawerItems;

    public NavDrawerListAdapter(Context context, ArrayList<NavDrawerItem> navDrawerItems){
        this.context = context;
        this.navDrawerItems = navDrawerItems;

    }

    @Override
    public int getCount() {
        return navDrawerItems.size();
    }

    @Override
    public Object getItem(int position) {
        return navDrawerItems.get(position);
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (convertView == null) {
            LayoutInflater mInflater = (LayoutInflater)
                    context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            convertView = mInflater.inflate(R.layout.drawer_list_item, null);
        }

        ImageView imgIcon = (ImageView) convertView.findViewById(R.id.icon);
        TextView txtTitle = (TextView) convertView.findViewById(R.id.title);
        TextView txtCount = (TextView) convertView.findViewById(R.id.counter);

        imgIcon.setImageResource(navDrawerItems.get(position).getIcon());
        txtTitle.setText(navDrawerItems.get(position).getTitle());

        // displaying count
        // check whether it set visible or not
        if(navDrawerItems.get(position).getCounterVisibility()){
            txtCount.setText(navDrawerItems.get(position).getCount());
        }else{
            // hide the counter view
            txtCount.setVisibility(View.GONE);
        }

        return convertView;
    }

}

and NavDrawerItem.java

public class NavDrawerItem {
    private String title;
    private int icon;
    private String count = "0";
    // boolean to set visiblity of the counter
    private boolean isCounterVisible = false;

    public NavDrawerItem(){}

    public NavDrawerItem(String title, int icon){
        this.title = title;
        this.icon = icon;
    }

    public NavDrawerItem(String title, int icon, boolean isCounterVisible, String count){
        this.title = title;
        this.icon = icon;
        this.isCounterVisible = isCounterVisible;
        this.count = count;
    }

    public String getTitle(){
        return this.title;
    }

    public int getIcon(){
        return this.icon;
    }

    public String getCount(){
        return this.count;
    }

    public boolean getCounterVisibility(){
        return this.isCounterVisible;
    }

    public void setTitle(String title){
        this.title = title;
    }

    public void setIcon(int icon){
        this.icon = icon;
    }

    public void setCount(String count){
        this.count = count;
    }

    public void setCounterVisibility(boolean isCounterVisible){
        this.isCounterVisible = isCounterVisible;
    }
}

drawer_list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="48dp"
    android:background="@drawable/list_selector">

    <ImageView
        android:id="@+id/icon"
        android:layout_width="25dp"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_marginLeft="12dp"
        android:layout_marginRight="12dp"
        android:contentDescription="@string/desc_list_item_icon"
        android:src="@drawable/ic_home"
        android:layout_centerVertical="true" />

    <TextView
        android:id="@+id/title"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_toRightOf="@id/icon"
        android:minHeight="?android:attr/listPreferredItemHeightSmall"
        android:textAppearance="?android:attr/textAppearanceListItemSmall"
        android:textColor="@color/list_item_title"
        android:gravity="center_vertical"
        android:paddingRight="40dp"/>

    <TextView android:id="@+id/counter"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/counter_bg"
        android:layout_alignParentRight="true"
        android:layout_centerVertical="true"
        android:layout_marginRight="8dp"
        android:textColor="@color/counter_text_color"/>

</RelativeLayout>

Answer 1

From your layout file, it seems you are trying to get a reference to a ListView that does not exist:

mDrawerList = (ListView) findViewById(R.id.list_slidermenu);

That's why you later get a NPE. If you followed that tutorial, check if indeed in your layout file (home.xml) you have a definition like the one in the tutorial:

<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">

<!-- Framelayout to display Fragments -->
<FrameLayout
    android:id="@+id/frame_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<!-- Listview to display slider menu -->
<!-- This is the listView you are trying to find in your code. -->
<ListView
    android:id="@+id/list_slidermenu"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:choiceMode="singleChoice"
    android:divider="@color/list_divider"
    android:dividerHeight="1dp"       
    android:listSelector="@drawable/list_selector"
    android:background="@color/list_background"/>
</android.support.v4.widget.DrawerLayout>