Android ImageButton中的java.lang.NullPointerException

Question

I have been trying to use an ImageButton on a fragment but I keep getting a NullPointerException and don't know how to fix it. I have read many similar topics and tutorials but really can't figure it out. Here is my fragment:

public class OneFragment extends Fragment{

ViewGroup viewGroup;
ImageButton startButton, stopButton;
private Context context = null;
boolean isPlaying;
Intent streamService;
SharedPreferences prefs;
TextView textView;

public OneFragment() {
    // Required empty public constructor
}

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

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    //start & play buttons
    context = getActivity();
    viewGroup = (ViewGroup) inflater.inflate(R.layout.fragment_one, container, false);
    stopButton = (ImageButton) viewGroup.findViewById(R.id.stopButton);
    startButton = (ImageButton) viewGroup.findViewById(R.id.startButton);
    prefs = PreferenceManager.getDefaultSharedPreferences(context);
    getPrefs();
    streamService = new Intent(super.getActivity(), StreamService.class);

    startButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            getActivity().startService(streamService);
            startButton.setEnabled(false);
        }
    });

    stopButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            getActivity().stopService(streamService);
            startButton.setEnabled(true);
        }
    });
    return viewGroup;
}


public void getPrefs() {
    isPlaying = prefs.getBoolean("isPlaying", false);
    if (isPlaying) startButton.setEnabled(false);
}}

and this is my log:

E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         java.lang.NullPointerException
                                                                             at info.androidhive.materialtabs.fragments.OneFragment.onCreateView(OneFragment.java:78)
                                                                             at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
                                                                             at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1026)
                                                                             at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1207)
                                                                             at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
                                                                             at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1572)
                                                                             at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:545)
                                                                             at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
                                                                             at android.support.v4.view.ViewPager.populate(ViewPager.java:1106)
                                                                             at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
                                                                             at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1474)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:607)
                                                                             at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:674)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                             at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
                                                                             at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
                                                                             at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
                                                                             at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
                                                                             at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
                                                                             at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
                                                                             at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2176)
                                                                             at android.view.View.measure(View.java:15518)
                                                                             at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1874)
                                                                             at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1089)
                                                                             at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1265)
                                                                             at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
                                                                             at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
                                                                             at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
                                                                             at android.view.Choreographer.doCallbacks(Choreographer.java:562)
                                                                             at android.view.Choreographer.doFrame(Choreographer.java:532)
                                                                             at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
                                                                             at android.os.Handler.handleCallback(Handler.java:725)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:92)
                                                                             at android.os.Looper.loop(Looper.java:137)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:5041)
                                                                             at java.lang.reflect.Method.invokeNative(Native Method)
                                                                             at java.lang.reflect.Method.invoke(Method.java:511)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
                                                                             at dalvik.system.NativeStart.main(Native Method)

If you have any ideas, I would really enjoy your help!

Answer 1

Move most of your stuff to onViewCreated because until then the view doesn't actually exist!

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    context = getActivity();
    viewGroup = (ViewGroup) inflater.inflate(R.layout.fragment_one, container, false);
    return viewGroup;
}


@Override
public void onViewCreated (final View view, final Bundle savedInstanceState) {
    super.onViewCreated (view, savedInstanceState);

    //start & play buttons
    stopButton = (ImageButton) viewGroup.findViewById(R.id.stopButton);
    startButton = (ImageButton) viewGroup.findViewById(R.id.startButton);
    prefs = PreferenceManager.getDefaultSharedPreferences(context);
    getPrefs();
    streamService = new Intent(super.getActivity(), StreamService.class);

    startButton.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        getActivity().startService(streamService);
        startButton.setEnabled(false);
    }
});

stopButton.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        getActivity().stopService(streamService);
        startButton.setEnabled(true);
    }
});
}

public void getPrefs() {
   isPlaying = prefs.getBoolean("isPlaying", false);
   if (isPlaying) startButton.setEnabled(false);
}}