Null pointer exception while integrating Facebook login with android app

Question

I have got null pointer exception while integrating facebook login with my android app. I followed this tutorial. I have attached written codes also. FacebookLogin.java

 public class FacebookLogin extends FragmentActivity{
    private static final int SPLASH = 0;
    private static final int SELECTION = 1;
    private static final int FRAGMENT_COUNT = SELECTION +1;
    private Fragment[] fragments = new Fragment[FRAGMENT_COUNT];
    private boolean isResumed = false;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);
        uiHelper = new UiLifecycleHelper(this, callback);
        uiHelper.onCreate(savedInstanceState);

        FragmentManager fm = getSupportFragmentManager();
        fragments[SPLASH] = fm.findFragmentById(R.id.splashFragment);
       // fragments[SELECTION] = fm.findFragmentById(R.id.selectionFragment);
        FragmentTransaction transaction = fm.beginTransaction();
        for(int i = 0; i < fragments.length; i++) {
            transaction.hide(fragments[i]);
        }
        transaction.commit();
    }
    private void showFragment(int fragmentIndex, boolean addToBackStack) {
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction transaction = fm.beginTransaction();
        for (int i = 0; i < fragments.length; i++) {
            if (i == fragmentIndex) {
                transaction.show(fragments[i]);
            } else {
                transaction.hide(fragments[i]);
            }
        }
        if (addToBackStack) {
            transaction.addToBackStack(null);
        }
        transaction.commit();
    }

    @Override
    public void onResume() {
        super.onResume();
        isResumed = true;
        uiHelper.onResume();
        isResumed = true;
    }

    @Override
    public void onPause() {
        super.onPause();
        isResumed = false;
        uiHelper.onPause();
        isResumed = false;
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        uiHelper.onActivityResult(requestCode, resultCode, data);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        uiHelper.onDestroy();
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        uiHelper.onSaveInstanceState(outState);
    }

    private void onSessionStateChange(Session session, SessionState state, Exception exception) {
        // Only make changes if the activity is visible
        if (isResumed) {
            FragmentManager manager = getSupportFragmentManager();
            // Get the number of entries in the back stack
            int backStackSize = manager.getBackStackEntryCount();
            // Clear the back stack
            for (int i = 0; i < backStackSize; i++) {
                manager.popBackStack();
            }
            if (state.isOpened()) {
                // If the session state is open:
                // Show the authenticated fragment
                showFragment(SELECTION, false);
            } else if (state.isClosed()) {
                // If the session state is closed:
                // Show the login fragment
                showFragment(SPLASH, false);
            }       }   }   
    @Override
    protected void onResumeFragments() {
        super.onResumeFragments();
        Session session = Session.getActiveSession();

        if (session != null && session.isOpened()) {
            // if the session is already open,
            // try to show the selection fragment
            showFragment(SELECTION, false);
        } else {
            // otherwise present the splash screen
            // and ask the person to login.
            showFragment(SPLASH, false);
        }   }
    private UiLifecycleHelper uiHelper;
    private Session.StatusCallback callback = 
        new Session.StatusCallback() {
        @Override
        public void call(Session session, 
                SessionState state, Exception exception) {
            onSessionStateChange(session, state, exception);
        }   };
}

SplashFragment.java

public class SplashFragment extends Fragment {
private Button skipLoginButton;
private SkipLoginCallback skipLoginCallback;

public interface SkipLoginCallback {
    void onSkipLoginPressed();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.splash, container, false);

    skipLoginButton = (Button) view.findViewById(R.id.login_button);
    skipLoginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (skipLoginCallback != null) {
                skipLoginCallback.onSkipLoginPressed();
            }            }        });
    return view;
}

public void setSkipLoginCallback(SkipLoginCallback callback) {
    skipLoginCallback = callback;
}}

main.xml

    <LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

   <!--  <fragment android:name="com.GenioCode.replay.SelectionFragment"
          android:id="@+id/selectionFragment"
          android:layout_width="match_parent"
          android:layout_height="match_parent" />-->
    <fragment android:name="com.GenioCode.replay.SplashFragment"
          android:id="@+id/splashFragment"
          android:layout_width="match_parent"
          android:layout_height="match_parent" />
</LinearLayout>

LogCat

    12-22 01:12:05.432: E/AndroidRuntime(1555): FATAL EXCEPTION: main
12-22 01:12:05.432: E/AndroidRuntime(1555): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.GenioCode.replay/com.GenioCode.replay.FacebookLogin}: java.lang.NullPointerException
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.os.Looper.loop(Looper.java:137)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread.main(ActivityThread.java:5103)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at java.lang.reflect.Method.invokeNative(Native Method)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at java.lang.reflect.Method.invoke(Method.java:525)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at dalvik.system.NativeStart.main(Native Method)
12-22 01:12:05.432: E/AndroidRuntime(1555): Caused by: java.lang.NullPointerException
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:656)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:556)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.Activity.performStart(Activity.java:5143)
12-22 01:12:05.432: E/AndroidRuntime(1555):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
12-22 01:12:05.432: E/AndroidRuntime(1555):     ... 11 more

Please help me to solve this exception.

Answer 1

private Fragment[] fragments = new Fragment[FRAGMENT_COUNT];

the length of your fragments array is 2;

so when you call

private void showFragment(int fragmentIndex, boolean addToBackStack) {
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction transaction = fm.beginTransaction();
        for (int i = 0; i < fragments.length; i++) {
            if (i == fragmentIndex) {
                transaction.show(fragments[i]);
            } else {
                transaction.hide(fragments[i]);
            }
        }
        if (addToBackStack) {
            transaction.addToBackStack(null);
        }
        transaction.commit();
    }

with fragmentIndex = 1, which is null, as you have commented the line

// fragments[SELECTION] = fm.findFragmentById(R.id.selectionFragment);

so it tries to show a null fragment (as the loop will still run twice, even though you have only one fragment)

that might give you a null Pointer Exception