Transformations.map() returns null values

Question

I am writing a program in which i have two LiveData objects as shown:

    public LiveData<placeImages> chosenImages;
    public LiveData<List<Bitmap>> chosenBitmaps= Transformations.map(chosenImages, (placeimags) ->
    {
        ArrayList<Bitmap> bitmaps=new ArrayList<Bitmap>();
        for(PhotoI photoI:placeimags.images)
        {
            try {
                Bitmap b=getThumbnail(app.getContentResolver(),photoI.imageURI);
                if(b!=null) {
                    bitmaps.add(b);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return bitmaps;
    });

The problem is whenever I observe the chosenImages LiveData it works fine, but when I observe chosenBitmaps LiveData it returns the following error:

2020-07-18 17:40:39.790 32164-32164/com.nisarg.locsav E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.nisarg.locsav, PID: 32164
    java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.lifecycle.LiveData.observeForever(androidx.lifecycle.Observer)' on a null object reference
        at androidx.lifecycle.MediatorLiveData$Source.plug(MediatorLiveData.java:141)
        at androidx.lifecycle.MediatorLiveData.onActive(MediatorLiveData.java:118)
        at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:437)
        at androidx.lifecycle.LiveData$LifecycleBoundObserver.onStateChanged(LiveData.java:395)
        at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361)
        at androidx.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:300)
        at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:339)
        at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:145)
        at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:131)
        at androidx.fragment.app.FragmentViewLifecycleOwner.handleLifecycleEvent(FragmentViewLifecycleOwner.java:51)
        at androidx.fragment.app.Fragment.performStart(Fragment.java:2737)
        at androidx.fragment.app.FragmentStateManager.start(FragmentStateManager.java:365)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1194)
        at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
        at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
        at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
        at android.os.Handler.handleCallback(Handler.java:794)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:176)
        at android.app.ActivityThread.main(ActivityThread.java:6635)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)

I think Transformations.map() is returning null value. I worked on a similar thing in a project before, same thing happened when i used Transformations.map(). Does anyone know why this is happening?

Answer 1

change public LiveData<placeImages> chosenImages; to

public LiveData<placeImages> chosenImages = MutableLiveData()

Answer 2

Presumably there is a MutableLiveData somewhere behind chosenImages?

What you describe happens when that MutableLiveData is not initialized by using the noargs constructor new MutableLiveData<placeImages>(), leaving the Transformations.map() derived LiveData also not initialized, hence the null.

The solution is to initialize the MutableLiveData on creation, even if it is initialized to null: new MutableLiveData<placeImages>(null) so that the derived also gets initialized.