我是android的初学者，我尝试通过以下教程制作Navigation Drawer。但是我在这里遇到了错误，请帮我继续进一步
MainActivity：
public class MainActivity extends AppCompatActivity {
private Toolbar toolbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
toolbar = (Toolbar) findViewById(R.id.app_bar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayShowHomeEnabled(true);
NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawerlayout), toolbar);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
if (id == R.id.navigate) {
startActivity(new Intent(this, SubActivity.class));
}
return super.onOptionsItemSelected(item);
}
}
activity_main.xml中：
<android.support.v4.widget.DrawerLayout
android:id="@+id/drawerlayout"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent">
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<include
android:id="@+id/app_bar"
layout="@layout/app_bar" />
<TextView
android:id="@+id/textview"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/app_bar"
android:text="@string/hello_world" />
</RelativeLayout>
<fragment
android:id="@+id/fragment_navigation_drawer"
android:name="com.example.kitty.emo.NavigationDrawerFragment"
android:layout_width="280dp"
android:layout_height="match_parent"
android:layout_gravity="start"
app:layout="@layout/fragment_navigation_drawer"
tools:layout="@layout/fragment_navigation_drawer" />
</android.support.v4.widget.DrawerLayout>
NavigationDrawerFragment.java：
public class NavigationDrawerFragment extends Fragment {
public static final String PREF_FILE_NAME = "testpref";
public static final String KEY_USER_LEARNED_DRAWER = "user_learned_drawer";
private ActionBarDrawerToggle mDrawerToggle;
private DrawerLayout mDrawerLayout;
private Boolean mUserLearnedDrawer;
private Boolean mFromSavedInstanceState;
private View containerView;
public NavigationDrawerFragment() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mUserLearnedDrawer = Boolean.valueOf(readFromPreferences(getActivity(), KEY_USER_LEARNED_DRAWER, "false"));
if (savedInstanceState != null) {
mFromSavedInstanceState = true;
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_navigation_drawer, container, false);
}
public void setUp(int fragmentId, DrawerLayout drawerLayout, Toolbar toolbar) {
containerView = getActivity().findViewById(fragmentId);
mDrawerLayout = drawerLayout;
mDrawerToggle = new ActionBarDrawerToggle(getActivity(), drawerLayout, toolbar, R.string.drawer_open, R.string.drawer_close) {
@Override
public void onDrawerOpened(View drawerView) {
super.onDrawerOpened(drawerView);
if (!mUserLearnedDrawer) {
mUserLearnedDrawer = true;
saveToPreferences(getActivity(), KEY_USER_LEARNED_DRAWER, mUserLearnedDrawer + "");
}
getActivity().invalidateOptionsMenu();
}
@Override
public void onDrawerClosed(View drawerView) {
super.onDrawerClosed(drawerView);
getActivity().invalidateOptionsMenu();
}
};
if (!mUserLearnedDrawer && !mFromSavedInstanceState) {
mDrawerLayout.openDrawer(containerView);
}
mDrawerLayout.setDrawerListener(mDrawerToggle);
}
public static void saveToPreferences(Context context, String preferenceName, String preferenceValue) {
SharedPreferences sharedPreferences = context.getSharedPreferences(PREF_FILE_NAME, context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString(preferenceName, preferenceValue);
editor.apply();
}
public static String readFromPreferences(Context context, String prefernceName, String defaultValue) {
SharedPreferences sharedPreferences = context.getSharedPreferences(PREF_FILE_NAME, context.MODE_PRIVATE);
return sharedPreferences.getString(prefernceName, defaultValue);
}
}
fragment_navigation_drawer.xml：
<FrameLayout 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"
tools:context="com.example.kitty.emo.NavigationDrawerFragment">
<!-- TODO: Update blank fragment layout -->
<TextView android:layout_width="match_parent" android:layout_height="match_parent"
android:text="@string/hello_blank_fragment" />
</FrameLayout>
最后 logcat的：
06-05 14:20:11.806 13896-13896/com.example.kitty.emo E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.kitty.emo, PID: 13896
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitty.emo/com.example.kitty.emo.MainActivity}: 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:5021)
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:827)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.kitty.emo.NavigationDrawerFragment.setUp(NavigationDrawerFragment.java:79)
at com.example.kitty.emo.MainActivity.onCreate(MainActivity.java:25)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1090)
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:5021)
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:827)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
at dalvik.system.NativeStart.main(Native Method)