findViewById返回null

Question

I'm writing an Android app with several fragments. On a phone, only one fragment will be displayed, and on a tablet, three fragments. Here are my layouts for the different devices:

Tablet layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mainTopLayout"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:baselineAligned="false"
    android:orientation="horizontal"
    android:background="@color/activity_background"
    android:tag="@string/layout_tablet_landscape" >

<FrameLayout android:id="@+id/leftContainerLayout"
    android:layout_width="0dp"
    android:layout_height="match_parent"
    android:layout_weight="0.33" />

<FrameLayout android:id="@+id/centerContainerLayout"
    android:layout_width="0dp"
    android:layout_height="match_parent"
    android:layout_weight="0.33" />

<FrameLayout android:id="@+id/rightContainerLayout"
    android:layout_width="0dp"
    android:layout_height="match_parent"
    android:layout_weight="0.33" />

</LinearLayout>

Phone layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mainTopLayout"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="@color/activity_background"
    android:tag="@string/layout_phone_portrait" >

<FrameLayout android:id="@+id/mainContainerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignParentTop="true"
    android:layout_above="@+id/mainBrewAd" />

<com.google.ads.AdView xmlns:ads="http://schemas.android.com/apk/lib/com.google.ads"
    android:id="@+id/mainBrewAd"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    ads:adUnitId="##############"
    ads:adSize="BANNER"
    ads:loadAdOnCreate="true"/>

</RelativeLayout>

Here's the code for adding the fragments to the layout in the main activity's OnCreate method:

if ((layoutTag == LAND_PHONE) || (layoutTag == PORT_PHONE)) {
        GravityFragment gravityFragment = (GravityFragment)fragmentManager.findFragmentById(R.id.mainContainerLayout);
        if (gravityFragment == null) {
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.add(R.id.mainContainerLayout, new GravityFragment());
            transaction.commit();
        }
    }
    if ((layoutTag == LAND_TABLET) || (layoutTag == PORT_TABLET)) {
        GravityFragment gravityFragment = (GravityFragment)fragmentManager.findFragmentById(R.id.leftContainerLayout);
        if (gravityFragment == null) {
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.add(R.id.leftContainerLayout, new GravityFragment());
            transaction.commit();
        }
    }

And here's the fragment's onStart method:

@Override
public void onStart() {
    super.onStart();
    //Init a brewcalc object
    brewCalc = new BrewCalc(getActivity());
    //Assign the click listener
    Activity topActivity = getActivity();
    Button calcButton = (Button)getActivity().findViewById(R.id.gravityCalculateButton);
    calcButton.setOnClickListener(calculateListener);
}

When I start it on a phone, everything works fine. When I start it on a tablet, I get a null pointer exception. Apparently, findViewById returns null in this case. Why is this?

Answer 1

Just Put your code in OnCreateView() Method in Fragment .

See the following example

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) 
    {
        activityContext = getActivity();
        View parentView = inflater.inflate(R.layout.sync_fargment,container,false);

        ImageView abc= (ImageView) parentView.findViewById(R.id.abc);
        ImageView xyz= (ImageView) parentView.findViewById(R.id.xyz);

    }

Answer 2

Please verify that you have put your layout files in corresponding layout folder for tablets. Please check out this document and it will describe how organize your layout files in tablets, http://developer.android.com/guide/practices/screens_support.html