Android - NullPointerException when trying to implement Adapter from Fragment

Question

I am loading a Fragment and in turn the Fragment creates an Adapter. The Adapter has an interface. The Fragment that loads implements the Adapter. There are no errors when I initiate mCallback.

Here is code in Adapter:

Scanlist mScanlist; 
ScanlistListener mCallback;

public Scanadapter(Activity a, ArrayList<Scanmodel> d, Resources resLocal){
    mActivity = a;
    mData = d;
    res = resLocal;
    sInflater = (LayoutInflater)mActivity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    mScanlist = (Scanlist) mActivity.getFragmentManager().findFragmentById(R.layout.scanlist);
    try {
        mCallback = (ScanlistListener) mScanlist;
        Log.v(TAG, String.valueOf(mCallback)+ " here");
    } catch (ClassCastException e) {
        throw new ClassCastException(mScanlist.toString()
                + " must implement ScanlistListener");
    }
}

private class OnItemClickListener implements OnClickListener{
    private int mPosition;
    OnItemClickListener(int position){
        mPosition = position;
    }
    @Override
    public void onClick(View v) {
        mCallback.scanlistclick(v, mPosition);
    }
}

public interface ScanlistListener{
    public void scanlistclick(View v, int i);
}

When I click on an item in the list im getting a NullPointerException yet the try statement does not give an error exception initiating the mCallback variable.

08-22 10:19:09.099: E/AndroidRuntime(8112): java.lang.NullPointerException
08-22 10:19:09.099: E/AndroidRuntime(8112):     at net.xxxxx.adapter.Scanadapter$OnItemClickListener.onClick(Scanadapter.java:140)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.view.View.performClick(View.java:4489)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.view.View$PerformClick.run(View.java:18803)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Handler.handleCallback(Handler.java:730)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Looper.loop(Looper.java:137)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.app.ActivityThread.main(ActivityThread.java:5455)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at java.lang.reflect.Method.invokeNative(Native Method)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at java.lang.reflect.Method.invoke(Method.java:525)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at dalvik.system.NativeStart.main(Native Method)

What am I doing wrong?

Answer 1

If your findFragmentByID does not find the layout element it was looking for, it will return null. You then assign null to mCallback. Assigning null with a cast will not trigger an exception. Perhaps step through your code with the debugger, and ensure that your view is being found and mCallback is not set to null.