设置项目单击侦听器时，Android null指针异常

Question

So im following this tutorial on android hive about creating a navigation drawer i keep getting a nullPointerException when i set an item click listener; does anyone know why this could be?

    private ArrayList<NavDrawerItem> navDrawerItems;
        private NavDrawerListAdapter adapter;
        @TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_maps);
            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();

/* this line is nullPointerException (line 87)*/            mDrawerList.setOnItemClickListener(new SlideMenuClickListener());

Stack Trace

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.adrian.xxxxxxxxxx/com.example.adrian.xxxxxxxxxxx.MapsActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
        at android.app.ActivityThread.access$800(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at com.example.adrian.xxxxxxxxxxxxx.MapsActivity.onCreate(MapsActivity.java:87)
        at android.app.Activity.performCreate(Activity.java:5231)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

Your activity_maps layout does not have a view with id list_slidermenu and findViewById() returns null. Invoking a method on null causes the exception.